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Larval recruitment in the crab Callinectes sapidus Rathbun: an amendment to the concept of larval retention in estuaries
Abstract
Mechanisms of larval export from estuaries and post-larval recruitment back are examined for the blue crab, Callinectes sapidus Rathbun. Zoeal Stages I, IV, and VII do not alter phototaxis or level of activity in response to temperature or salinity change, except in Stage I where reduction in salinity temporarily reduces activity level. In Stage I, inhibition of upward migration occurs at a halocline of 10 ppt salinity. Sensitivity to haloclines increases through zoeal development. A pycnocline in nature will not inhibit movement to the surface of Stage I larvae and thus cannot promote retention. Megalopa are negatively geotactic and exhibit high barokinesis with a low pressure threshold, adaptations which can promote movement to the surface depending on locomotor activity. A conceptual model, based on evidence from the literature and on experimental data presented here, is proposed for recruitment of Callinectes sapidus and invokes offshore recruitment as the dominant factor. Coastal circulation can maintain larvae of estuarine invertebrates nearshore and provide the potential for offshore recruitment. This mechanism may be essential to population dynamics in C. sapidus and potentially significant in maintaining genetic communication among estuarine populations of other species.
... Larvae which have hatched on the bottom and are migrating upwards in response to taxis and kinesis stimuli will encounter sharp salinity and temperature gradients through which they must pass if they are to reach the surface. Sulkin and Van Heukelem (1982) have shown that larvae of Callinectes sapidus Rathbun can pass through substantial gradients in salinity, but published reports on the ability of brachyuran larvae to penetrate sharp thermoclines are limited to one deep sea species (Kelly et al., 1982). ...
... Change in response to pressure does occur, often resulting in lower sensitivity and less precise control for depth regulation in later zoeal stages (Sulkin, 1973;Bentley and Sulkin, 1977;Wheeler and Epifanio, 1978;Sulkin et al., 1980). On the other hand, megalopae of the swimming crabs Macropipus sp, and Callinectes sapidus show high sensitivity to pressure, in the case of C. sapidusreversing an ontogenetic trend (Naylor and Isaac, 1973;Sulkin and Van Heukelem, 1982). However, because pressure sensitivity is so closely related to depth regulation in the water column and because the transitional megalopa stage is characterized by both larval and adult traits, it is tempting to attribute the low response threshold to pressure in megalopae of these two species to the swimming habit of the adult. ...
... Responses to hydrostatic pressure in the megalopa Because megalopae characteristically alternate swimming and crawling activity, it is difficult to measure swimming rates directly as was done for zoeal Stage I. The method used here for megalopae was the same as that reported by Sulkin and Van Heukelem (1982) for determining barokinesis in Callinectes sapidus megalopae and is based on the same principle as that employed by Naylor and Isaac (1973). The method utilizes the combined effects of kinesis and taxis and exploits the fact that when locomotory activity increases, megalopae will move upwards due to photo-and/or geotactic orientation responses. ...
... Note that these results are identical to those described for Panopeus herbstii and Leptodius floridanus (Sulkin, 1973). The application of hydrocarbons at levels of 0.1 and 1.0 ppm (water-accommodated fraction) caused net downward movement in Stages I to 111, with variable Bigford (1977Bigford ( , 1979 Cancer magister (-1 (-1 (-1 Jacoby (1982) Ebalia tuberosa ( -1 Schembri (1982) Geryon quinquedens (-1 (+l1 Kelly et al. (1982) Callinectes sapidus (-1 -( + l (-1 Sulkin et al. (1980); Sulkin and Van Heukelem (1982) Sulkin and Van Heukelem, unpubl. results thereafter. ...
... Using moderate intensities of diffuse white light to test for presence of barokinesis in zoeal Stages I, IV, and V11 of Callinectes sapidus, Sulkin et al. (1980) demonstrated persistent positive phototaxis through zoeal development. Sulkin and Van Heukelem (1982) reported the effect on phototaxis of temperature and salinity change in C. sapidus. Larvae of Stages I, TV, and V11 were light-adapted and tested at 0.25 W m-* in light filtered to 500 nm. ...
... The responses of Callinectes sapidus zoeae to laboratory-constructed haloclines are generally consistent with this conclusion. Sulkin and Van Heukelem (1982) determined the responses of Stages I, IV, and VII C. sapidus larvae to a series of vertically oriented haloclines ranging in magnitude from 2.5 to 10 % O S (the salinity below the halocline being a constant 30 Ym S). The experimental design was similar to that described above for thermoclines. ...
... Bottom (Bot); not significant (ns); significant (#); Summer (Su); Autumn (Au); Winter (Wi) and Spring (Sp). spawning areas and recruitment back to the estuary (Sulkin et al., 1980;Sulkin and Van Heukelem, 1982). The zoeae of C. sapidus caught within the estuary of the Patos Lagoon on the surface were due to the spawnning that occurred inside the estuary. ...
... Scheltema (1975) studied the estuaries of the South Atlantic and Mid-Atlantic Bight and found that if the megalopae became demersal on the continental shelf, they were passively transported by bottom currents to the estuary. Sulkin and Van Heukelem (1982) proposed a model for offshore recruitment based on the behavioral traits of C. sapidus larvae. They noted that radical behavior changes occurred in the megalopa phase, which resulted in a depth regulatory mechanism of high precision. ...
The abundance and distribution of larval phases of the Portunidae found in the estuary of the Patos Lagoon and the coastal region were studied during two years (1995 and 1999). A conical net (165 cm long, 60 cm mouth, and 330 µm mesh) equipped with a flowmeter was towed for three minutes at 2 knots at six stations within the estuary and four stations in the coastal region. Samplings were carried out on the surface and near the bottom. At each sampling location, the salinity and temperature were also recorded. In 1995, the zoeae of Arenaeus cribrarius (Lamarck, 1818), Callinectes sapidus Rathbun, 1896 and Achelous spinicarpus Stimpson, 1871 were caught, resulting in a total abundance of 121.98 ind.100 m-3 (90.95 ind.100 m-3 on the surface and 31.03 ind.100 m-3 near the bottom). A total of 452.27 ind.100 m-3 were caught in the megalopa phase (13.49 ind.100 m-3 on the surface and 438.78 ind.100 m-3 near the bottom). In 1999, only zoeae of C. sapidus were caught, resulting in a total abundance of 419.78 ind.100 m-3 (386.98 ind.100 m-3 on the surface and 32.8 ind.100 m-3 near the bottom). Megalopae of these three species were caught, resulting in a total abundance of 179.91 ind.100 m-3 (25.38 ind.100 m-3 on surface and 154.53 ind.100 m-3 near the bottom). Summer was the season with the highest abundance of larvae in both years. During spring and summer, spawning was observed in the estuarine region of the Patos Lagoon.
... This behavior could be endogenous or stimulated by cues in the environment. Megalopae are sensitive to pressure change and respond to increasing pressure by raising their level of activity which, in concert with negative geotaxis, results in upward movement (Naylor & Isaac 1973, Sulkin & Van Heukelem 1982). Increasing salinity may also stimulate activity in larval crustaceans (Sulkin 1984). ...
... Increasing salinity and pressure during flood would stimulate movement into the water column. Megalopae descend by passive sinking, at a rate of 1.8 0.55 cm S-' in the laboratory (Sulkin & Van Heukelem 1982). Frequent contact with the bottom and regular exposure to these stimuli could entrain a circatidal rhythm of activity. ...
Following larval development in coastal waters, postlarvae (megalopae) of the blue crab Callinectes sapidus Rathbun enter inlets and, against the net seaward flow of estuarine waters, move upstream to juvenile habitats. Abundance and vertical distribution of blue crab megalopae in the York River, Virginia, USA, a subsetuary of Chesapeake Bay, was quanitified to examine the hypothesis that megalopae are not transported simply as passive particles, but display behaviors that augment their immigration. Megalopal abundance and depth distribution and environmental variables were measured at shallow (3 to 4 m) sites in 1988 and 1989 and at a deep (10 m) site in 1990. Megalopae were more abundant in the water column during flood than ebb, indicating a net upstream flux of megalopae. Densities were always low during ebb. Highest densities occurred during night flood tides, when megalopae were aggregated near the surface. During day flood tides, megalopae were generally less abundant and distributed deeper in the water column. Occasionally, however, they were abundant during day flood, concentrated near the bottom in deep water. At shallow sites, megalopae were never abundant during day, apparently not ascending into well-lit water. Abundance and depth distribution of megalopae were not affected by current speed, wind speed, water temperature or salinity. A conceptual model of vertical migration of estuarine blue crab megalopae is presented. Megalopae which behave according to this model should lower their susceptibility to predation in the water column by selectively utilizing flood currents to rapidly reach juvenile nursery grounds while avoiding well-lit waters.
... Recently, there have been numerous studies investigating mechanisms of return of larval or postlarval stages to estuaries where the adult population is found (Epifanio et a%. 1984; Johnson 1982; Johnson et al. 1984; Hester 1983; Sulkin and Van Heukelem 1982; Sulkin et al. 1980). Crabs mature at 1 -1.5 yr of age and only live 1 more yr. ...
... A greater understanding of oceanography and species behaviour and ecology would seem to be required if survey results to estimate relative abundances of various life stages are to be satisfactorily interpreted. The hypothesis that year-class strength is established during the early life stages of a species (see May 1974) has been advocated by many authors studying crab (Mankin 1985; McKelvey et al. 1980; Reilly 1983 Reilly , 1985 Sulkin and Van Heukelem 1982; Wicham 1979a), using often very different arguments. However, it has also been proposed that predation (including cannibalism) of juvenile crab can determine yearclass strength (Amstrong and Gunderson 1985; Bailey 1982; Botsford and Wickham 1978; Peterson 1973; Waiwood and Elner 19821, again using often very different arguments. ...
Two general categories of crab fisheries exist, shallow-water, nearshore fisheries (e.g. Cancer, Callinectes) and deepwater, offshore fisheries (e.g. Paralithodes, Chionoecetes). Species in the former category are managed typically in a passive manner with no annual restriction of catch on the basis of population abundance, whereas species in the latter category tend to be managed actively through annual catch quotas. Active management of fisheries requires regular resource surveys and provides the most comprehensive data base for recruitment study. Recruitment fluctuations in crab populations are analyzed by considering (1) production of progeny from perceived parent populations, (2) mechanisms which determine magnitude of progeny return to fishery grounds, and (3) factors that determine survival of juveniles. It is concluded that year-class strength at recruitment can be established at any preceding life stage and that while one life stage may appear more critical than others for a given population in a given year, this is unlikely to be the case in future years or apply to different populations. There is little evidence that recruitment magnitude in crab populations has been influenced by past resource management. Management has not stabilized landings in any fishery over the long term, and environmental variables seem to be the major factor influencing recruitment level. The need for prediction of future fishery performance is greatest in actively managed fisheries, with short-term economic benefits and socially acceptable exploitation, not control of recruitment, being the most likely achievement of management.
... In the laboratory, altered larval distributions in response to the presence of haloclines have been documented for several taxa including echinoplutei (Metaxas and Young 1998a), ascidian tadpoles (Vázquez and Young 1996), barnacle nauplii (Harder 1968), littorinid (Harder 1968) and bivalve veligers (Mann et al. 1991), and decapod zoeae (Sulkin and Van Heukelem 1982). The effects of thermoclines on larval vertical distributions have not been demonstrated as consistently across taxa. ...
... Hydrodynamics are responsible for the retention of exported zoeae and megalopae over the Mid-Atlantic Shelf (even near the parental estuary) and for the reintroduction of megalopae into the estuaries (Johnson and Hess 1990;Little and Epifanio 1991;Epifanio 1995). Zoeae may be able to influence their probability of retention or dispersal by exhibiting strong vertical migration to remain near the surface (e.g., Sulkin and Van Heukelem 1982;Sulkin 1984). Also, megalopae may remain near suitable areas for settlement once inside the estuary by using well-timed verti-cal migration to take advantage of the predominant flows (Little and Epifanio 1991;Epifanio 1995). ...
For marine benthic invertebrates with meroplanktonic larvae, the relative importance of hydrodynamics and swimming behaviour in determining larval dispersal in the water column, particularly at small spatial scales, has not been determined. In the field, larval aggregations recorded at physical and biological discontinuities in the water column were attributed to hydrodynamics. Similar aggregations obtained in the absence of flow in the laboratory indicate a potentially significant role of behaviour. At large spatial scales, larval distribution in the plankton is mainly regulated by horizontal advection. However, the ability of larvae to behaviourally regulate their position at scales of micrometres to metres when exposed to turbulent fluid motion in the water column, as evidenced in the benthic boundary layer, is unknown. Evaluation of swimming in turbulent flows in the water column is an intriguing area of research, which involves several constraints. In the field, quantification of behaviour is limited by low success in tracking larvae and lack of appropriate observational tools. In the laboratory, the generation and quantification of flow regimes that are representative of those in the field remains a challenge. An approach that integrates biological and physical measurements within realistic ranges is necessary to advance our understanding of larval dispersal.
... Although relevant, these small-scale considerations are not new and several studies, particularly in estuarine systems, have investigated the consequences of local circulation upon the distribution of various organisms with planktonic larvae (e.g. Wood & Hargis 1971, Cronin & Forward 1982, Sulkin & van Heukelem 1982, Boicourt 1988). More recently, the necessity of testing hypotheses related to the demography of benthic species undergoing a larval phase, at smaller spatial scales, has been recognized and considered in other studies (i.e. ...
... Some crustacean larvae remain in estuaries by timing their vertical swimming to tides (Forward and Tankersley 2001). In estuaries of eastern North America, vertical migration of larvae of the crab Rhithropanopeus harrisii results in little net motion over tidal cycles in estuarine channels (Cronin and Forward 1982), whereas larvae of the crab Callinectes sapidus initially swim upward and are flushed out of the estuary (Sulkin and van Heukelem 1982). Export from estuaries is not universal or inevitable. ...
Crustaceans are increasingly being used as model organisms in all fields of biology, including neurobiology, developmental biology, animal physiology, evolutionary ecology, biogeography, and resource management. Crustaceans have a very wide range of phenotypes and inhabit a diverse array of environments, ranging from the deep sea to high mountain lakes and even deserts. The evolution of their life histories has permitted crustaceans to successfully colonize this variety of habitats. Few other taxa exhibit such a variety of life histories and behavior. A comprehensive overview of their life histories is essential to the understanding of many aspects of their success in marine and terrestrial environments. This book provides a general overview of crustacean life histories. Crustaceans have particular life history adaptations that have permitted them to conquer all environments on earth. Crustacean life cycles have evolved to maximize fecundity, growth, and ageing, in a wide range of environmental conditions. Individual contributions contrast benefits and costs of different life histories including sexual versus asexual production, semelparity versus iteroparity, and planktonic larvae versus direct development. Important aspects of particular behaviors are presented (e.g. migrations, defense and territorial behaviors, anti-predator behavior, symbiosis).
... Earlystage larvae, however, showed upward orientation in the water column (negative geotaxis), whereas advanced larvae exhibited downward orientation (positive geotaxis). Together, these results predict a near-surface distribution of early-stage zoeae and a deeper position as larval development progresses (Sulkin et al. 1980, Sulkin & Van Heukelem 1982. But once larvae have molted to the megalopal stage, they resume negative geotaxis (in the laboratory) and also display an endogenous swimming rhythm with maximum activity during the diurnal phase of the light/dark cycle. ...
The blue crab Callinectes sapidus Rathbun has been the subject of scientific investigation for more than 100 years. The crab is a key predator in shallow estuarine and coastal environments, and supports a large commercial fishery along the Atlantic and Gulf Coasts of the United States. Much of the initial research on C. sapidus was purely descriptive and provided only a patchwork perspective on the natural history of the crab. This is in contrast to more modern work that has defined a clear, interrelated series of processes that underlie the early life history of the species. The intent of the present review was to discuss contemporary understanding of the early life history of blue crabs in the context of a coherent time line of development. The review begins with a short section on general aspects of the life history, which provides a background for the overall discussion. This is followed by a segment on courtship and mating, with particular emphasis on chemical communication between mating pairs. Three subsequent sections then deal, respectively, with spawning migrations within the estuary, larval release in the adjacent coastal ocean, and transport of larvae on the inner continental shelf. A following segment, describes settlement of the larvae in the lower estuary and eventual transport of early juvenile stages to estuarine nursery areas. The final section provides a summary and conclusions. Points of emphasis in the review include the following: (1) the role of pheromones in courtship and mating; (2) the discovery of two distinct phases in the spawning migration; (3) the importance of chemical cues in assuring synchronized hatching of eggs; (4) the predominant effect of wind- and buoyancy-driven processes in controlling larval transport in the coastal ocean; (5) the development of mathematical models that allow critical analysis of transport processes; and (6) the combined physical and behavioral processes that facilitate transport of megalopa larvae from the estuarine mouth to nursery habitat in the lower estuary. © 2019 National Shellfisheries Association. All rights reserved.
... Until recently, it was expected that episodic recruitment was the result of storms and/or relaxation events where these currents reverse (Dudas et al. 2009;Garcia-Reyes and Largier 2012). Many estuarine invertebrate larvae exhibit behavior which allows them to be retained in estuaries (Strathmann 1982;Sulkin and Van Heukelem 1982;Ogburn et al. 2009;Kunze et al. 2013;Peteiro and Shanks 2015); however, recent evidence suggests that larvae of some marine invertebrates and fish also have behavioral adaptations that allow them to remain in the nearshore zone along the coast and recruit to the shore despite the influence of these predominant currents (Morgan et al. 2009;Morgan et al. 2012;Shanks et al. 2014). The estuarine populations of both burrowing shrimp species we studied have early-stage larvae that are exported from the estuary and then return to the relatively small entrances to these estuaries after development in coastal ocean waters. ...
Recruitment is a strong determinant of year class strength and adult population density especially for sessile benthic invertebrates where post-settlement mortality and competition are low or relatively stable over time. A series of surveys were undertaken to characterize recruitment and post-settlement processes for two species of burrowing shrimps, Neotrypaea californiensis and Upogebia pugettensis in order to determine how they influenced broader adult populations in US west coast estuaries. On average, U. pugettensis decapodids settled earlier (April–July), recruited almost exclusively to areas with conspecific adults, and grew more rapidly during their first summer than N. californiensis. Neotrypaea californiensis decapodids settled and recruited over a longer period (June–November) and were distributed across the tidal flat. While initially more abundant in areas with conspecific adults, they also either survived better or redistributed as small juvenile shrimp to areas where adults were absent. Linear relationships were found between abundance of newly recruited (0+ age class) shrimp and that of older 1+ shrimp a year later. Positive slopes were close to one for N. californiensis but less than one for U. pugettensis, suggesting lower survival. Annual recruitment varied dramatically but was more consistent for both species in Yaquina Bay. Patterns in strong recruitment years amongst estuaries, particularly for U. pugettensis, suggest the presence of multi-estuary metapopulations linked via larval dispersal. These results have important implications for shrimp population management including control for shellfish aquaculture, but also conservation of estuarine habitats due to the strong influence of these ecosystem engineers on the benthic community.
... The megalopae often attach to floating debris and are tidally transported back into estuarine nursery areas where the final transformation into adulthood occurs (Churchill 1919). Blue Crab larvae are ideal test organisms to determine toxicities of oil, dispersants, and/or other bioremediation treatment combinations because the pelagic megalopae are generally planktonic and drift within the top 2 m of offshore waters (Shanks 1985, Sulkin andVan Heukelem 1982). Larval life-stages of crustaceans are often the organisms most sensitive to contaminants (Ahsanullah andArnott 1978, Conner 1972); larvae of Crangon crangon L. (Sand Shrimp ), Carcinus maenus L. (Green Crab), and Homarus gammerus L. (European Lobster) were 1 to 3 orders of magnitude more sensitive to mercury, copper, and zinc than adults. ...
During the Deepwater Horizon incident in 2010, ∼1.8 million gallons of Corexit® dispersants were approved for use directly onto the released oil. Callinectes sapidus (Blue Crab) megalopae are pelagic and, therefore, likely to be one of the first organisms exposed to spilled oil and applied dispersants in open-ocean and nearshore waters. In this study, we examined acute toxicity of Corexit 9500, Corexit 9527, and MicroBlaze® (a microbial surfactant) alone and in combination with crude oil. We adapted methods from the established 48-h copepod toxicological assay and exposed Blue Crab megalopae for 48 h to varying dosages of each treatment. Oil treated with dispersant was more toxic than either oil or dispersant alone (48-h LC50 = 29.8 mg/L vs. 55.9 mg/L and 37.5-59.1 mg/L, respectively), and MicroBlaze was essentially non-toxic (48-h LC50: 7643 mg/L). Corexit 9527 was more toxic than Corexit 9500 both in solutions with oil and alone (48-h LC50 = 37.5 mg/L vs. 51.8 mg/L and 59.1 mg/L, respectively). Exposure to these toxicants not only induced mortality at certain dosage levels, but life-stage transitioning also seemed to be effected. The decreased ability to metamorphose, however, was not affected in a typical gradient manner, as with mortality; those that were exposed to a toxicant, overall, exhibited a decreased occurrence of metamorphosis (37% average decrease). This study provides essential baseline data needed for further investigations to determine optimal dosing of dispersants and balancing of dispersant use and dosage with anticipated crab-fishery impacts.
... Temperature changes during flood tide were too inconsistent to serve as a cue for the ascent (De Vries et al., 1994). Relative rates of increase in pressure and salinity change in a consistent pattern during flood tide (De Vries et al., 1994), and megalopae ascend in response to these changes (Sulkin and Van Heukelem, 1982;Tankersley et al., 1995). However, the relative rate of increase in pressure during flood tide is about an order of magnitude below the lowest relative rate that evokes an ascent response . ...
This overview combines our recent studies with existing information to develop more complete conceptual models of selective tidal-stream transport (STST) of ovigerous female and post-larvae of the blue crab Callinectes sapidus. During the first phase of the spawning migration, non-ovigerous females migrate seaward from brackish water to the mouths of estuaries following insemination. After oviposition, females with mature embryos undertake the second phase of the spawning migration, in which they undergo ebb-tide transport for movement seaward to release larvae and then migrate back into estuaries using flood-tide transport. Following larval development offshore, post-larvae or megalopae undergo flood-tide transport for up-estuary movement in which they ascend into the water column during flood tides at night and are on or near the bottom at all other times. This behavioral pattern is not due to a circatidal rhythm in activity since megalopae have a circadian rhythm. The timing of this endogenous rhythm is paradoxical because megalopae are active during the day phase and inactive at night. Neither exposure to a cycle in salinity change that simulates the natural tidal cycle nor step decreases in salinity alter this circadian rhythm. The behavior underlying flood-tide transport consists of behavioral responses to a sequence of cues. Megalopae ascend into the water column in response to the relative rate of increase in salinity during flood tide. Water turbulence due to flood-tide currents induces sustained swimming, and the decline in turbulence during slack water at end of flood tide induces settlement out of the water column. Environmental cues during ebb tide do not induce STST. Since light inhibits swimming, flood-tide transport does not occur during the day and is reduced when the time of slack water after flood tide occurs after sunrise. Future studies are needed to determine the behavioral basis of STST of females, and especially the reversal from ebb-tide to flood-tide transport.
... São apontadas como vantagens da exportação larvar a diminuição da exposição à predação, uma maior estabilidade das propriedades físico-químicas do ambiente em que se encontram, a manutenção de trocas genéticas entre as populações segregadas espacialmente (Sulkin & Heukelem, 1982) e a maior disponibilidade de alimento nas áreas costeiras adjacentes. Para além disso, a dispersão aumenta as probabilidades de transporte de larvas para diferentes áreas, contribuindo para a persistência de espécies nesses locais (Bilton et al., 2002). ...
... Mature females migrate to higher salinity waters near the bay mouth to spawn (Van Engel 1958). After hatching near the estuary mouths, a combination of larval behavior and net flow of surface water out of the estuary results in near-surface aggregations (Sulkin et al. 1980;Sulkin and Van Heukelem 1982) and transports larvae into coastal waters (Smyth 1980;Dittel and Epifanio 1982;McConaugha 1988;Roman and Boicourt 1999). Once C. sapidus larvae enter coastal waters, physical mechanisms likely retain larvae near parent estuaries in the Middle Atlantic Bight (Boicourt 1982;Johnson et al. 1984;Epifanio et al. 1989;Roman and Boicourt 1999;Epifanio and Garvine 2001;Steppe and Epifanio 2006;Epifanio 2007;Tilburg et al. 2007). ...
Transport of Callinectes sapidus (blue crab) megalopae from the continental shelf into estuaries may influence recruitment variability of this species. Observations of the vertical distribution of C. sapidus megalopae near the mouths of Chesapeake and Delaware Bays were used to infer vertical swimming behaviors that may influence ingress to these estuaries. Megalopae and oceanographic conditions were sampled at locations from ~10 km inshore of the estuary mouths to ~40 km offshore in coastal shelf waters on September 7–13, 2005 and September 3–7, 2006. Megalopae were present in greater abundance and at shallower depths during night compared to day at all locations in Chesapeake and Delaware Bays, suggesting that megalopae make diel vertical migrations within the estuary and on the continental shelf near the estuarine mouths. Within the mouth of the estuaries, only limited evidence suggests that megalopae increase in abundance in the upper water column during nocturnal flood tides in Delaware Bay. These findings suggest that wind forcing and density-induced subtidal flows are more likely mechanisms for ingress to Chesapeake and Delaware Bays than tidal transport.
... This combination of behavioral pattern was observed by Rice (1964) in the megalopae of the shore crab Carcinus maenas (Linneaus). The megalopae of Callinectes sapidus (Rathbun) also display many of these same patterns (Naylor & Isaac 1973, Sulkin & Van Heukelem 1981. ...
... Many invertebrate larvae are known to display phototactic responses at some stage prior to settlement (Thorson, 1964). Ontogenetic shifts in photo-and geo-taxis have been observed for many sessile invertebrate larvae (e.g., barnacles - Lang et al., 1979;polychaetes -Marsden, 1988polychaetes -Marsden, , 1990Young and Chia, 1982;ascidians -Svane, 1987;Durante, 1991;molluscs -Kaartvedt et al., 1987;Barile et al., 1994;echinoderms -Pennington and Emlet, 1986;and crustaceans -Ritz, 1972;Sulkin and Van Heukelem, 1982;Forward, 1989;Olmi, 1994), and may either enhance or restrict dispersal. Preferences of invertebrate larvae for particular light intensities have also been shown to assist larvae to select particular microhabitats at the time of settlement (Weinberg, 1979;van Duyl et al., 1981;Hadfield, 1986;Svane and Young, 1989;Durante, 1991;Svane and Dolmer, 1995). ...
On coral reefs scleractinian corals show strong patterns of vertical zonation, yet the underlying mechanisms creating and maintaining vertical zonation are poorly understood. Here we examine the potential contribution of light-dependent settlement in scleractinian coral planulae to patterns of vertical zonation. The effect of intensity and spectral quality of light on the settlement of six species of scleractinian corals (Goniastrea favulus Dana, Goniastrea aspera Verrill, Acropora tenuis Dana, Oxypora lacera Verrill, Montipora peltiformis Bernard, and Platygyra daedalea Ellis and Solander) with contrasting depth distributions was examined in laboratory trials. Light-dependent settlement was shown by planulae from five of the six species examined. Planulae from individual species showed a response to either light quality or light quantity, but not both. Settlement patterns shown by planulae from all six species were consistent with the vertical distribution patterns of adults in the field. Settlement of planulae from con-generic species with similar adult distribution patterns did not respond to variation in light intensity or spectral quality in a uniform manner, indicating the optimal light environment for settlement is species specific. The settlement patterns shown by planulae from five of the six species examined were more complex than required for selection of cryptic or exposed micro-habitats at settlement. The ecological function of such complex responses to light at settlement may be to identify optimum habitats for adult survival.
... 1984;Johnson 1985b). The behavior of the megalopae (Sulkin and Van Heukelem 1981) and their distribution in the plankton (Smyth 1980;Johnson 1985a) both suggest that these larvae are inhabitants of the neuston. Larvae that inhabit the neuston can be transported onshore by internal waves (Shanks 1985). ...
In areas of mesotides (tidal range 2 to 4 m) and narrow continental shelves (~30 km) internal waves can transport (Le., convey from one place to another) the larvae of coastal organisms shoreward. Research reported here was in an area of microtides (tidal range 80 km), the South Atlantic Bight. Half of the sampled sets of internal waves were aligned parallel to shore and probably originated at the shelf break. The higher densities oflarvae and flotsam in the slicks over these internal waves (convergence zones) than in the rippled water between slicks (divergence zones) indicates that these waves were transporting larvae and flotsam shoreward. All nontransporting internal waves were aligned at a sharp angle to shore and may have formed over shoals oriented perpendicular to shore. To further test the hypothesis that internal waves can transport larvae, surface plankton were col lected from the waters over, in front, and behind a set of internal waves. The density of PortulIU8 spp. megalopae was significantly higher in waters in front of the set than behind. The average densities of a variety of larval fish and invertebrates were significantly higher over the internal waves than in front of the set of waves. These data indicate that internal waves can cause shoreward transport of larvae and flotsam. Precompetent larval fish were not carried shoreward by this set of waves while competent stages (Le., juvenile through postflexion) were transported shoreward.
... In western Atlantic estuaries such as Delaware Bay and Chesapeake Bay, the blue crab CalLinectes sapidus Rathbun has a life cycle that entails migration by adult females to the estuary mouth for subsequent release of zoeae. Early zoeae remain near the surface and are advected from the estuary to develop in high salinity waters of the continental shelf (Sandifer 1975, Sulkin et al. 1980, S u l h n & van Heukelem 1982, Sullun et al. 1982, Provenzano et al. 1983, Epifanio et al. 1984, Sulkin 1984, van den Avyle & Fowler 1984 ). The mechanism of C, sapidus recruitment back into the estuary and associated tributaries is presently being debated, but apparently entails southeasterly wind patterns near the mouth for larval retention (D. R. Johnson et al. 1984, D. R.) coupled with late zoeal and megalopal behavioral responses to light, gravity and pressure (, Sulkin 1984, Brookins & Epifanio 1985, D. F. Johnson 1982). ...
... Para os crustáceos decápodos, a salinidade pode afetar o crescimento, a sobrevivência e o desenvolvimento larval (Costlow et al., 1960;Anger et al., 1990;Anger, 1996;Lárez et al., 2000;Anger et al., 2000;Anger, 2003;Luppi et al., 2003), assim como também a atividade alimentar (Anger, 2001), a taxa de acumulação de carbono (Anger et al., 1998) e a composição bioquímica das larvas de algumas espécies (Torres et al., 2002). Este parâmetro induz ainda respostas comportamentais específicas (Forward, 1989) influenciando na distribuição vertical das larvas na coluna d'água (Sulkin & Van-Heukelem, 1982;Sulkin, 1984;Cronin & Forward, 1986), além selecionar o tipo de estratégia reprodutiva adotada pelas espécies como, por exemplo, a "exportação" larval (Christy, 1982;Forward et al., 1982;Anger, et al., 1994). No entanto, para algumas espécies com dispersão larval para as águas costeiras, a salinidade pode servir como indicadora de condições estuarinas, orientando as megalopas durante o recrutamento (O'Connor & Epifanio, 1985). ...
The present work studied the effect of salinity on the survival and duration of larval development of the mangrove crab, Ucides cordatus (from the Caeté River estuary, North of Brazil) until the megalopal phase in seven salinity treatments (0, 5, 10, 15, 20, 25 e 30). Salinity significantly affected the survival of the zoea larvae, however it did not affect the duration of the larval development (20.77 ± 1.56 days). In salinity 0, 5 and 10 all zoea larvae died. Only from off salinity 15, complete development until the megalopal phase occurred. The survival rate was highest in salinity 30 (72%) and lowest in 15 (16%). The reduced survival rate of the U. cordatus zoea larvae, in low salinities, indicates the necessity of larval dispersion from the estuary to coastal waters, where salinity conditions for larval development are more favorable. Otherwise, if there was no dispersion, the reduced salinity of estuarine waters in the rainy season would cause a high mortality thereby affecting the recruitment, maintenance and growth of the U. cordatus population in the mangroves.
... Megalopae in October samples were molt staged using maxillipeds, in addition to uropods. Loglikelihood analysis of molt stage frequencies (Sokal & Rohlf 1981) was used, instead of ANOVA, because of incomplete replication of habitat types at the downriver location. Individual megalopae were widely dispersed and only together in the sampling apparatus for a short time; thus the molt stage of each megalopa was assumed to be an independent sample. ...
Physiological state (molt stage) of planktonic and benthic blue crab (Callinectes sapidus) postlarvae (megalopae) was quantified within and outside Chesapeake Bay at various spatial scales. Physiological state of planktonic megalopae advanced significantly from the continental shelf off the Chesapeake Bay mouth, through the bay mainstem, and into upriver stations in the York River, a tributary of Chesapeake Bay. These results imply that settlement and metamorphosis of blue crab megalopae is dependent on location relative to the shelf. The physiological evidence supports the export-reinvasion theory of blue crab recruitment and is inconsistent with a larval retent~on hypothesis. In the tributary, benth~c megalopae were significantly more advanced in molt stage state than planktonic rnegalopae. Temporal variation in molt stage was observed over days and months. In addition, time to metamorphosis was significantly and positively correlated with molt stage. Advance-ment in physiological state of megalopae during reinvasion of the estuary may serve as an indicator of likelihood of settlement.
... Species that exhibit barokinesis in response to small changes in pressure are those in which larvae are retained in estuaries, where high sensitivity to small changes in tidal height is essential (e.g., Rhithropanopeus harrisii, Bentley and Sulkin, 1977;. However, other species that exhibit low sensitivity include late stage Callinectes sapidus (Sulkin and Van Heukelem, 1982), Cancer irroratus (Bigford, 1977) and Leptodius floridanus (Sulkin, 1973;Wheeler and Epifanio, 1978), as well as all stages of H. oregonensis (Arana and Sulkin, 1993). These species all develop in the coastal ocean where precision in depth regulation is not as important to transport. ...
Hemigrapsus sanguineus is an invasive species of crab (family: Grapsidae) in the north Atlantic basin. The species has spread rapidly since it was first discovered in North America in the late 1980s; however, the mechanisms of this range expansion remain unclear. This study attempts to predict the vertical distribution and, thus, ultimate transport of H. sanguineus larvae by examining larval responses to gravity and pressure. Geotaxis was determined by measuring the response of individual larvae to gravity in the absence of other tactic stimuli. Barokinesis was determined by measuring changes in swimming speed of larvae upon step-wise changes in pressure. Geotactic response of the larvae changes ontogenetically; early stage larvae are negatively geotactic (orienting towards the surface), while late stage larvae are positively geotactic (orienting towards the bottom). Larvae show a response to pressure that would aid in depth regulation. Early-stage larvae increase activity upon a change in pressure and orient their movement to gravity. However, the larvae show a relatively low sensitivity to pressure change. The evidence predicts an export-and-return model of larval transport, similar to that of Uca spp. in the Middle Atlantic Bight. This model supports the hypothesis that H. sanguineus larvae have the potential for high dispersal and will continue to invade new regions through larval transport.
... There are a number of papers expressing doubts about, or even disregarding, behavior of larvae while in the water column (de Wolf, 1974;Scheltema, 1975, with older literature;Andrews, 1979;Boicourt, 1982;Seliger et al., 1982;and Hannan, 1984). In contrast, dispersal in the open sea and recruitment to the bottom on the shelf have been investigated less often (Lough, 1976;Kelly et al., 1982;Sulkin and Van Heukelem, 1982). ...
A 1953 investigation of merozooplankton in Kiel Bay, which is subject to estuarine exchange between the Baltic Sea and Danish waters, is reviewed. Larvae of 18 polychaete and 6 echinoderm species were abundant and were spatially and temporally distributed as if they had behaved similarly to passive, neutrally buoyant bodies. On three diel stations larvae occurred in very pronounced layers even with weak hydrographic stratification; furthermore, their depth changed with internal waves but not with the day-night cycle. Four cruises during the summer and a station occupied fortnightly through the fall showed that the depths of general occurrence of the larvae were governed by hydrographic processes rather than active orientation. Larvae were tied to water masses, defined by the T-S relation and of known history, and were subject to vertical eddy diffusion (mixing) as well as horizontal transport. Young larvae did not rise towards the surface. Older larvae generally did not occur closer to the sea bed than younger ones, but instances of a passive deposition of metamorphosing larvae to the sea bed could be inferred. "Animal navigation" in the estuarine circulation was not observed. The results are of general application since hydrographic conditions similar to those in Kiel Bay, especially in the upper layers, are common in neritic seas.
... by Lambert and Epifanio (Lambert and Epifanio, 1982), Carcinus maenas by Queiroga (Queiroga, 1996) and Upogebia africana by Wooldridge and Loubser (Wooldridge and Loubser, 1996), among others, indicate a spatial distribution pattern in which first zoeae and megalopae are found inside the estuaries whereas intermediate stages are found offshore. The lower exposure to predation, stability of salinity and temperature, increased species dispersal and genetic exchange between isolated habitats, have been recognized as selective advantages for export of larvae (Strathmann, 1982;Sulkin and Van Heukelem, 1982;Bilton et al., 2002). However, the synchrony of larval emission induces larval aggregation, which putatively increases the survival due to saturation of predators, and concentration of larval release in specific periods can be related to separation of larval patches of different ages, to avoid cannibalistic behaviour that is common in larvae with different predation capabilities (Paula, 1989(Paula, , 1998. ...
... Particularly lacking have been investigations of possible cues involved in habitat selection at the time of settlement and the periods when postlarvae are capable of exercising such a choice, i.e., the competency period (see Pechenik, 1990). The zoeal and megalopal stages of decapods correspond to the developmental (precompetent), and searching (competent) phases, respectively, of invertebrate larvae (Meadows & Campbell, 1972;Sulkin & Van Heukelum, 1982), and both behaviorally and morphologically the megalopal stage is transitional between the plankton and the benthos. For most reptant species there is greater mobility at this stage than any other in the life cycle, making it admirably suited to the task of locating and settling in an appropriate area. ...
Gregarious settlement by megalopae of the porcelain crabs Petrolisthes cinctipes (Randall) and P. eriomerus Stimpson was investigated to determine possible means of attraction and postsettlement benefits. Megalopae of P. eriomerus were attracted to conspecific adults confined to chambers that prevented tactile or visual contact, suggesting that postlarvae respond to a waterborne cue. There was also clear evidence of an extended period of competency for settlement, since newly molted megalopae of P. cinctipes held with conspecific adults settled within 2–4 days while those without adults delayed settlement for 2–3 wk. Loss of swimming ability at settlement is followed by degeneration of the pleopods and color changes. Following settlement juvenile instars appear to gain protection by continuing to hide between and beneath conspecific adults; this behavior significantly reduced predation in a laboratory experiment.
Dispersal of benthic crustaceans primarily occurs by larvae, which can be transported far from parents. However, larval dispersal is reduced by depth regulation in a sheared water column, where surface and bottom currents flow at different rates or directions, and navigation by postlarvae recruiting to adult habitats. Larvae undertake migrations between adult and larval habitats that range from retention near adult habitats to cross-shelf migrations. The extent of these migrations is regulated by depth preferences and vertical migrations that are timed exogenously or endogenously by diel and tidal cycles over planktonic development. Depth regulation is cued primarily by gravity, hydrostatic pressure, and light, and secondarily by temperature, salinity, and turbulence. Settlement stages navigate to suitable settlement sites using hierarchies of acoustic, chemical, visual, and celestial cues that are effective at different distances. The extent of larval migrations between adult and larval habitats as well as diel vertical migrations may be set by the vulnerability of larvae to abundant planktivorous fish in estuaries and nearshore waters. The timing of larval release and vertical swimming by larvae changes across tidal regimes to conserve migrations between adult and larval habitats across species ranges while minimizing predation.
The aquatic contributions to this volume have focused on the dynamics of either patches on the substratum (benthic communities) or patches in the water column (pelagic communities). This contribution couples these two foci, exploring the importance to benthic organisms of the dynamics of patches in the water. We will outline benthic processes that depend on the transport of materials in the water column. We will describe an empirical technique for quantifying the mixing and transport of patches in nature, and will present some examples of our measurements for wave-swept rocky shores (habitats important in ecological research but difficult to study hydrodynamically). Then we will discuss how future modeling efforts might incorporate these findings.
Estuarine crabs release their larvae freely into the water column in spite of the risk of these larvae being transported out of the estuary onto the continental shelf. Many species have evolved behavioral traits that in combination with horizontal advection retain the larvae near favorable adult habitat. Leakage of these species onto the shelf results in loss of the larvae to adult population. Behavioral models resulting in retention include: 1) constant maintenance of position deep in the water column, 2) downstream advection of surface-dwelling, immature larvae, and 3) tidally rhythmic vertical migration of larvae. Larvae of some species appear to be adapted for routine dispersal onto the continental shelf. These larvae must be transported back to suitable estuarine habitat for adult existence. Models for transport back to the estuary include: 1) active horizontal swimming by mature larvae or juveniles, 2) seaward advection of surface-dwelling, immature larvae followed by landward advection of bottom-dwelling, mature larvae, 3) transport of mature larvae in convergence zones associated with landward movement of tidally induced, internal waves, and 4) wind-generated surface transport of mature larvae. A number of case studies involving well-studied estuarine species are reviewed in the paper.
Two quite different sorts of selection could favor export of offspring from estuaries: (1) animals which spread offspring over several estuaries increase relative to non-spreaders in the population when favorability of estuaries varies independently; (2) animals which export offspring from an estuary during the larval stage increase relative to non-exporters in the population when survival or growth of larvae is greater outside the estuary. Adaptation to local conditions and the occurrence of homing in many life histories are indirect evidence against the first hypothesis. Conclusions are premature, but predictions are that (1) varying favorability among estuaries rarely favors the spread or dispersal of larvae among estuaries, (2) waters outside estuaries are often safer for larvae than waters inside estuaries, and (3) when there is selection for export of larvae from estuaries, it is selection for migration to areas more favorable to larval development rather than dispersal. In this view, the large scale spread of offspring is an accidental though important consequence of a long planktonic period; the spread of offspring confers no short term advantage.
In the attack on the complexities of the recruitment variability problem, knowledge of the physical transport mechanisms serves to reduce the dimensionality and sharpens the questions driving field research. In coastal regions, topographic and oceanographic boundary constraints contribute to this structuring by limiting the transport domain and clarifying the transport pathways. A measure of the state of our understanding of planktonic transport is the ongoing controversy concerning active versus passive mechanisms. The tendency has been to regard them as mutually exclusive, whereas observations support their acting together, with advection often dominating. Innovative field and laboratory studies on planktonic behaviour are needed to allow extention of numerical transport experiments beyond the point where an intractable number of possibilities are presented for testing via field experiment. In addition, an Eulerian description of water motion will be required on the small scales necessary for the estimation of Lagrangian mean velocities and for the incorporation of spatial variations of diffusion into transport formulations.
Behavioral and salinity tolerance studies of early and late stage larvae of Uca pugnax were used to infer possible effects of salinity on the dispersal and recruitment of fiddler crabs. Larvae were induced to swim through salinity discontinuities of 0-10‰ magnitude. Stage I zoeae were inhibited from moving upward through haloclines at Δ salinity = 6, 8, and 10‰, whereas stage V larvae tended to remain near the bottom of the water column and were inhibited from moving upward only at Δ salinity = 10‰. Both stage I and V larvae passed downward through all haloclines. Larvae of Uca pugnax were reared in 10, 20, and 30‰ salinity water. Larvae were unsuccessful at molting in 10‰ water. Zoeae reared at 20‰ showed a slight delay in molting compared with larvae at 30‰. More stage V zoeae molted to megalopae at 30‰ than at 20‰, but more megalopae underwent metamorphosis to crab stage 1 at 20‰ than at 30‰. These results support the hypothesis of transport of fiddler crab larvae from spawning areas to major estuaries and continental shelf regions. Only very sharp, steep haloclines would inhibit the vertical migrations of larvae. Our results suggest that larvae are retained nearshore in high salinity water during development and that metamorphosis is delayed until larvae reach lower salinity water characteristic of adult habitats.
The Asian shore crab, Hemigrapsus sanguineus, is native to coastal and estuarine habitat along the east coast of Asia. The species was first observed in North America near Delaware Bay (39°N, 75°W) in 1988, and a variety of evidence suggests initial introduction via ballast water early in that decade. The crab spread rapidly after its discovery, and breeding populations currently extend from North Carolina to Maine (35°–45°N). H. sanguineus is now the dominant crab in rocky intertidal habitat along much of the northeast coast of the USA and has displaced resident crab species throughout this region. The Asian shore crab also occurs on the Atlantic coast of Europe and was first reported from Le Havre, France (49°N, 0°E) in 1999. Invasive populations now extend along 1000 km of coastline from the Cotentin Peninsula in France to Lower Saxony in Germany (48°–53°N). Success of the Asian shore crab in alien habitats has been ascribed to factors such as high fecundity, superior competition for space and food, release from parasitism, and direct predation on co-occurring crab species. Laboratory and field observations indicate that H. sanguineus is a generalist predator with potential for substantial effects on sympatric populations of mollusks and crustaceans. However, broad ecosystem effects and actual economic impact are unclear. The literature on H. sanguineus is limited in comparison to better known invasive species like the European green crab (Carcinus maenas) and the Chinese mitten crab (Eriocheir sinensis). Nevertheless, there are substantial bodies of work on larval biology, trophic ecology, and interspecies competition. This paper presents a review of the biology and ecology of invasive populations of the Asian shore crab H. sanguineus in North American and European habitats.
Megalopae of Callinectes sapidus from offshore waters can shorten the interval to metamorphosis to first crab in response to cues common to settlement habitat within estuaries. Observations were made on postlarvae (megalopae) collected offshore from the Chesapeake Bay (USA) over a period of several days in October 1992, fairly late in the recruitment season. Megalopae exposed to a combination of water from the marsh surface and low salinity accelerated metamorphosis by 10%. Physiological condition of postlarvae collected both in surface waters and from the bottom was compared, using molt stage, progress to molt, and survivorship as indicators. No differences in physical condition were detected, and no megalopae were in active premolt. The pattern of progress to metamorphosis implies that both benthic and surface postlarvae are able to delay metamorphosis, and that the initiation of behaviors that move megalopae from surface waters is temporally segregated from advance to metamorphosis.
The present paper develops a mathematical model for the transport and recruitment of blue crab (Callinectes sapidus) larvae, and applies it to the inner continental shelf of the Middle Atlantic Bight near Delaware Bay, U.S.A. Blue crab larvae develop through seven or eight planktonic zoeal stages to a megalopa stage suitable for recruitment to adult populations of east coast estuaries. The larvae are concentrated near the surface, and the currents are primarily forced by alongshelf winds and river discharge through major estuaries. Model currents are prescribed based on a realistic synthesis of their observed relationship to wind and river discharge. Besides the resulting advection, particle diffusion and biological mortality are added to determine the fate of larvae released from their parent estuary. Groups of particles were released across the source region of the outflowing buoyancy-driven current in the model estuary mouth. Most larvae were swept alongshelf to the south with the buoyancy-driven coastal current, and thus were lost as recruits to the population of their parent estuary. However, some larvae released close to the seaward edge of the emerging coastal current were able to cross the coastal current front and move seaward into inner shelf water during upwelling-favorable (northward) wind events. Some of these, in turn, were suitably placed near the parent estuary mouth so that they could be advected landward as megalopae into the estuary during a subsequent downwelling-favorable (southward) wind event and thus join the adult population. The model results for megalopae returns were computed from consecutive daily release of 1000 particles, and were compared with 4 years of blue crab megalopa settlement data for Delaware Bay. The model results for 1989 and 1990 matched the observed data remarkably well, with both years showing dominance by a single return event of a few days duration. For 1991 and 1992, the observed results showed multiple return events, and the model results did not follow these well.
Results of a three-year survey of the occurrence of Callinectes sapidus larvae in the mouth of Delaware Bay indicated that stage I zoea larvae were most abundant insurface water as compared to mid-depths and near bottom. The major peak in abundance of stage I zoea larvae occurred in early August with a secondary peak in early September. Peaks in abundance of megalopae occurred five weeks after the respective peaks in zoeal abundance. Zoea stages II–VIII were not collected in the bay mouth. Results of sampling every 3 h over consecutive tidal cycles showed that stage I zoea larvae were most common in the water column on ebbing tidal currents. Megalopae were most common in the water column on flooding tidal currents, suggesting a tidally related, vertical migration. It was concluded that stage I zoea larvae are flushed from the estuary and undergo development on the continental shelf. Megalopae are then transported back to inshore waters by a combination of winds and currents and invade the estuary by means of migration into the water column on flooding tidal currents and migration to the bottom on ebbing tidal currents.
Successful development from hatching to metamorphosis occurred only at temperatures ≥12°C. With increasing temperature, both overall survival and range of salinity tolerance increased, whereas development duration decreased exponentially. Development time increased at unfavourably low or high salinities (where mortality also increased). Zoea I was very euryhaline, with an optimum in slightly brackish water (25‰S). During subsequent zoeal development, the larvae became increasingly stenohaline, and their optimum shifted to seawater. Megalopa, was euryhaline again and developed fastest in lower salinities (15-25‰S). Metamorphosis is, after a gradual adaptation during megalopa development, possible in principle at any salinity. An optimum, however, was found at 15-25‰S. These patterns of ontogenetic change in salinity tolerance was interpreted as an adaptation to 1) hatching in brackish water of outer estuaries; 2) offshore transport during zoeal development, with late stages living predominantly under marine conditions; 3) onshore (near-bottom) transport of the megalopa; 4) settlement in any part of an estuary. -from Author
The ontogeny ofbehavioral responses of lar vae of the crabs Rhithropanopeus harrisii and Neopa nope sayi to rates of change in salinity were analyzed with a video system. A salinity increase evoked an ascent in both species. For R. harrisii the threshold rate of in crease was 1.1 X l0@ ppt s@ ‘¿� for the first and last zoeal stages and changed little with acclimation salinity. N. sayi larvae were more sensitive, as thresholds were 2.8 x l0@ppt s@'for StageI zoeaeand 7.0 X l0@ppt s@' for Stage IV. This difference in sensitivity may relate to the magnitude of salinity gradients in the estuarine/ coastal areas inhabited by the larvae. At threshold rates ofsalinity increase the absolute amount ofchange before a response was lower for Stage I zoeae (0.09â€"0. 11 ppt) than Stage IV zoeae (0.21â€"0.29 ppt) for both species. De creases in salinity did not induce the expected descent response in either species at rates up to 4.7 X l0_2 ppt sW'. The different responses in a salinity gradient may have resulted because the rate threshold and absolute amount of change before a response to a salinity increase were below those for a salinity decrease. Considering larval sinking rates and normal environmental salinity gradi ents, larvae of both species can respond to rates and amounts of salinity increase in their environment. The ascent response may be important for keeping larvae up in the water column and reducing the likelihood that they will encounter the bottom.
The influence of environmental variability on Cancer magister year class strength was examined using trawl survey data from the Washington coast, 1983–88. Average June–September estimates of early instar abundance within two coastal estuaries and the adjacent nearshore are compared with alongshore and cross-shelf transport vectors during the preceding 4- to 5-mo pelagic larval phase. Settlement in the study area varied interannually by nearly 40-fold and was inversely related to net alongshore (northward) transport during January–May. Settlement was confined to a relatively narrow band along the coast and in estuaries; new recruits were rarely encountered beyond 46 m depth, corresponding to a distance of ~15 km from shore. We found no evidence for sustained westward Ekman transport to account for progressive seaward transport of zoeae, as proposed by other authors. Rather, persistent landward transport of near-surface waters will tend to minimize advective loss of larvae to seaward while promoting retention in proximity to suitable juvenile and adult habitat nearshore. Temporal and spatial continuity of northward transport during the winter months, coupled with proximity to high-risk areas downstream, suggests that Washington C. magister populations are frequently dependent on larvae originating to the south for recruitment.
A conceptual hypothesis relating physical forcing to dispersion and retention was developed for blue crab larvae within the Mississippi Bight. The spawning period for blue crabs in the northern Gulf of Mexico is protracted. Hatching of eggs occurs near the barrier islands and mouths of coastal bays from March through October. Larvae are released on ebbing tides and spend the next 30 to 50 d offshore where they develop through seven zoeal stages before undergoing metamorphosis to megalopae. Duration of the megalopal stage is variable but generally persists from 6 to 20 d. Blue crabs recruit to Gulf estuaries as megalopae. During the critical planktonic phase in their life history, larvae are subject to the vagaries of seasonal circulation patterns which can either return them to nearshore where they can successfully settle, or lose them at sea. Archived currents from a 3-dimensional, primitive equation, sigma-coordinate model of the Gulf of Mexico, driven by climatological winds and damped to surface salinity and temperature, were used to study advection of blue crab larvae in the Mississippi Bight. Data suggest that seasonal circulation patterns driven by average wind stress provide a window of opportunity for blue crab larval dispersion offshore and return nearshore during the appropriate period in their development for settlement as megalopae. In the Mississippi Bight, this window usually occurs between April and October. Large basin-scale events, such as Loop Current intrusions and spin-off eddy generation, may interrupt this circulation pattern and change the settlement success rate. Variations in the seasonal forcing, due to anomalous winds, or basin-scale events may contribute to fluctuations in levels of harvestable adult blue crabs.
Research on larval biology and life history strategies of marine invertebrates has been influenced by the perspectives of Thorson's (1946; 1950) categorization of developmental modes and of theoretical models (e.g., r- and K-selection, bet hedging, and energetic trade-offs) which explain demographic patterns. Using these two perspectives, recent developments in five main problems are reviewed: (1) validations and challenges to the perspective that developmental/larval mode primarily determines life history strategies; (2) the significance of variability and flexibility in the larval phase and other aspects of life history strategies; (3) the need for studies of heritability of life history traits; (4) the importance and difficulty of measuring larval mortality; and (5) the need to measure covariation and evolutionary constraints of larval and other life history variables. This review indicates that the analysis of life history strategies of marine invertebrates is in a transition period without a clear new perspective. However, eight recommendations for new research are presented.
Sets of sole larvae and juveniles were subjected, under DD, LL or DL conditions, to a variety of constant salinities, salinity gradients and cyclic variations of salinity. Animals were observed during several days in cylindrical vertical tanks and their swimming activity was recorded automatically with opto‐electronic IR barriers. Analysis of actograms, periodograms and actotaxigrams demonstrated effects of salinity on behaviour. Salinity variations induced an exogenous rhythm of activity and changes in the distribution patterns of larval and juvenile sole. Nevertheless, sole activity and distribution depended more on nycthemeral light variations than on salinity variations. The ecological consequences of salinity variations in the settlement of juvenile sole in coastal and estuarine areas are discussed.
The copepod Lepeophtheirus salmonis Krayer (Copepoda: Caligida), the salmon louse, is a parasite of salmonids. The vertical distribution of the infective stage, the copepodid, was studied in salinity gradients with one step increase of 15‰ (154%e> on top of 304‰), 5‰ (25–4‰ on top of 30–4‰) and 2%> (28–4‰ on top of 304‰) in 1-m perspex columns. Copepodid distribution in a linear gradient, where the salinity increased from 154‰ at 0 cm depth to 304‰ at 87–5 cm, was also recorded. Homogeneous 304‰ salinity columns served as the control. In these, the animals gathered in the top section of the water column in response to 1 h of light from above, and spread downwards in response to 4 h of darkness. In columns with a 15‰ step increase in salinity with depth, copepodids aggregated just underneath the discontinuity irrespective of light conditions. In step salinity gradients of 5‰ and 2‰ S, under both light regimes, animals were significantly more numerous in the step sections compared with the control. In the linear gradient, significant numbers of copepodids accumulated at approximately 20‰ salinity when subjected to 1 h of light. In the dark, there were no significant aggregations. Copepodids were found in 15–17–2‰ salinity in all linear gradient experiments.
MARINE species commonly have broadly dispersing juveniles called larvae. Their return to the adult populations is highly variable1-3, often generating large fluctuations in population size4-6, yet the causes of the variation are poorly understood. Historically, attention has been focused on the roles of variable reproductive output by adults and variable mortality during larval development7,8. The limited success of these factors as general explanations prompted a more recent focus on the influence of variable transport of the larvae9-13. Here we show that nearly a decade of settlement variation of the barnacle, Semibalanus balanoides (L), closely matched predictions based solely on a transport hypothesis: differences in transport generate recruitment variation by determining whether larvae complete development near a favourable habitat. The irregular nature of coastlines, particularly the presence of bays and estuaries, generates substantial regional variation in coastal transport that may generate correspondingly large variation in recruitment to marine populations.
Larvae of the Chinese mitten crabEriocheir sinensis were reared in the laboratory from the time of hatching and through metamorphosis. Development normally consists of a Prezoea,
5 Zoea stages, and a Megalopa. Occasionally, an additional (stage VI) Zoea and, in one case, an additional Megalopa (transitional
to the first crab stage) were observed. Detailed morphological descriptions of all larval and the first two juvenile instars
are given, and larval morphology is compared with that of two closely related species,Eriocheir japonicus andEriocheir rectus, descriptions of which have recently become available. The zoeal stages of these species can be distinguished by their different
number of aesthetascs and setae on the antennules, and different setation of maxillipeds 1 and 2. The Megalopa shows differences
in the shape of the rostrum and again in the morphology of the antennule. These and other morphological differences (mainly
in setation and spinulation of the zoeal carapace) betweenE. sinensis andE. japonicus larvae suggest that they may be very closely related but separate species; this contradicts a recent study of adult morphometrics
and molecular genetics (Li et al., 1993), suggesting that they are only varieties of a single species.
This study describes the use of newly hatched larvae of Callinectes sapidus (blue crab) in a 48-h acute toxicity test and compares their sensitivity to two other estuarine crustaceans (Mysidopsis bahia and Palaemonetes pugio) commonly used for evaluation of effects of potentially toxic materials. C. sapidus larvae were twice as sensitive to sodium dodecyl sulfate as 24-h post-release M. bahia, and five times more sensitive than 24-h-old P. pugio larvae. We found the blue crab toxicity test to be simple, rapid and accurate and it provides low variability and high reproducibility. Since the data indicate high sensitivity of this commercially important species to a reference toxicant and the potential impact on its survival during a critically sensitive developmental stage, we propose future research further evaluating C. sapidus as a potential toxicity test species.
Surface and bottom plankton samples taken with a Clarke-Bumpus Quantitative Plankton Sampler at monthly intervals over a two-year period were examined for decapod crustacean larvae. These samples were taken at 12 stations over a distance of about 80 miles from fresh water in the Pamunkey River, through the meso-and polyhaline York River, to euhaline conditions at the mouth of Chesapeake Bay. Some additional samples were collected with paired BCF Bongo Samplers.
Planktonic stages representing at least 37 species of decapods were identified. Larvae ofUca spp.,Crangon septemspinosa, Hexapanopeus angustifrons, Neopanope sayi andRhithropanopeus harrisii were abundant in the plankton of the York River system and lower Chesapeake Bay, and larvae of a number of other species were common. Larvae ofEurypanopeus depressus, formerly one of the abundant xanthid crabs of the bay, were rarely found. This apparent decline in abundance ofE. depressus may be associated with the recent introduction of the sacculinid parasite,Loxothylacus panopaei, into Chesapeake Bay.Callianassa spp. (other thanC. atlantica),Lepidopa cf.L. websteri andNaushonia crangonoides were recorded from the bay for the first time.
Decapod larvae were nemerous throughout the estuary during summer. However,Crangon septemspinosa larvae were abundant from late winter throughout the spring, and peak numbers were generally of the same order of magnitude as total concentrations of larvae of the other species at the bay stations during summer.
The number of decapod species represented in the plankton and the portion of the year during which their meroplanktonic larvae were present decreased with distance upriver. These trends are in large part related to the decreasing salinity upstream, although other factors may be involved as well. The distribution data also suggest that areas of peak larval abundance probably indicate regions of abundance of breeding adults.
Larvae of a number of common species were considerably more abundant in bottom collections than near the surface. This tendency of many larvae, especially later stages, to congregate near the bottom where the net flow is upstream may be one of the factors operating to insure retention of decapod larvae within the Chesapeake Bay.
Vertical movements by pelagic larvae of marine benthic invertebrates are directed by light and gravity. The spatial orientations of these responses are known to be affected by light intensity, temperature, hydrostatic pressure, feeding, and salinity (reviewed by Forward, 1976a). This last parameter, which is highly variable in coastal plain estuaries, can alter both phototactic and geotactic behavior. A negative phototaxis can be induced in nornually photopositive larvae of van ous estuanine organisms by a sudden exposure to low sahinities. The salinity change necessary for such a phototactic reversal may range from 4.5 % to 66% dilution of seawater ( Edmondson and Ingram, 1939 ; Lynch, 1949 ; Lyon, 1906 ; Ranade, 1957) . Furthermore, Edmondson and Ingram ( 1939) found that barnacle nauplii regained positive phototaxis in ten minutes after a salinity decrease. Evidence that an increase in salinity alters the sign of phototaxis is unreported. A negative geotaxis enables larvae to remain up in the water column in the absence of light by directed swimming which conipensates for a tendency to sink (Sulkin, 1973). The depth in the water column at which larvae are found, how ever, is related to developmental stage. Generally, later larval stages remain lower than earlier stages (e.g., Bousfiehd, 1955 ; Carniker, 1951 ; Lynch, 1947 ; Sandifer, 1975; Sulkin, 1973). Work with salinity discontinuities in nature (Gnindley, 1964) and in the laboratory (Harder, 1968 ; Lance, 1962 ; Lyster, 1965 ; Roberts, 1971 ; Scarratt and Raine, 1967), as well as laboratory experiments with fluctuating sahinities (Haskin, 1964 ; Hughes, 1969 ; Hughes and Richard, 1973) , have shown that a downward movement is the common response to a salinity decrease. Likewise, an increase in salinity will induce an upward nuovement ( Haskin, 1964 ; Hughes and Richard, 1973). Little attempt, however, has been made to determine whether such nuovenients result from a response to light and/or to gravity. The present study examines the effect of sudden salinity changes upon photo taxis and geotaxis in larvae of the brachyuran crab Rhithropanopezis harrisii (Gould). This specieswaschosenfor studybecause its larvaeoccurin coastal plain estuaries, where they are subjected to natural salinity variations. In addition, much is known about the larvae : the effects of salinity on larval developnuent (Costlow, Bookhout, and Monroe, 1966), osnioregulatory ability (Kalber and Costlow, 1966), the ontogeny of phototaxis (Forward, 1974; Forward and Cost low, 1974), the shadow response (Forward, 1976b). the effect of temperature on phototaxis and geotaxis (Ott and Forward, 1976), polarotaxis (Via and Forward,
A survey of the literature indicates that zooplankton aggregate at salinity or temperature interfaces. The experiments reported here offer evidence that such interfaces may be preferential sites for certain marine organisms. Experiments were made by layering water of various densities and introducing planktonic organisms into the layers. The zooplankters under study included protozoa, and adults or larvae, or both, of ctenophores, chaetognaths, cirripeds, amphipods, polychaetes, brachiopods, mysids, molluscs, appendicularians, and teleost eggs. Almost all organisms tested accumulated at the interfaces of water layers.
An den Stränden zwischen Ensenada (Mexiko) und Süd Oregon (USA) zeigen Adulti von Emerita analoga (Decapoda, Hippidae) ähnliche Abundanz- und Wachstumsmuster. Nördlich und südlich dieses Gebiets verändern sich diese Muster. Es wird vermutet, daß die Mechanismen, mit Hilfe derer die pelagischen Larven zum Strand zurückkehren, nicht ausschließlich auf Zufalls-eddies zurückgeführt werden können, welche die Larven in einer langsam sich bewegenden Wassermasse zu einer stromaufwärts gelegenen Position zurückführen. Es wird angenommen, daß Arten wie Emerita analoga nur an exponierten Küsten mit einem Gegenstrom leben können. Die beiden Strömungen würden die Larven auf- und abwärts entlang der Küste tragen, aber einige Larven stets an ihre Ausgangsstätte zurückführen. Zwischen Ensenada und Süd Oregon existiert ein Gegenstrom, der Davidson-Strom, und ein ähnlicher Unteroberflächenstrom. Nördlich und südlich dieser Region scheint die Art in der Lage zu sein, Gebiete von der Gegenstrom-Region her zu besiedeln, aber die Nachkommenschaft dieser peripheren Populationen wird normalerweise nicht in die nächste Generation aufgenommen. Es wird argumentiert, daß die Temperatur weniger effektiv ist als die Strömungen in Bezug auf die Begrenzung der Verbreitung küstennaher Faunenelemente mit pelagischen Larven.
A model of larval dispersal of coastal benthic invertebrates is proposed: Eddy mixing spreads larvae in the onshore-offshore direction during the period before competence to settle and continues to take larvae both onshore and offshore after they are competent to settle; predators kill larvae during both precompetent and competent periods. Distribution of larval stages of several species are consistent with the model. The model correctly predicts (1) increased competent period with increased precompetent period in comparisons among species, and (2) a competent period greater or equal to the precompetent period. The model predicts a decrease in the ratio of competent to precompetent period as precompetent period increases, which is not apparent in available data. Also, some observed competent periods are longer than those predicted by the model. Reasons for these departures from predictions are suggested. The model differs from previous optimization or maximization models for planktonic larvae by including onshore-offshore mixing and therby linking selection on the precompetent and competent periods.
Expatriated species of zooplankton found in the mid-Atlantic Bight include Arctic-Boreal species derived from shelf waters northeast of Cape Cod, transition zone species from the adjoining Slope Water and tropical-subtropical species that commonly reside in the Gulf Stream, and Sargasso Sea. Introduction of expatriates is largely associated with the pattern of advective movements of water onto the shelf: Arctic-Boreal species are brought in from the northeast largely by over-shelf transport; transition zone species by Slope Water penetration at the surface when horizontal density gradients are minimal and at mid-depth in response to physical processes such as estuarine-type circulation, wind-driven upwelling, cold shelf water ‘bubble’ formation and movement out into the Slope Water or to shelf-Slope Water interactions associated with warm core rings; warm water species by injection of warm core ring surface water in over the shelf. There is little evidence that Carolinian species are introduced into the mid-Atlantic Bight directly around Cape Hatteras. In general, the occurrence of expatriate warm water species is more important in terms of species numbers and total biomass when compared to the occurrence of expatriate cold water species. The Bight region can be divided into three regions with regard to oceanic influences: (1) the band of low salinity water along the coast south of the mouth of the Hudson River, extending to the mouth of the Chesapeake; (2) the Continental Shelf edge extending from about 37 ° 30′N to 40 °N and extending shoreward towards the eastern half of the Long Island and Block Island Sound, but not including the region south-east of Cape Cod and Nantucket: (3) the southern sector, including the shelf edge south of 37 °N and extending landward south of Chesapeake Bay. Each of these regions is characterized by types of expatriate species and by hydrographic features.
Behavioral responses to gravity and hydrostatic pressure have been investigated in two species of xanthid crabs Leptodius floridanus (Gibbes) and Panopeus herbstii Milne-Edwards to determine whether such responses provide a mechanism for depth regulation in the absence of light.In laboratory experiments, the four zoea stages and one megalopa stage of each species assume a differential vertical distribution in darkness, with succeeding stages showing a deeper overall distribution. Passive sinking rates increase in succeeding zoea stages and drop to an intermediate level after the molt to the megalopa stage. All zoea stages exhibit a negative geotaxis in the absence of light; the megalopa shows a positive geotaxis. The first zoea stage of Leptodius floridanus responds to an increase in hydrostatic pressure (up to 1 atmos above ambient) with an increase in swimming rate. This pressure response is shown to be reversible and not subject to short-term acclimation. The swimming rate of the last zoea stage does not increase in response to an increase in pressure.It is concluded that the responses of these larvae to gravity and hydrostatic pressure together with their characteristic passive sinking rates provide a mechanism for depth regulation in the absence of light that varies during ontogeny.
Survival rate, although comnuonly used as an index of an organisnu's ability to nueet a given environmental challemuge ( Costbow, Bookhuout, and Monroe, 1960, 1962, 1966 ; Roberts, 1971) , is a rather crude estimate of the ability of an organism to adapt to its environment. Behavioral responses associated with the mainte muanceof an animal in ifs optinial environnient are more suitable measures of adaptability (Shelford, 1915 ) . Orienfative behavior reflects, at least in part, flue integration of an animal's various adaptive physiological changes in metabolism, reproduction, hormomualbalance, etc., to challenges of the external milieu. Responses of various zoophankfers to salinity disconfinuities were first studied by Harder ( 1952a, 1952b, 1954, 1957, 1968) . The ability to detect and avoid waters of reduced salinity has now beemudenuonstrated experimentally for several meroplankters as well as hohoplamikters. A response to salinity discontinuities has been demonstrated in Mercenaria ;nencenania trochophores (Turner amid George, 1955 ) , Phyllodoce trochuopluores ( Lysfer, 1965) , some copepods and the zoeal instars of Pisidia longicornis (Lamuce, 1962) , and Zoea I of Homarus amenicanus ( Scarratt and Raimue,1967). In the present study, the response of Pagunus longicarpus zoeah instars to salinity discontinuities of different nuagmuifudeswas examined. The avoidance be havior was described and quantified. The imuteraction effect of developmental age and magnitude of the discontinuity omuthe response was examined. Further an attempt was made to determine if flue receptor(s) is localized on certain structures. MATERIALS AND METHODS
Distribution of Callinectes larvae in surface (neuston) and subsurface shelf waters in the Middle Atlantic Bight was determined from quarterly zooplankton collections taken during a 2-year study. Observations confirmed the presence in offshore waters of a large larval pool from which recruitment may take place. Larvae were predominantly late zoeae and megalope, with peak abundances in late summer collections reaching 16,000 per 100 m3 in neuston collections. During summer, crab larvae were distributed across the shelf with the majority at 10-80 km offshore. Abundances were sig nificantly greater in neuston than subsurface zooplankton collections and generally greater in neuston collections taken at night. Water temperature and distance from shore were factors most closely correlated with abundance of larvae in the neuston. Megalopae of Callinectes were present at outer shelfstations in winter and spring and together wi th megalopae of Portunus and other forms were of southern origin. Basedon experimentally determined temperature-salinity preferences reported in the literature for Callinectes larvae, metamorphosis may be delayed in cooler offshore waters, thus increasing chances of long-range transport.
Three aspects of the geographic distribulions of swimming crabs in the genus Callinectes are examined. C. maracaiboensis, previously thought to be endemic to the Lake Maracaibo area, occurs in Jamaica, Colombia, and Curaçao. Although typically found in fresh and brackish waters, available evidence indicates that C. maracaiboensis cannot complete its life cycle in waters of low or highly variable salinity, and could not have evolved in Lake Maracaibo when it was cut off from the Caribbean.The larvae of C. sapidus, the species most successful in temperate waters, can neither hatch nor molt much below 20°C, and females in the northerly part of the range spawn only during the warm season. The occurrence of Callinectes in a geographic area implies settlement from the plankton, rather than adult immigration. Callinectes make their farthest poleward penetration in West Atlantic areas having warmer summers and cooler winters than those at comparable latitudes in the East Pacific and East Atlantic. Thus, latitudinal distribution seems to be limited by the effects of summer temperatures on larvae, rather than winter temperatures.Caribbean high islands, typically possessing brackish and fresh as well as marine environments, have most or all potentially occurring Callinectes. Low islands, typically without significant amounts of water of reduced salinity, lack C. bocourti, C. maracaiboensis, C. sapidus, and C. danae. Analysis of geographic records shows that low salinity water may be an ecological requirement for some Callinectes, which are distributed similarly to freshwater shrimps, genus Macrobrachium. The most eurytopic Callinectes are absent from areas of the Caribbean populated by their less eurytopic congeners due to their lack of suitable reduced salinity refuges.
The primarily benthic megalopa larvae of Callinectes sapidus and Macropipus holsatus swim upwards for a short distance from the bottom when subjected to pressure increased above ambient values, particularly when illuminated from above. Larvae exposed to stepwise increments of pressure exhibit a threshold in the barokinetic response which is lower in Callinectes (0.4 atm) than in Macropipus (0.8–1.0 atm). Such behaviour would tend to inhibit swimming in surface waters of stratified estuaries where net flow is seawards, but would promote slight upward swimming when on the bottom in deeper water which has a net flow inwards, thus substantiating other evidence that the megalopa is a stage at which recruitment to estuaries takes place. The lower threshold of the barokinetic response in Callinectes may be related to its occurrence in more uniformly shallow estuaries on the south and east coasts of N. America.In both species swimming is markedly inhibited after a slight reduction in pressure. Such responses would be of value to larvae running the risk of being washed ashore in surf Sudden reductions in pressure by surf action would induce sinking into the undertow which would carry larvae offshore into deeper water.
Roughly 90,000 species of marine bottom invertebrates reproduce by pelagic larvae, thus spending the most sensitive period of their life under the influence of light. Data on their responses to light have here been brought together for larvae of 141 species. In their early pelagic stages. 82 % of these species respond positively to light and migrate to the surface layers, 12 % seem to be indifferent to light. and 6 % respond negatively to light and continue to do so during their whole pelagic life. In their later and oldest pelagic stages most of the initially photopositive larvae will turn over to photonegativity, seeking down to deeper lying bottoms. but larvae of many intertidal species will remain photopositive till they stop swimming, thus sticking to the surface layers where they can get into contact with intertidal localities.
The distribution patterns exhibited by some larval decapod crustaceans within the York River system and adjacent lower Chesapeake Bay, Virginia, suggest means by which young stages can be recruited to the adult benthic and bentho-pelagic populations. Species considered are four caridean shrimp (Crangon septemspinosa, Hippolyte pleuracantha, Ogyrides limicola and Palaemonetes spp.) and 12 brachyuran crabs (Callinectes sapidus, Cancer irroratus, Hexapanopeus angustifrons, Neopanope sayi, Ovalipes ocellatus, Pinnixa chaetopterana, Pinnixa sayana, Pinnotheres maculatus, Pinnotheres ostreum, Rhithropanopeus harrisii, Sesarma reticulatum and Uca spp.).
Effects of water of reduced salinity on the vertical migration of the zooplankton of Southampton Water were investigated in the laboratory using the adults of six copepod species (Acartia tonsa, A. bifilosa, A. discaudata, A. clausi, Centropages hamatus, Temora longicornis) and the larvae of one decapod species (Porcellana longicornis). Additional observations were made on Centropages typicus adults taken off Plymouth.
Salinity discontinuity layers had a marked effect on the vertical migration of zooplankton. A single discontinuity layer formed by placing less dense diluted sea water over full strength sea water acted as a barrier to animals attempting to swim toward the surface of an experimental water column and no individuals were able to enter the zone of reduced salinity when an extreme dilution was used. Comparison of the behaviour of various species and of different sexes or developmental stages of a particular species suggests that vertical distribution was partly dependent on the salinity tolerances of individuals. Upward migration was also restricted by a vertical series of discontinuity layers.
The swimming activity of copepods was depressed in homogeneous water columns of reduced salinity. Activity depended both on the degree of dilution and on the period of immersion. There were indications that behaviour varied according to the salinity tolerances of the different species.
Investigations of the effect of sudden temperature change on the phototaxis of Stage I and IV zoeae upon stimulation from horizontal and vertical directions with 500-nm light indicate a temperature-induced geotactic response in larvae of the crab Rhithropanopeus harrisi (Gould). For the horizontal tests both zoea stages were reared at 20 °C. Stage I showed positive phototaxis at temperatures between 15 ° and 35 °C, while Stage IV responded over the range of 10–30 °C. For the vertical tests, larvae, reared at 25 °C, were stimulated with overhead lights. Stage I zoeae ascended at 15 °, 20 ° and 25 °C and descended at 5 °, 10 °, 30 ° and 35 °C. Stage IV zoeae ascended at 20 ° and 25 °C and descended at 5 °, 10 °, 15 °, 30 ° and 35 °C. Although the descent at high temperatures could result from a negative phototaxis, a reversal in phototactic sign at high temperatures was not found in the horizontal experiments and the same vertical movement pattern is observed in total darkness. Upon exposure to high temperatures near the water surface, larvae would descend by means of a positive geotaxis rather than a negative phototaxis. This response involves active swimming by Stage IV larvae and passive sinking by Stage I.
Seasonal occurrence of brachyuran developmental stages is predictable in meroplankton collected inside the barrier islands
of North Carolina near inlets and along longitudinal transects of the larger estuaries. Nets of 1 mm mesh employed in the
sampling program limited catches to larger zoeae and megalopae, and of these only a fraction can be identified with precision.Callinectes megalopae were taken in every month of the year and through almost the entire lenghth of the estuary. Evidence indicates
that this postlarval stage is active in surface waters at night, contributing substantially to dispersal of the species, probablyC. sapidus in this case.Hexapanopeus angustifrons megalopae were limited to the warm season in samples from a single station near Beaufort Inlet.Rhithropanopeus harrisii zoeae and megalopae were distributed abundantly in oligo-mesohaline waters of Neuse and Pamlico Rivers. Showing no nocturnal
stratification, the stages were present continuously from June to October. Megalopae of all these species, usually considered
benthic in habit, are readily captured in surface waters at night.
Larven der Blauen Krabbe (Callinectes sapidus Rathbun) wurden bei 25° C und 30 ‰ S (= p. p. m.) gezüchtet, bis das Megalopalarvenstadium erreicht war. Die Megalopalarven wurden sodann in 6 verschiedenen Salzgehalten und 4 verschiedenen Temperaturen gehalten, um die Wirkung von Salzgehalt und Temperatur sowie von deren Kombinationen auf Überlebensrate, Entwicklung und Metamorphose festzustellen. Bei 20°, 25° und 30° C überlebten über 70 % der Megalopalarven in Medien von 10 bis 40 ‰ S. Bei 15° C betrug die Überlebensrate 10 bis 50 % in Salzgehalten von 20 bis 40 ‰. Die Megalopalarven metamorphosierten nicht bei 15° C und 10 ‰ sowie bei 20° C und 5 ‰. Die Dauer des Megalopastadiums schwankte von 5 Tagen bei 30° C bis zu 67 Tagen bei 15° C. Die Temperatur beeinflußte die Lebensdauer der Megalopalarven stärker als der Salzgehalt. Bei 20° und 30° C wurde die Dauer des Megalopastadiums durch veränderte Salinitätsbedingungen nicht beeinflußt. Bei 15° C bewirkte eine Erhöhung des Salzgehalts jedoch eine Verzögerung der Metamorphose des ersten Krabbenstadiums. Durch statistische Analysen sind die Wirkungen von Salzgehalt und Temperatur auf Sterblichkeit und Entwicklungsrate der Megalopalarven für eine größere Anzahl von Umweltsbedingungen, als sie im Laboratorium geprüft werden konnten, ermittelt worden, so daß die Überlebensrate der Megalopalarven innerhalb eines großen Temperatur- und Salzgehaltsbereiches mit großer Wahrscheinlichkeit vorausgesagt werden kann. Es ist anzunehmen, daß durch die zeitliche Verlängerung des Megalopastadiums und die hohe Überlebensrate in Gewässern mit niedriger Temperatur und hohem Salzgehalt ein Transport der Megalopalarven durch Meeresströmungen über weite Entfernungen ermöglicht wird. Diese Annahme würde unter anderem die Verbreitung dieser Gattung über einen großen Teil der Gewässer entlang der atlantischen Küste sowie im Golf von Mexiko erklären.
Genetic variability of the blue crab,Callinectes sapidus, was estimated for populations in Chesapeake and Chincoteague Bays. Genetic similarity between these populations was attributed
to larval intermixing in the mid-Atlantic Bight.
Oregon has one of the smaller and best known coastal upwelling systems. It is about 50 km wide, but upwelling is most intense within 15 km of the shore, and episodes of active upwelling primarily affect the circulation and hydrography of the upper 20 m. It is in this nearshore, surface zone that phytoplankton and zooplankton are most abundant. Phytoplankton biomass is 5 to 20 mg chl-am⁻³, and zooplankton 50 to 200 mg dry weight m⁻³. Vertically stratified sampling along transects perpendicular to the shore has produced a new picture of the upwelling process and suggests relationships between circulation and the population biology of planktonic animals.