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Production and reworking of sediment by parrotfishes (family Scaridae) on the Great Barrier Reef, Australia
Abstract
Sediment produced by parrotfishes (family Scaridae) may comprise new bioeroded material and/or reworked sediment. The relative contribution of the two components was examined in two bioeroding Chlorurus species, C. gibbus and C. sordidus, from Lizard Island in the Northern Great Barrier Reef. The relative importance of reworked sediment was determined based on direct estimates of sediment ingestion. In C. gibbus, 2.4% of the sediment produced is reworked. In C. sordidus, reworking contributes 27.2%. Comparisons of sediment size-distributions in epilithic algal communities, gut contents and defaecation sites suggest that both C. gibbus and C. sordidus markedly decrease the particle size of sediment as a result of ingestion and trituration in the pharyngeal apparatus.
... In instances where rates of erosion exceed rates of carbonate production (e.g., Eakin 1996), the loss of reef structure may also threaten reef capacity to mediate wave exposure, thereby compromising their ability to protect coastlines and inshore habitats (Sheppard et al. 2002;Cuttler et al. 2018;Perry et al. 2018). Conversely, erosion can remove dead reef substrate, which can free up bare space for coral settlement (McCauley et al. 2014) and create sediments and rubble that contribute to the stability of shorelines and reef habitats (Bellwood 1996;Perry et al. 2011). Understanding the factors that govern erosion is, therefore, key to understanding physical, biological, and chemical processes on coral reefs. ...
... External bioeroders include grazing parrotfish, pufferfish, and urchins, which play an important role in creating new substrate for coral recruitment (Mumby and Harborne 2010;McCauley et al. 2014), generating calcium carbonate sediments (Hutchings 1986), and shaping reef structure (Hutchings 1986;Hutchings et al. 2005). Grazing by parrotfish and urchins contributes up to 90% and 84% of the estimated total bioerosion in the Indo-Pacific and Caribbean, respectively (Chazottes et al. 1995;Bellwood 1996;Perry et al. 2014). Accordingly, one-time "snapshot" assessments of grazer abundance and activity have been used to indirectly estimate total rates of erosion (e.g., Graham and Nash 2013;Hoey et al. 2016), although this may not adequately capture temporal and spatial variability in grazing pressure (Perry and Hepburn 2008;Johansson et al. 2010;Lange et al. 2020). ...
... Along each 100 Â 10 m transect, observers identified and recorded all parrotfishes to species and estimated total length (TL) to the nearest 10 cm. Parrotfish bite rates (bites min À1 ), bite volume (cm 3 ) and proportion of bites leaving scars were estimated using focal individual observations (following Bellwood 1996, Bruggemann et al. 1996; see Supporting Information Results). The mean bite mass for each parrotfish species was calculated by multiplying the mean bite volume by the mean substrate density for Indo-Pacific coral reefs (1.47 g cm À3 , Perry et al. 2011). ...
Erosion is a key process in shaping the physical structure of coral reefs, yet due to erosion being semi‐cryptic and difficult to quantify, information remains limited. Here, we investigate erosional processes along Ningaloo Reef, an extensive fringing coral reef in Western Australia. We employed both direct and indirect methods to measure erosion in wave‐exposed reef slopes and protected lagoonal habitats. Direct measurements of erosion on coral blocks were among the highest found globally, with total erosion of 3.07 kg m⁻² yr⁻¹ (4% from micro, 0.6% from macro, and 94% from external), whilst indirect rates were estimated at 2.4 ± 0.20 kg m⁻² yr⁻¹ (78% from parrotfish, 22% from urchins). Indirect erosion rates were influenced by the species and size of parrotfish, with Chlorurus microrhinos removing 0.44 ± 0.19 kg m⁻² yr⁻¹ (22% of parrotfish erosion). Scanning electron microscopy and computed tomography show that micro and macroborer erosion contributions to direct erosion were low, most likely due to heavy grazing by parrotfish and the short deployment period of experimental substrates. A substantial portion of external erosion on blocks (0.53 ± 0.23 kg m⁻² yr⁻¹) could not be attributed to bioeroders and was poorly correlated with wave exposure, suggesting processes not quantified contribute to this unaccounted aspect of erosion. Our results confirm that bioerosion by parrotfish is especially significant at Ningaloo Reef, and large‐bodied individuals of C. microrhinos are key in conserving this key ecological process.
... This is because sediments are shaped by a complex suite of processes which operate and interact on, in, and around reefs. Sediment distributions can be influenced by physical factors such as currents and waves Ogston et al., 2004;Storlazzi et al., 2004;Browne et al., 2013a;Cartwright et al., 2021), and reef geomorphology (Golbuu et al., 2003(Golbuu et al., , 2011Kench and Brander, 2006), as well as biological drivers such as the feeding activity of fishes (Bellwood, 1996;Bowden et al., 2022;Perry et al., 2022) and the type of benthic biota (Reeves et al., 2018;Pessarrodona et al., 2021). This complex array of factors leads to marked variability in the amount and type of sediment in different coral reef reservoirs (e.g. the water column, on-reef surfaces, and off-reef sediment aprons) (Storlazzi and Jaffe, 2008;Harris et al., 2014;Tebbett et al., 2017a). ...
... Given the substantial role of parrotfishes in reworking sediments (Bellwood, 1996;Perry et al., 2022), we estimated the potential magnitude of their contribution to this geo-ecological function. To do this, we used a two-step, depth stratified underwater visual census and combined this with sediment reworking rate data from the literature. ...
Sediments are found on all coral reefs around the globe. However, the amount of sediment in different reservoirs, and the rates at which sediments move between reservoirs, can shape the biological functioning of coral reefs. Unfortunately, relatively few studies have examined reef sediment dynamics, and associated bio-physical drivers, simultaneously over matching spatial and temporal scales. This has led to a partial understanding of how sediments and living reef systems are connected, especially on clear-water offshore reefs. To address this problem, four sediment reservoirs/sedimentary processes and three bio-physical drivers were quantified across seven different reef habitats/depths at Lizard Island, an exposed mid-shelf reef on the Great Barrier Reef. Even in this clear-water reef location a substantial load of suspended sediment passed over the reef; a load theoretically capable of replacing the entire standing stock of on-reef turf sediments in just 8 h. However, quantification of actual sediment deposition suggested that just 2 % of this passing sediment settled on the reef. The data also revealed marked spatial incongruence in sediment deposition (sediment trap data) and accumulation (TurfPod data) across the reef profile, with the flat and back reef emerging as key areas of both deposition and accumulation. By contrast, the shallow windward reef crest was an area of deposition but had a limited capacity for sediment accumulation. These cross-reef patterns related to wave energy and reef geomorphology, with low sediment accumulation on the ecologically important reef crest aligning with substantial wave energy. These findings reveal a disconnect between patterns of sediment deposition and accumulation on the benthos, with the 'post-settlement' fate of sediments dependent on local hydrodynamic conditions. From an ecological perspective, the data suggests key contextual constraints (wave energy and reef geomorphology) may predispose some reefs or reef areas to high-load turf sediment regimes.
... Among the fishes that inhabit coral reef and seagrass ecosystems, some wrasses (Eupercaria: Labridae) have emerged as critically important ecosystem engineers and bioeroders that play important roles as herbivorous grazers in their respective habitats (Alwany et al., 2009;Bellwood, 1995Bellwood, , 1996Bonaldo et al., 2014;Bruggemann et al., 1996;Grupstra et al., 2022). In particular, the parrotfishes (labrid tribe: Scarini) are microphages that feed on endolithic and epilithic autotrophic microbes living on and within coral skeletons, as well as on the surface of macroalgae and other sessile marine organisms (Clements & Choat, 2018;Clements et al., 2017). ...
... This beak is composed of some of the strongest biological material on the planet and allows many parrotfishes to scrape and excavate the calcium carbonate skeletons of corals (Marcus et al., 2017). Studies have shown that this constant grazing by some labrid species exerts tremendous pressure on the growth rate of coral colonies and that the resulting calcium carbonate sediment that is excreted during the grazing process forms the foundation of shallow water tropical coastlines worldwide (Bellwood, 1995(Bellwood, , 1996Perry et al., 2015). ...
The upper and lower jaws of some wrasses (Eupercaria: Labridae) possess teeth that have been coalesced into a strong durable beak that they use to graze on hard coral skeletons, hard-shelled prey, and algae, allowing many of these species to function as important ecosystem engineers in their respective marine habitats. While the ecological impact of the beak is well-understood, questions remain about its evolutionary history and the effects of this innovation on the downstream patterns of morphological evolution. Here we analyze 3D cranial shape data in a phylogenetic comparative framework and use paleoclimate modeling to reconstruct the evolution of the labrid beak across 205 species. We find that wrasses evolved beaks three times independently, once within odacines, and twice within parrotfishes in the Pacific and Atlantic Oceans. We find an increase in the rate of shape evolution in the Scarus+Chlorurus+Hipposcarus (SCH) clade of parrotfishes likely driven by the evolution of the intramandibular joint. Paleoclimate modeling shows that the SCH clade of parrotfishes rapidly morphologically diversified during the middle Miocene. We hypothesize that possession of a beak in the SCH clade coupled with favorable environmental conditions allowed these species to rapidly morphologically diversify.
... As a by-product of framework modification, reef fishes also contribute considerably to the production and modification of sedimentary carbonate (Bellwood, 1996;Perry et al., 2020; Figure 1). Parrotfishes excrete the ingested reef framework mainly in the sand-size fractions (Hoey & Bellwood, 2007), while pufferfishes and triggerfish generate rubble grade material through physical breakage of reef framework (Glynn & Manzello, 2015;Guzman & Lopez, 1991). ...
... Sediment grain-size diminution may occur mechanically following ingestion by fish, either through pharyngeal apparatus grinding as in parrotfishes (Bellwood, 1996), or as it passes through the gizzard-like stomach of some surgeonfishes (Nelson & Wilkins, 1988). Diminution may also occur via partial dissolution in the acidic stomach fluids of, for example, carnivorous fishes (Alheit, 1983;Lobel, 1981), as has also been shown in holothurians with alimentary fluid pH values of 6.7 (Hammond, 1981). ...
Coral reef fishes perform essential and well-documented ecological functions on reefs, but also contribute important geo-ecological functions, which influence reef carbonate cycling regimes. These functions include reef framework modification (through bioerosion and breakage), and the production, reworking, and transport of reefal sediments. To explore how these functions vary across reefs and regions, we compiled a dataset of available taxa-specific function rates and applied these to fish census data from sites in the Pacific Ocean (PO), Indian Ocean (IO), and Greater Caribbean (GC), each region displaying a gradient in fish biomass. The highest overall function rates occur at the highest fish biomass sites in the PO (Kingman Reef) and IO (Chagos Archipelago), where bioerosion dominates framework modification and sediment generation (up to 7 kg m À2 year À1). At the lowest biomass PO and IO sites, framework modification and sediment generation are driven mainly by breakage and occur at lower rates (~2 kg m À2 year À1). Sediment reworking rates are high across all PO and IO sites (~1-5 kg m À2 year À1) and higher than other function rates at low biomass sites. Geo-ecological function rates are generally low across the GC sites, despite total fish biomass being comparable to, or even exceeding, some PO and IO sites, with sediment reworking (up to~1 kg m À2 year À1) being the dominant function. These site-level differences partly reflect total fish biomass, but fish assemblage size structure and species identity are critical, with a few fish families (and species) underpinning the highest function rates and regulating the "health" of the fish-driven carbonate cycling regime. Reefs with high fish-driven framework modification, sediment production and reworking rates define one end of this spectrum, while at lower biomass sites little new sediment is produced and sediment reworking dominates. While additional species-level rate data are urgently needed to better constrain function rates, these transitions align with ideas about the progressive shutdown of carbonate production regimes on ecologically perturbed reefs, with important
... This fish group is essential for maintaining reef health, acting directly on algal productivity and coral recruitment (Bonaldo and Bellwood 2011). Structurally, these animals also reshape the landscape built by scleractinian corals by scraping the hard carbonate surfaces and transporting sediment around the reef (Bellwood 1996;Goatley and Bellwood 2012). ...
Biological invasions have modified habitat structure, forcing changes in ecosystem functions. Structural complexity modulates diversity and trophic pathways, but the roles of invasive species in mediating coral reef habitat attributes and trophic effects are poorly understood. We investigated the influence of invasive corals on reef structural complexity and their implications on reef fish trophic structure. To assess habitat complexity and trophic relationships, we used a digital probe to map reef rugosity and characterized benthic cover and fish abundances by video and visual estimates. We calculated a coral skeleton complexity index (for individual invasive and native colonies) by building high-resolution three-dimensional models with photogrammetry techniques. The study was conducted between February 2018 and March 2019 in Cascos Reef, located on the east coast of Brazil. We reveal that the complex morphology of the invasive coral Tubastraea spp. skeleton had a significant positive effect on reef rugosity, contributing to substrate complexity at a sub-metric scale. However, this likely did not promote reef fish diversity but altered the assemblage structure patterns, demonstrated by a negative relationship between coral colony complexity index and abundance of trophic groups such as roving herbivores and omnivores and a positive relationship with planktivores. Thus, our findings support that habitat attribute modification promoted by invasive corals can influence the benthos-fish dynamic, promoting some fish groups to the detriment of others, with pervasive implications for ecosystem functions. Global changes are increasing invasions worldwide, enhancing the need for effective policies for regulation and management to ensure human well-being and ecosystem services.
... Traditional gut content analyses on the species-rich coral reef clade of parrotfishes, i.e. the genera Bolbometopon, Cetoscarus, Chlorurus, Scarus, and Hipposcarus (Streelman et al. 2002), typically report that the bulk of ingested items are unidentifiable calcareous material (Gobalet 2018;Bellwood 1996;Choat et al. 2002). A highly derived pharyngeal jaw coupled with beak-like mandibular jaws supported by powerful adductor mandibulae muscles are responsible for this reducing of calcareous material (Clements and Bellwood 1988;Bellwood and Choat 1990;Gobalet 2018). ...
... Traditional gut content analyses on the species-rich coral reef clade of parrotfishes, i.e. the genera Bolbometopon, Cetoscarus, Chlorurus, Scarus, and Hipposcarus (Streelman et al. 2002), typically report that the bulk of ingested items are unidentifiable calcareous material (Gobalet 2018;Bellwood 1996;Choat et al. 2002). A highly derived pharyngeal jaw coupled with beak-like mandibular jaws supported by powerful adductor mandibulae muscles are responsible for this reducing of calcareous material (Clements and Bellwood 1988;Bellwood and Choat 1990;Gobalet 2018). ...
Diet in fish is influenced by multiple factors including nutritional requirements, trophic morphology and spatial and temporal variation in resource availability. We examined spatial variation in trophic resources on substrata grazed by scarinine parrotfishes by combining quantitative microhistology with 16S and 18S small subunit rRNA barcoding of feeding substrata in six parrotfish species on outer-shelf reefs of the Great Barrier Reef, Australia. We then compared four of these taxa with conspecific data from mid-shelf reefs differing in incident wave energy, parrotfish assemblage structure and benthic cover of hard corals, crustose coralline algae (CCA) and macroalgae. The dominant biota on outer-shelf feeding substrata in terms of both surface coverage and frequency of occurrence were filamentous cyanobacteria. The density of filamentous cyanobacteria on outer-shelf feeding substrata as measured by microscope did not differ either among the six parrotfish species or within-species cross-shelf. Endolithic and epilithic filamentous cyanobacteria from the order Nostocales were the most frequently observed filamentous cyanobacteria, suggesting that these represent a key feeding target for these parrotfishes. In addition to filamentous Nostocales cyanobacteria, taxa that were consistently present on both mid-shelf and outer-shelf feeding substrata were the euendolithic micro-chlorophytes Ostreobium and Phaeophila, diatoms, fungi, CCA, Peyssonnelia, dinoflagellates of the family Symbiodiniaceae, the sponge taxa Clionaida and Poecilosclerida and the filamentous algae Sphacelaria and Polysiphonia. Our results reveal key nutritional drivers underlying feeding by parrotfish on carbonate reefs and provide further support for the hypothesis that microscopic photoautotrophs are a major dietary target for grazing parrotfishes.
... Direct contributions derive postmortem from skeletal fauna such as molluscs and foraminifera (Bosence, 1989), and from the disaggregation of the calcified segments of calcareous green and red algae (Neumann and Land, 1975). The grazing activities of some species of parrotfish and sea urchins, which excrete carbonate particles after ingestion (Hunter, 1977;Bellwood, 1996), also represent a major sediment source, and species of triggerfish and pufferfish can produce coral rubble as they break coral colonies whilst foraging . All bony marine fish (teleosts) additionally excrete silt and clay-grade carbonate generated through intestinal secretions (Perry et al., 2011b;Salter et al., 2012). ...
Standardised methodologies for assessing reef-derived sediment generation rates do not presently exist. This represents a major knowledge gap relevant to better predicting reef-derived shoreline sediment supply. The census-based SedBudget method introduced here generates estimates of sediment composition and grain-size production as a function of the abundance and productivity of the major sediment-generating taxa at a reef site. Initial application of the method to several reefs in the northern Chagos Archipelago, Indian Ocean, generated total sediment generation estimates ranging from (mean ± SE) 0.7 ± 0.1 to 4.3 ± 1.3 kg CaCO 3 m ⁻² yr ⁻¹ . Sediment production was dominated by parrotfishes (>90% at most sites), with site-variable secondary contributions from sea urchins (up to 20%), endolithic sponges (~1–7%) and benthic foraminifera (~0.5–3.5%). These taxa-level contributions are predicted to generate sediments that at all sites are coral- (83–94%) and crustose coralline algae-dominated (range ~ 5–12%). Comparisons between these estimates and sedimentary data from proximal reef and island beach samples generally show a high degree of consistency, suggesting promise in the SedBudget approach. We conclude by outlining areas where additional datasets and revised methodologies are most needed to improve rate estimates and hope that the methodology will stimulate research on questions around sediment production, transport and shoreline maintenance.
... Parrotfish are recognized as critical ecosystem engineers on coral reefs, and as such are priority species in reef management and conservation (Bellwood et al., 2004;Mumby et al., 2006;Hughes et al., 2007;Morgan & Kench, 2016;Wolfe et al., 2020;Tricas & Boyle, 2021). The functional roles of parrotfish in coral reef bioerosion and sediment production are well documented (Gygi, 1975;Hutchings, 1986;Bellwood, 1995Bellwood, , 1996Bonaldo et al., 2014;Kuffner & Toth, 2016;Mallela & Fox, 2018;Yarlett et al., 2021). Parrotfish carbonate erosion rates and sediment production estimates are used in calculating carbonate budgets to model coral reef geomorphology, which is of significance as sea levels rise (Perry et al., 2012;Morgan & Kench, 2016;Lange et al., 2020;Kench et al., 2022;Morais et al., 2022). ...
Parrotfish are key agents of bioerosion and sediment production in coral reef ecosystems; however, their dietary targets and therefore potential sources of variation in carbonate cycling lack resolution. Here we address this knowledge shortfall in our current understanding of parrotfish diets by testing the concept that protein-rich micro-photoautotrophs are the target prey for many Scarinine parrotfishes. We focus at fine spatial scales on the feeding substrata of 12 syntopic Indo-Pacific parrotfish species at mid-shelf sites around Lizard Island, Great Barrier Reef, Australia. We followed individual parrotfish on snorkel until biting was observed, and then extracted a reef core around each bite. The surface of each bite core was scraped to ~1 mm for quantitative microscopic analysis (up to 630 × magnification) and for 16S and 18S rRNA metabarcoding. The most dominant photoautotrophic group in terms of surface cover was filamentous cyanobacteria, followed by crustose coralline algae. Epiphytic, epilithic, endophytic and endolithic filamentous cyanobacteria were consistent bite core biota. Although the density of filamentous cyanobacteria on bite cores was largely consistent among the 12 parrotfish species, the quantitative microscopic data and rRNA metabarcoding revealed distinct differences between parrotfish species in the taxonomic composition of core biota. Our data provide further evidence that these syntopic parrotfish species partition feeding resources.
... They play a crucial role in coral reef ecosystems [2,3], and as consumers of benthic algae, directly affect the structure and composition of benthic communities, and positively affect coral survival and growth [4]. Parrotfish are also involved in calcium carbonate cycling in coral reefs [5,6], and decompose coral reef skeletons into sand-sized sediments [7,8]. They maintain a coral-dominated community structure by feeding on fast-growing algae and can also influence reef development and complexity by decomposing reef carbonates [2,9]. ...
Species markers can be quickly and accurately assessed using DNA barcoding. We investigated samples from the parrotfish family Scaridae using DNA barcoding in Hainan. A total of 401 DNA barcodes were analyzed, including 51 new barcodes generated from fresh material, based on a 533 bp fragment of the cytochrome c oxidase subunit I (CO I) gene. There were 350 CO I barcode clusters that matched 43 species from the Barcode of Life Data Systems (BOLD) and GenBank databases. The results showed the following average nucleotide compositions for the complete dataset: adenine (A, 22.7%), thymine (T, 29.5%), cytosine (C, 29.5%), and guanine (G, 18.2%). The mean genetic distance between confamilial species was nearly 53-fold greater than that between individuals within the species. In the neighbor-joining tree of CO I sequences, Chlorurus sordidus and C. spilurus clustered together, and all other individuals clustered by species. Our results indicated that DNA barcoding could be used as an effective molecular tool for monitoring, protecting, and managing fisheries, and for elucidating taxonomic problem areas that require further investigation.
The patterns of erosion and defaecation by 2 species of parrotfish Chlorurus gibbus and C. sordidus (family Scaridae) were examined at 2 sites on Lizard Island, Great Barrier Reef, Australia. Feeding behaviour and associated external bioerosion were examined at 3 spatial scales: reef zone, microhabitat and bite site. Feeding patterns in both species showed strong selectivity for specific reef zones, microhabitats and substratum types. Both species fed predominantly in the shallows, with strong preferences for coral stumps. C. gibbus strongly selected the reef crest and turf algae at both sites. C. sordidus showed differential selectivity at the 2 sites. It is suggested that these site and species differences are a result of interactions with territorial herbivores. In C. sordidus the patterns of feeding and defaecation across the reef were extremely similar. There was no net movement of material. In contrast, C. gibbus showed a strong preference for feeding in shallow reef zones, with defaecation marked by movement away from feeding areas to specific defaecation sites, usually in deeper areas of the reef. This resulted in an active net movement of carbonate off the reef.
Abundances of the surgeon fish Acanthurus lineatus (Linnaeus) within a single reef-system were estimated using a hierarchical sampling design during 1982. Additional sampling was carried out during 1983 and 1984 at a restricted number of sites. This species is aggressive toward other herbivorous fishes and is strongly site-attached. On the study reef (Lizard Island; 1440S; 14528E), A. lineatus was restricted to reef-crest sites below mean low water, mainly on reefs not directly exposed to prevailing winds. Within this reef zone and depth-defined stratum, A. lineatus was distributed heterogenously with high abundance, (approximately 14 fish per 300 m2) at a single sample-locality covering 600 m of reef crest. Abundances of herbivorous species (members of the families Acanthuridae and Scaridae) at other localities did not correlate with abundance patterns of A. lineatus. Subsampling within study localities revealed considerable heterogeneity in the abundance patterns of herbivorous fishes, especially within the area of high A. lineatus density. A detailed behavioural study of interactions among herbivorous fishes at two adjacent sites within the locality of high A. lineatus abundances revealed a complex pattern of site-general and sitespecific features. A. lineatus excluded smaller scarids from its feeding territories at one site, but not at another. Scarids attaining large size (>350 mm standard length) were present at one site and consistently fed within A. lineatus territories; large scarids were rare at the second site, even though the distances involved were small. In addition, the small surgeon fish A. nigrofuscus, a consistent target for A. lineatus aggression, was rare at one site but moderately common at the other. Finally, the abundant surgeon fish Ctenochaetus striatus was present at high densities at both sites and fed within A. lineatus territories. This species was not attacked by A. lineatus nor did it attack other herbivorous fishes within the vicinity. Small but consistent differences in reef structure were detected at each site. Local-scale heterogeneity in these interactions makes it difficult to develop generalizations concerning the role of territorial herbivores such as A. lineatus within reef systems. We hypothesize that very local differences in the within-habitat component of acanthurid and scarid abundances and distributions may reflect site-associated variability in recruitment patterns, post-recruitment mortality or behaviour that is independent of A. lineatus activities. Replicated removal experiments which include explicit tests for local site-effects and better descriptions of recruitment in larger herbivorous fishes are required before these interpretations can be evaluated.
The assumption that parrotfishes represent a single group of grazing herbivores is addressed by morphological, functional and ecological analyses. This assumption is rejected. The 24 scarine parrotfishes from the Great Barrier Reef, Australia, are divided into two functional groups: excavators and scrapers. The osteology and myology of the jaws of all 24 species were examined, and a detailed description of a representative species from each group is provided. The relative weights of the major jaw structures of five representative species were quantified. Morphological differences between species were interpreted in a functional context and were assessed based on observations of feeding in the field. Species in the two functional groups show major group-related differences in their bite speeds, microhabitat utilization patterns, and form and extent of substratum excavation during grazing. Group-related differences are also apparent in feeding rates, foray sizes and bite rates. The division of the species into two functional groups is supported by both morphological and behavioural observations. The ecological significance of two functional groups within the family is discussed in relation to the role of parrotfishes on reefs, particularly in terms of bioerosion.
The osteology and myology of the jaws and pharyngeal apparatus of two herbivorous labroid fishes are described. Odax pullus (family Odacidae) is a temperate-water species, feeding primarily on fucoid and laminarian macroalgae; Scarus rubroviolaceus (family Scaridae) is tropical in distribution and grazes algae from coral and rock surfaces. The intestinal morphology and feeding behaviour of the two species are also described. These observations are discussed in terms of the functional and ecological significance of the morphological differences between these two superficially similar species.
Current data were collected at 3 stations in the Great Barrier Reef Lagoon of Australia between Lizard Island and Carter Reef, an outer ribbon reef, (approximately 14S) over a 2 year period. During the southeast Trade wind season (March–September), net circulation at all stations was to the northwest, parallel to the coast and reefs, with little cross-shelf movement. This motion was periodic at about 20 days and highly coherent with the wind. During the non-Trade wind season (October–February) the net circulation depended on the variable wind regime and exhibited frequent current reversals and cross-shelf motion. Tidal currents were superimposed on the net circulation and were mainly cross-shelf but with a tidal excursion of only about 5 km on a flood tide. Tidal currents close to Carter Reef were not cross-shelf but remained parallel to the reef, suggesting that the major tidal flux is through the reef passages. Net circulation close to Carter Reef was not coherent with net circulation at the stations in more open waters, during both Trade and non-Trade seasons. Current speeds were typically 10–30 cm s-1. Passive plankters entering the water from Carter Reef are therefore likely to remain close to the outer ribbon reefs and be moved parallel to them. Based on the above, we predict that in the Trade wind season, passive plankters would be advected further from their point of origin than during the non-Trade wind season, but there would be more cross-shelf advection during the latter.