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The Biology of Eurypharynx pelecanoides (Pisces, Eurypharyngidae)
Abstract
This paper is based on 760 Atlantic specimens of pelican eels caught from off Iceland to 48°S in depths between 2750 and c. 500 m. Stomach contents from 120 specimens show a varied diet dominated by crustaceans, fish and squid. The frequent occurrence of Sargasso weed consumed by Eurypharynx indicates a lack of prey-item discrimination. The forward directed eyes appear to enable stereoscopic prey localization, possibly aided by the well developed lateral line. The huge gape is unsuitable for suction feeding. It is proposed that prey engulfing is accomplished by a forward thrust of the head by which the produced water pressure will expand the buccal cavity to a volume several times that of the fish itself. When ripening, males especially show morphological changes in some characters, indicating that the pelican eel breeds only once, a condition which seems to be the rule among eels.
... The suborder Anguilloidei is a mostly deep-sea clade that includes the shallow-water spaghetti eels (Moringuidae) and the catadromous freshwater eels that spawn pelagically in the deep sea (Anguillidae), as well as the primarily deep-sea sawtooth eels (Serrivomeridae), snipe eels (Nemichthyidae), one-jaw eels (Monognathidae), bob-tail snipe eel (Cyematidae), red bob-tail snipe eel (Neocyematidae), pelican eel (Eurypharyngidae), and swallower eels (Saccophayngidae) (Santini et al. 2013;Tang and Fielitz 2013;Poulsen et al. 2018). The deep-sea anguilloids are especially notable for their pelagic habitat, the wide range of jaw structure, and specialized coloration including black, red (in Neocyema), and transparent (in Monognathus) species, all of which likely evolved in response to their deep-sea habitats (Nielsen and Smith 1978;Nielsen and Bertelsen 1985;Bertelsen and Nielsen 1987;Bertelsen et al. 1989;Nielsen et al. 1989;Poulsen et al. 2018). ...
... Adult-sized individuals of the remaining deep sea anguilloids are most commonly collected in bathypelagic environments Froese and Pauly 2000). The gulper eels in the genera Eurypharynx and Saccopharynx are the only known bioluminescent members of Anguilloidea and have a caudal light organ that has been suggested to be a prey lure (Nielsen and Bertelsen 1985;Nielsen et al. 1989;Priede 2017). The gulper eels are most commonly found in the bathypelagic zone and have an intense black coloration in life that has been demonstrated to be ultra-black coloration based on melanincontaining melanosomes in Eurypharynx (Nielsen and Bertelsen 1985;Nielsen et al. 1989;Davis et al. 2020). ...
... The gulper eels in the genera Eurypharynx and Saccopharynx are the only known bioluminescent members of Anguilloidea and have a caudal light organ that has been suggested to be a prey lure (Nielsen and Bertelsen 1985;Nielsen et al. 1989;Priede 2017). The gulper eels are most commonly found in the bathypelagic zone and have an intense black coloration in life that has been demonstrated to be ultra-black coloration based on melanincontaining melanosomes in Eurypharynx (Nielsen and Bertelsen 1985;Nielsen et al. 1989;Davis et al. 2020). Davis et al. (2020) demonstrated that the very low reflectance of ultra-black camouflage would be particularly advantageous as camouflage from the relatively dim light produced by bioluminescence as compared to solar light and suggested that this was the functional benefit of an ultra-black coloration. ...
The deep-sea environment is associated with a wide range of anatomically specialized morphologies allowing camouflage in this low or no light environment. Specialized ultra-black coloration has been documented in the pelican eel, Eurypharynx pelecanoides, but has not been explored in the other largely deep-sea inhabiting pelagic anguilloid eels. Histological examination of the integument revealed a layer of free melanosomes in the superficial dermis consistent with specialized ultra-black camouflage in the swallower eels Saccopharynx, the bobtail snipe eel Cyema, the sawtooth eels Serrivomer, and the snipe eels Avocettina and Nemichthys. The anatomy in these taxa is consistent with the previously described ultra-black morphology, except that Nemichthys, Avocettina, and Serrivomer have both large amounts of free melanosomes and melanophores. Consideration of this morphology in the context of anguilloid eel evolution in the deep-sea environment suggests repeated independent evolution of ultra-black coloration within the anguilloids, and greater development in the taxa more specifically associated with the bathypelagic habitats and the production of bioluminescence.
... The four families were previously classified as an elopomorph order "Saccopharyngiformes" [2,9] although the lineage is a derived subclade within the Anguilliformes and ordinal status therefore not appropriate [8]. The Saccopharyngidae (swallowers) and Eurypharyngidae (gulpers) have long-thin tails with luminous organs at the end, greatly extendable guts for holding large prey, and long jaws [10,11]. The Cyematidae (bob-tail snipe eels) have drastic shortening of the body [9] and the Monognathidae (one-jaws) have reabsorbed upper jaws and poisonous fangs in the adults [11,12]. ...
... The Saccopharyngidae (swallowers) and Eurypharyngidae (gulpers) have long-thin tails with luminous organs at the end, greatly extendable guts for holding large prey, and long jaws [10,11]. The Cyematidae (bob-tail snipe eels) have drastic shortening of the body [9] and the Monognathidae (one-jaws) have reabsorbed upper jaws and poisonous fangs in the adults [11,12]. Neocyema, a genus known from only five specimens [13], also has a shortened body that superficially resembles Cyema atrum (Fig 1). ...
... The Neocyema eel specimen from off Greenland included in this study was likely caught in pelagic waters by getting entangled in the mesh of the trawl (found on the deck, not in the cod-end) before or after bottom-trawling occurred at depths of about 1180 m [13]. Cyema atrum, Eurypharynx pelecanoides and Saccopharynx eels have also been collected at depths shallower than 3000 m [9,73], with E. pelecanoides being collected between 600 and 2300 m in the Atlantic [11]. Whether saccopharyngiforms may use diel vertical migration (DMV) behavior to move to shallower depths at night for feeding, and back to deeper depths during the day to avoid predation, is not known, although some anguilliform leptocephali appear to use DMV [74]. ...
Deep-sea midwater “saccopharyngiform” eels of the families Cyematidae, Monognathidae, Eurypharyngidae and Saccopharyngidae (order Anguilliformes) are extraordinary fishes having major skeletal reductions and modifications compared to the general anguilliform body structure. Little is known about most aspects of the systematics, phylogeny, and ecology of these families, and few of the approximately 30 species described from adult specimens have been matched with their leptotocephalus larvae. Based on mitogenomic sequence data from rare new specimens, we show that the long-speculated-about larval form referred to as “Leptocephalus holti”, which was thought to possibly be the larva of the rare orange-colored eels of Neocyema (5 known specimens; speculated to belong to the Cyematidae) are actually the larvae of the one-jaw eels of the family Monognathidae. One of the 5 types of L. holti larvae that were collected in the Pacific is genetically matched with Monognathus jesperseni, but multiple species exist based on larval sequence data and the morphology of adult specimens. A rare leptocephalus from the Sargasso Sea, with unique morphological characteristics including many small orange spots on the gut, was found to be the larva of Neocyema, which is presently only known from the Atlantic Ocean. We demonstrate that Neocyema constitutes a separate family being most closely related to Eurypharyngidae and Saccopharyngidae based on mitogenomic DNA sequences and unique mitochondrial gene orders.
... The species is known from the lower mesopelagic and bathypelagic zones, likely from 500 to 3,000 m depth (Nielsen et al. 1989) but has also been reported from depths of less than 200 m (Moore et al. 2003a). E. pelecanoides feeds on a wide variety of prey but particularly decapods, fish and squid -perhaps actively hunting with a well-developed lateral-line system and binocular vision, provided by eyes near the tip of its snout, to detect prey items which are then engulfed in its very large mouth (Nielsen et al. 1989). ...
... The species is known from the lower mesopelagic and bathypelagic zones, likely from 500 to 3,000 m depth (Nielsen et al. 1989) but has also been reported from depths of less than 200 m (Moore et al. 2003a). E. pelecanoides feeds on a wide variety of prey but particularly decapods, fish and squid -perhaps actively hunting with a well-developed lateral-line system and binocular vision, provided by eyes near the tip of its snout, to detect prey items which are then engulfed in its very large mouth (Nielsen et al. 1989). ...
During 2007–10, a series of midwater trawl surveys was conducted, at meso- and bathypelagic depths, in The Gully, a submarine canyon and Marine Protected Area immediately east of Sable Island, and around the canyon’s mouth. One of the major outputs of that program, appearing in the primary literature, is a multivariate statistical analysis of the structure of the fish assemblage. That work is here supported by supplementary information on the preparation of the data matrices for the multivariate analyses and by examination of both the spatio-temporal distribution and the length frequency of each of the 19 species included in the analysis. The examination revealed up-canyon trends in fish abundance (declining in 17 species), in depth distributions and in average lengths (increasing in 10 species – usually because of a rapid up-canyon decline in numbers of small individuals).
... As adults, these species are true deep pelagic organisms, as neither is known to undergo vertical diurnal migration. Their buoyant pelagic eggs may be found nearer the surface, and as their leptocephalus larvae mature, they gradually settle into deeper layers (Nielsen et al. 1989). Eurypharynx pelecanoides attains a maximum size of about 75 cm and occurs in the deep pelagic usually to about 3000 m (Smith 1997b). ...
... Eurypharynx pelecanoides attains a maximum size of about 75 cm and occurs in the deep pelagic usually to about 3000 m (Smith 1997b). Its diet is varied and seems to be dominated by crustaceans, though fishes, cephalopods, and other food items have been found among stomach contents (Nielsen et al. 1989). Cyema atrum attains a maximum size of about 15 cm and occurs to about 3000 m. ...
The ocean’s midwaters (the mesopelagic and bathypelagic zones) make up the largest living space on the planet, but are undersampled
and relatively poorly understood. The true distribution of many midwater species, let alone the abiotic factors most important
in determining that distribution, is not well known. Because collecting specimens and data from the deep ocean is expensive
and logistically difficult, it would be useful to be able to predict where species of interest are likely to occur so that
sampling effort can be concentrated in appropriate areas. The distribution of two representative midwater fishes, the gulper
eel Eurypharynx pelecanoides and the bobtail eel Cyema atrum (Teleostei: Saccopharyngiformes), were modeled with MaxEnt software to examine the viability of species distribution modeling
(SDM) for globally distributed midwater fishes using currently available environmental data from the ocean surface and bottom.
These species were chosen because they are relatively abundant, easily recognized, and unlikely to have been misidentified
in database records, and are true midwater fishes, not known to undertake significant vertical diurnal migration. Models for
both species show a generally worldwide distribution with some exceptions, including the Southern Ocean and Bering Sea. Variable
contributions show that surface and bottom environmental variables correlate with species presence. Both species are more
likely to be found in areas with low levels of silicate. SDM is a promising method for better understanding the ecology of
midwater organisms.
... The new specimen shows that the skull and mandible of Hupehsuchus is characterized by a mixture of features resembling feeding adaptations in pelicans and rorqual whales, suggesting that the genus shared the feeding style of these two animals. Pelicans, rorquals, and pelican eels share a common feeding style of capturing prey in a gular pouch together with a large amount of water as they move forward, while their flexible jaw symphyses permit expansion of the lower jaw [19][20][21] . However, there are differences among these groups in terms of the mechanisms of gular space widening and in how they eliminate excess water from the buccal cavity. ...
... By contrast, rorquals use rotation of the solid mandibular rami alone 19 . The pelican eel differs from the other two in expanding both the mandible and the upper jaw 21 . For convenience, we will hereafter refer to the feeding styles of pelicans and the pelican eel as lunge feeding, as is also characteristic of rorquals (see Methods). ...
Traditional wisdom holds that biotic recovery from the end-Permian extinction was slow and gradual, and was not complete until the Middle Triassic. Here, we report that the evolution of marine predator feeding guilds, and their trophic structure, proceeded faster. Marine reptile lineages with unique feeding adaptations emerged during the Early Triassic (about 248 million years ago), including the enigmatic Hupehsuchus that possessed an unusually slender mandible. A new specimen of this genus reveals a well-preserved palate and mandible, which suggest that it was a rare lunge feeder as also occurs in rorqual whales and pelicans. The diversity of feeding strategies among Triassic marine tetrapods reached their peak in the Early Triassic, soon after their first appearance in the fossil record. The diet of these early marine tetrapods most likely included soft-bodied animals that are not preserved as fossils. Early marine tetrapods most likely introduced a new trophic mechanism to redistribute nutrients to the top 10 m of the sea, where the primary productivity is highest. Therefore, a simple recovery to a Permian-like trophic structure does not explain the biotic changes seen after the Early Triassic.
... The bioluminescent properties of the caudal organ of the pelican eel (Eurypharynx pelecanoides) are even less evident. It is not equipped with filaments and observations on luminescence from this organ (Owre & Bayer, 1970) have not been confirmed (Nielsen et al., 1989). However, observations and video recordings of luminescence from the white line organs in the pelican eel have revealed them to be capable of brilliant luminescent flashes of such short duration that they are probably under neural control (E. ...
... Aglomerularism in the two saccopharyngiform eels S. ampullaceus and E. pelecanoides may be explained similarly. All three species are pelagic in the deep sea but lack a gas bladder (Böhlke 1966;Nielsen and Bertelsen 1985;Nielsen et al. 1989;Sasaki et al. 2006). These species, therefore, are pelagic by some means other than a gas bladder, and we speculate that aglomerularism evolved for neutral buoyancy. ...
We report the presence of an aglomerular kidney in the pelagic deep-sea fish Saccopharynx
ampullaceus (Saccopharyngiformes: Saccopharyngidae). The thin kidney is unpaired and ribbon-like rostrally, while it is thicker caudally
with a rod-like shape. Light microscopic observation of serial sections revealed no glomeruli at all. The kidney is composed
of renal tubules, sinusoids and capillaries of the renal portal system and extensive interstitial lymphoid tissues. Each renal
tubule is surrounded by well-developed renal portal sinusoids, and the tubules are well separated from each other. There is
a large space dorsal to the vertebrae, similar to the situation in the closely related Eurypharynx pelecanoides. We consider that S. ampullaceus possesses an aglomerular kidney to gain neutral buoyancy. The urinary bladder of S. ampullaceus is a distinct vesicular structure, unlike that of E. pelecanoides.
The paper provides a review features of the strike Teleostei methods – suction, ram and bite, in different representatives at the definitive stage of development with characteristic morphology traits their feeding apparatus at qualitative level. In the paper we describe new traits of construction jaw apparatus some Teleostei species, those testifie in favour application them several methods of the strike separately or at a time during feeding.
Aim
The criteria used to define the International Union for Conservation of Nature (IUCN) Red List categories are essentially based on demographic parameters at the species level, but they do not integrate species' traits or their roles in ecosystems. Consequently, current IUCN‐based protection measures may not be sufficient to conserve ecosystem functioning and services. Some species may have a singular combination of traits associated with unique functions. Such functionally distinct species are increasingly recognized as a key facet of biodiversity since they are, by definition, functionally irreplaceable. The aim of this study is to investigate whether threatened species are also functionally rare and to identify which traits determine extinction risk.
Location
European continental shelf seas.
Time period
1984–2020.
Major taxa studied
Marine fish.
Methods
Using newly compiled trait information of 425 marine fish species in European waters, and more than 30 years of scientific bottom trawl surveys, we estimated the functional distinctiveness, restrictedness and scarcity of each species and cross‐referenced it with their IUCN conservation status.
Results
In European continental shelf seas, 38% of the species threatened with extinction (9 out of 24 species) were identified as the most functionally distinct. By mapping extinction risk in the multidimensional species trait space, we showed that species with the greatest risk of extinction are long‐lived and of high trophic level. We also identified that the most functionally distinct species are sparsely distributed (4% of the total area on average) and have scarce abundances (<1% of the relative mean abundance of common species).
Main Conclusions
Because a substantial proportion of threatened species are functionally distinct and thus may play unique roles in ecosystem functioning, we stress that species traits—especially functional rarity—should become an indispensable step in the development of conservation management plans.
Hitherto, aglomerular kidneys have been found in some species within three groups of teleosts: Elopomorpha, Paracanthopterygil, and Percomorpha. Here we report the presence of an aglomerular kidney In the benthopelagic deep-sea fish Ateleopus japonicus (Ateleopodidae: Jellynose fish). The kidney consists of a pair of strap-like bands located In the typical retroperitoneal position. Light microscopic observation of serial longitudinal and cross sections revealed no glomerulus. The kidney is composed of renal tubules, sinusoldal capillaries of the renal portal system, and extensive Interstitial lymphold tissues. Each renal tubule is highly convoluted and surrounded by the well-developed renal portal system. The tubules are well separated within the kidney. An updated species list of teleosts with aglomerular kidneys Is presented. Water content of A. Japonicus was found to be very high for a marine fish (ca. 90%), and we discuss the functional significance of the aglomerular kidney in A. Japonicus In terms of neutral buoyancy.
Although the Greenland fish fauna has been studied for more than 200 years, new species continue to be discovered. We here take the opportunity of the International Polar Year 2007–08 (IPY) to present an updated check-list of the fishes of Greenland and discuss whether the growing diversity can be explained by global warming. A total of 269 species from 80 families are known from the Greenland Exclusive Economic Zone (EEZ), based on published literature and specimens in museum collections. Since the latest publication covering all known Greenland fishes [Nielsen & Bertelsen 1992], 57 species have been added. Nineteen of these (Harriotta raleighana, Centroscymnus coelolepis, Bathytroctes microlepis, Einara edentula, Ceratoscopelus maderensis, Argyropelecus gigas, Maurolicus muelleri, Polyipnus asteroides, Nansenia oblita, Melanostomias bartonbeani, Polymetme corythaeola, Coryphaenoides mediterraneus, Merlangius merlangus, Guttigadus latifrons, Entelurus aequoreus, Helicolenus dactylopterus, Epigonus telescopus, Lophius piscatorius, Linophryne bicornis) are reported here for the first time. Twenty-nine of the species were added on the basis of taxonomic revisions and/ or identification of specimens caught before 1992, whereas 28 species have been caught in Greenland waters for the first time since 1992. Ten species were new to science described since 1992. Only five of the added species are Arctic – i.e. mainly caught north of the Davis and Denmark Straits. Of the 28 species caught after 1992, five species (Maurolicus muelleri, Merlangius merlangus, Helicolenus dactylopterus, Lophius piscatorius, Entelurus aequoreus) from the southern regions (Atlantic) are mainly from shallow waters (< 400 m) and their arrival is likely to be a result of increasing temperatures. The explanation of the many new records of deep-water fishes is most likely increasing fishing efforts down to depths of 1500 m. The deep waters off Greenland (> 1500 m), however, remain almost unstudied.
The pelagic (589 spp.) and demersal (505 spp.) oceanic ichthyofaunas of the North Atlantic Basin have very different compositions at ordinal and family level. Yet the pattern of relationships between species' maximum size and maximum fecundity from data available (10% of the pelagic, 19% of the demersal species) was similar. A positive relationship between fecundity and weight was confirmed among most teleosts, but was not followed by the elasmobranchs represented. Species' reproductive styles are reviewed in ordinal groupings within a framework of the overall body size/fecundity distribution. Species size (maximum weight) spectra were synthesized for both pelagic and demersal assemblages to assess the allocation of potential reproductive effort throughout the North Atlantic oceanic ichthyofauna. The only available examples of species size spectra and biomass spectra from the pelagic and demersal ichthyofauna in this ocean basin imply geographic and bathymetric variation in overall reproductive effort among fishes whose fecundity is size dependent. Further implications concerning reproductive effort are discussed in the light of food availability.
Convergence is an important evolutionary phenomenon often attributed solely to natural selection acting in similar environments. The frequency of mutation and number of ways a phenotypic trait can be generated genetically, however, may also affect the probability of convergence. Here we report both a high frequency of loss of gas bladder (swim bladder) mutations in zebrafish and widespread convergent loss of gas bladders among teleost fishes. The phenotypes of 22 of 27 recessive lethal mutations, carried by a sample of 26 wild-caught zebrafish, involve loss or noninflation of the gas bladder. Nine of these bladderless mutations showed no other obvious phenotypic abnormalities other than the lack of an inflated gas bladder. At least 19 of the 22 bladderless mutations are genetically distinct, as shown by unique morphology or complementation. Although we were not able to obtain eggs for all 21 required crosses to demonstrate definitively that the remaining three mutations are different from all other bladderless mutations, all available evidence suggests that these mutants are also distinct. At least 79 of 425 families of extant teleosts include one or more species lacking a gas bladder as adults. Analysis of the trait's phylogenetic distribution shows that the gas bladder has been lost at least 30-32 times independently. Although adaptive explanations for gas bladder loss are convincing, a developmental bias toward bladderless phenotypes may also have contributed to the widespread convergence of this trait among teleosts. If gas bladder development in teleosts is as vulnerable to genetic perturbation as it is in zebrafish, then perhaps a supply of bladderless phenotypes has been readily available to natural selection under conditions for which it is advantageous not to have a gas bladder. In this way, developmental bias and selection can work together to produce widespread convergence.
Study of three lyomere larvae (two specimens of Leptocephalus latissimus Schmidt and one of Leptocephalus pseudolatissimus Bertin) indicates that lyomeres develop a more complete hyobranchial apparatus than hitherto known, and that absence of ventral elements of the hyoid and gill arches in adults represents secondary developmental losses from a normal teleost pattern rather than primitive absence of structures. The larvae have interhyal and ceratohyal cartilages, a ventral cartilage in the first gill arch, and, at least in L. latissimus, an opercular cartilage. The toothed upper jaw of the larva clearly occupies the morphological position of the maxillary, as in eel larvae. The specimen of L. pseudolatissimus is apparently the fourth metamorphosing lyomere to be reported. Most of its characters are still larval, but the jaws and suspensorium had begun to elongate. The ventral hyobranchial cartilages are still present but the opercular cartilage seems absent. Damage and poor preservation obscure structural details of the upper jaw. The kidney is thin and cylindrical; it is somewhat thickened posteriorly where the main renal blood vessels enter it. The high structure that Bertin termed kidney is hepatic or pancreatic tissue. Strong similarity of the two kinds of larvae suggests close relationship. Wide ranges of variation in somite counts suggest that each larva may represent a polytypic group rather than a single species. The variation and species composition of lyomere genera are still poorly understood. Available evidence, including metamorphosing specimens, supports the provisional identification of Leptocephalus pseudolatissimus as the larval stage of Eurypharynx. Somite and vertebral counts suggest that Leptocephalus latissimus may be the larva of Saccopharynx, but taxonomic uncertainties and the lack of metamorphosing stages leave the generic allocation of this larva open to doubt.
Two nearly perfect specimens of the little-known gulper eel, Eurypharynx pelecanoides, were collected within 24 hours from two localities in the Hispaniola Basin, in opening-and-closing plankton nets. The depths from which they were caught were approximately 971 and 1532 m, where the bottom was at depths, respectively, of 3870 and 3541 m. Data on temperatures and salinities at the fishing depths were obtained with a Bissett-Berman STD probe. The lateral-line organs and caudal organ, which differ structurally from published accounts, are illustrated. Photographs and a drawing contributing to our comprehension of the form of this fragile and spectacular species are included.
Three new species of the genus Monognathus-M. isaacsi, M. ahlstromi, and M. jesse-are described from the Pacific Ocean, bringing the number of the species to six. M. isaacsi differs from the other species in having a relatively large head and dark brown pigmentation on the whole body. M. ahlstromi has a characteristic paddle-shaped caudal fin, and M. jesse has a lanceolate caudal fin. A key to the six species and their distribution in the Pacific and Atlantic are given. Leptocephali of Monognathus sp. are identified and described for the first time. The status and relationships of the Monognathidae are discussed. Metamorphic forms of Saccopharynx and Eurypharynxare described. The identity of Leptocephalus latissimus to Saccopharynx and of L. pseudolatissimus to Eurypharynx is confirmed. An unknown leptocephalus closely resembling that of Cyema is described, and the possibility of a new genus in the Cyemidae is suggested. Relationships of the Cyemidae to the Nemichthyidae are refuted, and relationships of the Cyemidae to the Saccopharyngiformes are supported. The deepsea gulpers of the order Saccopharyngi formes (Monognathidae, Saccopharyngidae, and Eurypharyngidae) are among the most curious and extremely modified bathypelgic fishes, and very little is known about them. Bohlke (1966) reviewed the literature on the attempts to relate them to diverse groups of fishes. I describe three new species of Monognathus (M. isaacsi, M. ahlstromi. and M. jesse) and four metamorphic stages of Monognathus sp. A key to the six known species of the genus Monogna thus is given. This is the first record of the family from the southern, central, and eastern Pacific. The three species of Monognathus described by Bertin (1934, 1938) from the Atlantic and Indo Pacific regions are only juveniles. The lack of adult monognathids even led Bohlke (1966) to suspect that the then known monognathids might be postlarval saccopharyngids. The specimen named as M. isaacsi is in a more advanced stage than any of the other specimens of the six species. Many of its features clearly indicate that this family is distinct from the Saccopharyngidae. A leptocephalus stage in the life history ofMono gnathus is reported for the first time. Information on the ethmoid tooth, food, and distribution of Monognathus is given. The status and relation ships of the family Monognathidae to the Sac Copharyngiformes are discussed.
Although some of the details of the structure of Lyomeri have been rather carefully studied, as for instance the minute structure of the gut, kidneys, of some bones and cartilages by Nussbaum-Hilarowiez, and the proportions of parts of the body by Bertin, the main characters of Lyomeri have not attracted the attention of scientists and the systematic position of the group is most uncertain.The Lyomeri have a branchial apparatus built differently from other fiches; the Lyomeri have no proper gill cover and no bones supporting such cover. In Eury-pharynx there are six functional visceral clefts and five holobranchs (in Saccopharynx the hind cleft and the hind gill are reduced). The bone forming the upper jaw of Lyomeri lies medially to the hyomandibular, mandibular and the quadrate. The radials of the lobate pectorals articulate with the basal cartilages of the lobes but not with the pectoral girdle. The rays of the fins of Lyomeri are ossified, but soft and not segmented. The disposition of the series of the lateral line on the head is unusual. There is a series along the edge of the upper jaw, a series along the suspensorium, and one parallel to the suspensorium along the cheek, from the orbit to the posterior end of the opercular fold. There is no lateral line along the mandible.Several more characters of Lyomeri stress the peculiarity of this group. The Lyomeri have no supraoccipital, and no lateral ethmoids; the prootics of Eurypharynx extend anteriorly into the interorbital septum beyond the hypophysis and the trigeminus-facialis ganglion; parts of the neurocranium are movably articulated; the cover bones of the roof of the cranium are unusual; there are no vomer arid parasphenoid in Saccopharynx (these bones are present, but much reduced in Eurypharynx).It seems possible, from the present study, that the upper jaw of Lyomeri is formed by the elements of the mandibular arch, that the anterior visceral cleft of Lyomeri is not a branchial but a prehyoid cleft, and that the cartilages supporting the septum behind this cleft are the ‘missing’ ventral elements of the hyoid arch*.However, as in the specimens studied by me the cephalic nerves are partly lost their course could not be followed.Thus these two main features of Lyomeri still remain uncertain.An attempt at a general characterization of Lyomeri and of the two fanlilich of this group is made in Part II of this paper. In Part III a brief comparison of Lyomeri with the Osteichthyes generally, and with some peculiar Actinopterygians, is given.In Part I the specimens of Lyomeri of the British Museum are described in sonic detail, their systematic position is discussed, and a brief revision of the species of Saccopharynx and Eurypharynx is given.This work was done in rather an uncomfortable time and conditions. It was much delayed by the absence of books, and of many fishes preserved in the British Museum which had been evacuated for safety, and by the lack of various other facilities.All the drawings and diagrams were made by the writer.The system of fishes used here for the main groups is that adopted by Forster-Cooper (1940), and for the orders that used by Regan (1929).The names of parts of the skeleton used here are mainly as used by de Beer (1937); references to the terminology of Holmgren and Stensio (1936) are also given.I am most grateful to the Zoological Society of London and to the Linnean Society for permission to use their libraries. I am much indebted to Dr. E. Trewavas of the British Museum for most helpful discussions and advice, and for kindly reading part of the manuscript. I tender my most sincere thanks to Prof. G. R. de Beer, F.R.R., for kindly discussing with me this paper and for his criticisms and suggestions.
Sur une series de Leptocephales appartenant au genre Saccopharynx Mitchill
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On the anatomy and relations of the Eurypharyngidae
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Les poissons Apodes appartenant au sOlls-ordre des lyomeres
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Explorations in fhe life offishes
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