Ophidiiformes (Cusk-Eels and Relatives)
(Cusk-eels and relatives)
Number of families 4 or 5
Evolution and systematics
The order Ophidiiformes is a group of slender, elongate, eel-like, and mostly bottom-dwelling fishes that is relatively unremarkable in external morphological features but exhibits highly evolved behavioral and reproductive traits. The order contains 355 named species, but there are numerous undescribed forms in scientific collections that have not been formally named. Collectively, there are probably 380–400 ophidiiform species in 90 genera, living mostly in marine environments but also including some freshwater and estuarine species. The order has the deepest-dwelling fish known to science, Abyssobrotula galatheae, captured at a depth of approximately 5 mi (8,370 m) below the ocean surface! The ophidiiform family Carapidae (the pearlfishes) comprises about 31 species in seven genera, including some species that are parasitic in the body cavities of marine invertebrates, such as starfish and sea cucumbers. The family Ophidiidae, the cusk-eels, is the most diverse family (about 218 species in 48 genera) and includes several large benthic species that are fished commercially. The family Bythitidae contains 96 species in 32 genera and includes live-bearing species that reside in shallow waters. The family Aphyonidae contains 22 live-bearing species (in six genera) that occur in deeper waters. Another family, the Parabrotulidae (three species in two genera), often is included in the order Ophidiiformes, but its placement there is the subject of controversy. Some researchers consider parabrotulids to be close relatives of the perciform family Zoarcidae, the eelpouts. Parabrotulids, however, share with Ophidiiformes many skeletal and soft anatomical features that are lacking in zoarcids. Thus, the taxonomic limits of the order Ophidiiformes remain uncertain and require further research.
The cusk-eels and their allies are classified in the super-order Paracanthopterygii, a large and morphologically diverse assemblage that includes the freshwater trout-perch and its relatives (Percopsiformes), the codfish and its relatives (Gadiformes), and the goosefish and its relatives (Lophiiformes). Some ichthyologists doubt that the Paracanthopterygii is a natural grouping (that is, monophyletic, or derived from a common ancestor), and the superorder has been reorganized several times. At present the superorder is defined by the common possession by its members of certain skeletal traits of the caudal fin and the cranium as well as the presence of a small bone (termed a supraneural) that lies in the dorsal musculature above the anterior vertebrae. Some paracanthopterygian fishes also possess certain morphological traits that are characteristic of the higher perchlike fishes (Acanthomorpha). These traits include the presence of true fin spines, particular patterns of small bones and ligaments associated with anterior vertebrae, and patterns of ossification of the pelvic bones. Thus, it is generally accepted that the Paracanthopterygii are acanthomorph fishes, currently classified within the Holacanthopterygii, a newly proposed name for a higher category that includes stephanoberyciform, zeiform, beryciform, and all other perchlike fishes. Within the Paracanthopterygii, ophidiiform fishes may be aligned most closely with gadiformes. Ophidiiforms and gadiforms share certain structural and developmental features of the caudal skeleton and gut anatomy. The phylogenetic relationships among paracanthopterygian orders remain speculative, however.
Extensive morphological analysis has not produced a satisfactory hypothesis of the evolutionary history of the ophidiiform fishes, and the phyletic relationships among ophidiiform families are largely unknown. Within the order, ichthyologists have shown natural groupings at family, subfamily, and generic levels, and there are solid proposals for sister group relationships among some families. Evidence for monophyly of the entire ophidiiform lineage is lacking, however. A number of characters have been proposed to classify ophidiiform families, including the position of the pelvic fins, but most of these proposals have proved unsatisfactory. Ichthyologists currently subdivide ophidiiform fishes into two groups, based on the presence or absence of viviparity and the anatomical features that are associated with live bearing.
The suborder Bythitoidei (containing the live-bearing families Bythitidae and Aphyonidae) is considered to be a natural grouping, since it is apparent that these fishes share highly specialized reproductive traits. Within the suborder, aphyonids and bythitids are sister taxa, each possessing uniquely specialized morphological characters (termed "synapomorphies"). Ichthyologists have been unable to specify synapomorphies supporting monophyly of the suborder Ophidioidei (containing the oviparous Carapidae and Ophidiidae), however. Within the Ophidioidei, the Carapidae has been shown to be monophyletic and is considered the closest relative of all other ophidiiforms. The Ophidiidae has not been shown to be monophyletic, but there are natural groupings within the family. The ophidioid subfamily Ophidiinae (the true cusk-eels) is considered monophyletic, because all its members share certain characteristics of the pelvic girdle. The ophidioid subfamilies Brotulinae and Brotulotaeniinae each contain a single, well-defined genus. The ophidioid subfamily Neobythitinae, containing 38 genera and about 170 species, is probably not a natural group and requires further research.
Fossil evidence of paracanthopterygian fishes is not extensive. A freshwater Paleocene fossil taxon, Mcconichthys, may be associated with the ophidiiform lineage, but this conclusion is disputed by some researchers. Fossil ophidiiform fishes, especially fossilized otoliths, or ear bones, are abundant in some Tertiary deposits. To date, several ophidiiform taxa have been described from the Paleocene and Eocene and from more recent deposits, including species of Hoplobrotula and Ampheristus.
Most ophidiiforms are long, relatively slender fishes with big heads, often small eyes, long dorsal and anal fins, and a caudal portion that tapers to a point. Sizes range from the
tiny Microbrotula, which matures at 1.5 in (38 mm) in length, to commercially exploited species such as Genypterus blacodes that attain lengths of 3.3–6.6 ft (1–2 m). The cusk-eels often are strongly pigmented along the dorsal or anal midline, with lateral black or brown markings or bands extending the length of the body. In other ophidiiforms, the body is covered uniformly with small pigment spots, or pigment is entirely lacking. The mouth usually is large, and the upper jaw (maxilla) reaches a point at or beyond a vertical drawn through the eye.
The caudal fin is small, sometimes reduced to a bony point, never forked, and often inconspicuous when it is confluent with the dorsal and anal fins. The pectoral fins often are long, sometimes exceeding the length of the head. The pelvic rays sometimes are absent. When present, the pelvic rays usually are long and conspicuous. Scales can be absent, but when present, they generally are small. The order is defined by the following set of external characteristics. (1) The pelvic fins (when present) have only one or two rays. (Some species have an additional spinelike splint.) (2) The pelvic fins are inserted anteriorly, just below the opercular margin or sometimes farther forward. (3) The pelvic fin bases typically are close together.(4) The dorsal and anal fin bases are long, reaching the caudal fins in most species. (5) There are no dorsal or anal fin spines. (6) Bones (termed "pterygiophores") supporting the anal- and dorsal-fin rays outnumber total vertebrae. (The ratio of dorsal and anal rays to total vertebrae is about 1.5:1.)(7) The nostrils are paired on each side of the head.
In the pearlfishes (Carapidae), the anal-fin rays are longer than the dorsal-fin rays, and the upper jaw lacks a supramaxillary bone. In ophidiids the supramaxillary bone is present, and the anal-fin rays are either equal in length or shorter than the dorsal-fin rays. Within these families, subfamilies are distinguished on the basis of the presence or absence of pelvic fins, scales, barbels, and other internal traits. Carapids and ophidiids are egg-laying fishes, and males lack the external intromittent organ that is characteristic of bythytoid fishes. In addition, the nostrils of ophidioid fishes are positioned higher on the snout than those of bythytoid fishes. Bythytids possess a swim bladder, and most species have scales; aphyonids lack both scales and a swim bladder. Bythitid subfamilies are distinguished by caudal-fin morphological features. Aphyonids are fragile, transparent fishes that have weakly developed skeletal systems and possess other traits characteristic of larval ophidiiforms. Thus, many ichthyologists believe that aphyonids are neotenic. Neotenic organisms are those that retain larval features as adults, and the process of neoteny is thought to be important in the evolution of many animal species, including humans.
Most ophidiiform fishes are distributed broadly in all oceans, sometimes to abyssal depths and extending to shallow seas and estuaries. Ophiidiforms occur from Greenland south to the Weddell Sea, but most species are found in warmer waters of the tropics and subtropics. Carapids, ophidiids, and aphyonids are strictly marine, whereas some bythitid species are estuarine or reside in freshwater. Some bythitids have extremely restricted home ranges in caves and sinkholes.
Ophidiiform fishes generally are secretive and tend to associate with structures or with other animals. Most opidiiforms are free-living benthic species hovering close to the bottom and residing in mucus-lined mud or sand burrows, rock or coral crevices, or sea caves or associated with bottom-dwelling invertebrate communities, including deep sea vent fauna. Some species are pelagic or benthopelagic. One mesopelagic genus is associated uniquely with a species of deep-sea jellyfish. Pearlfishes are either free-living or inquiline species. Free-living species (the pyramondontimes and some echiodontines) are pelagic or bottom dwelling in deep oceanic water or on the continental shelf. Some echiodontines are believed to be associated with tube worm communities. The commensal carapids (Onuxodon, Carapus, and Encheliophis) are all shallow-dwelling species that reside within the body cavity of invertebrate hosts, such as pearl oysters, giant clams, tunicates, sea stars, and sea cucumbers. Species in the bythitid genera Lucifugia and Ogilbia reside in freshwater caves and sinkholes.
The air bladder, anterior vertebrae, and associated ligaments and muscles are modified in many ophidiiform fishes to produce sound. In some species, the air bladder is partly ossified and serves as a resonating chamber. Sound production is thought to be associated with reproduction, as some species have been observed to produce sound just before mating. Hydroacoustic surveys often show large assemblages of vocal cusk-eels. Ophidiiform fishes generally have a close association with the bottom, hovering near holes, ledges, and drop-offs or hiding in mud and sand. Burrowing is accomplished by tail-first entry into soft and movable sediments. Ophidiiform fishes usually hide in burrows or crevices or within or around invertebrate hosts during daylight hours and then exit at night to forage. One bythitidae species has been seen living inside a hot thermal vent at great depths. Many ophidiiform fishes have highly evolved commensal associations with invertebrates. In the pearlfishes, some species are obligatory inquilines that never leave their hosts and feed on a host's internal organs.
Feeding ecology and diet
Ophidiiform fishes consume a wide variety of invertebrate and fish prey. Most are bottom dwellers that are strongly nocturnal, suggesting that they forage for benthic organisms during evening hours. This feeding behavior is facilitated by well-developed sensory pores on the head; a large, inferior mouth; and long, fleshy barbels on the chin in some species that are believed to aid in locating prey. Food consists of worms, crustaceans, echinoderms, and small bottom fishes such as gobies and small flatfishes. In turn, ophidiiform fishes are important food sources for many larger fish predators including skates, rays, sharks, eels, cod, hakes, goosefishes, and flounders. In addition, cusk-eels and their relatives living in shallow waters are prey to wading birds.
Ophidiiform fishes either deposit eggs or bear live young. The oviparous (egg-bearing) species include the pearlfishes (Carapidae) and the cusk-eels (Ophidiidae). Eggs of most carapids and ophidiids are unknown. Those that have been identified either are spawned in open water as individual, free-floating eggs or are deposited in a mucilaginous raft or gummy matrix, much like eggs of the goosefish and its allies (Lophiiformes). The egg rafts float at the ocean surface until they hatch, usually within several days. Larvae of carapids and ophidiids are pelagic, typically floating near the surface and sometimes traveling great distances from their hatching locality. Scientists believe that larvae of these species have the ability to regulate growth and metamorphosis (the transformation to juvenile and adult form). This developmental strategy allows species to disperse over great distances into habitats that are underutilized, thus reducing competition for limited resources.
Pearlfish larvae are unique in their possession of a long, highly ornamented predorsal filament (actually, the first dorsal fin ray), known as the vexillum. The vexillum may have a sensory function, because it contains stout cranial nerve fibers and its position around the head and mouth can be controlled by the larva. Vexillifer larvae have been identified for almost all pearlfish genera. Larvae are elongate, and their bodies typically are sparsely pigmented. After a long pelagic period, vexillifer larvae of species that are commensal or parasitic in invertebrates as adults lose the vexillum and shrink in length to transform abruptly to a so-called tenuis stage. The tenuis larvae seek out their benthic invertebrate hosts before maturing into adults.
Most cusk-eel larvae do not undergo an abrupt transition in morphological features in their development. One remarkable exception is a strange larva captured near South Africa. The larva has an enormous appendage that is highly pigmented and resembles the tentacles of a jellyfish. The appendage contains the larval gut. This so-called exterilium larva ("the larva with an outside gut") is tentatively identified as an unknown ophidiid species. Larvae of the ophidiid sub-family Neobythitinae have elongate dorsal and pelvic rays, but all other larval cusk-eels lack elongate rays. These larvae generally resemble adults, with a conspicuous coiled gut, pelvic fins placed far forward on the throat, long dorsal and anal rays, and big heads and mouths.
Larvae of the live-bearing bythytoid fishes are poorly known, and only a few have been described. Larvae of Brosmophysis (Bythitidae) and Barathronus (Aphyonidae) generally resemble ophidiid larvae, in that they lack elongate rays and possess coiled guts and long dorsal and anal fin bases. Aphyonid larvae hatch at larger sizes than do bythitid larvae. Some bythytoid embryos have specialized feeding appendages termed "trophotaena," through which they gain maternal nourishment. Some bythytid embryos consume or suckle bulbs of ovigerous tissue during development to supplement embryonic nutrition through the yolk sac.
The IUCN lists seven ophidiiform species as Vulnerable due to their rarity and limited habitats. All are live-bearing bythytids: Lucifugia (Stygicola) dentata, L. simile, L. spelaeotes, L. subterranea, L. teresianarum, Ogilbia pearsei, and Saccogaster melanomycter. One species, O. galapagosensis, is listed as Data Deficient.
Significance to humans
Most ophidiiform fishes are unknown to the public and are rare, deep-water species taken only in research cruises or commensal and parasitic forms that hide inside the bodies of invertebrate hosts or otherwise secretive species living in caves or small crevices in the reef. Four species in the genus Genypterus are large; their flesh is tasty, and they are fished commercially. In Chile and New Zealand, landings of G. blacodes reach 33,000 tons (30,000 metric tonnes) annually. Brotula barbata is a food source of growing importance to the peoples of several west African countries.
List of SpeciesKey brotula
Ogilbia cayorum Evermann and Kendall, 1898, Key West, Florida, United States. The holotype is a female. It was unknown at the time of the original description that the species is sexually dimorphic. Thus, considerable taxonomic confusion occurred when the male of the species was described separately. Taxonomic confusion persists, since it is believed currently that numerous undescribed Ogilbia species reside in the western tropical Atlantic Ocean. The extent to which their distributions overlap with the Key brotula is unknown. Thus, care must be taken in identification.
other common names
English: West Indies brotula.
A small and moderately elongate ophidiiform fish with long anal and dorsal fins. The body and fins are uniformly pigmented, appearing yellow to olive-brown. Adults can attain about 3.9 in (10 cm) in length. The caudal fin is distinct and separate, not confluent with the dorsal- and anal-fin bases. The pelvic fins are positioned at the isthmus, anterior to the pectoralfin bases. Each pelvic fin has one fleshy ray that is longer than the length of the head. The eye is small and the mouth large, with the upper jaw extending well posterior to the eye. The body is covered with small overlapping scales that do not appear on the cheeks (opercular bones).
The species is known from the Florida Keys, the Bahamas, and other localities in the Caribbean, including Belize, Cuba, Puerto Rico, and Jamaica south to Venezuela. Many of these distributional records may be for undescribed and similar species with overlapping distributions in this region.
The holotype of the Key brotula was collected by seine on an algae-covered shoal over coral rubble. The coral fragments were encrusted with organisms. The species usually is collected at depths of less than 9.8 ft (3 m), often by uprooting and shaking clumps of Halimeda. This secretive bythitid also may reside in reef crevices, rocky holes, and mangrove roots in shallow waters and out to the edge of the reef. It is rarely seen by divers and typically is collected only in research surveys at poison stations, sometimes locally abundant. For example, more than 15 individuals were collected at one station in Belize. The species probably is tolerant of large fluctuations in temperature and salinity.
Specimens kept in aquaria constantly hide under shells or rubble. The species probably is nocturnal, judging by its small eyes and cryptic nature. There are few data on its natural habits.
feeding ecology and diet
There are no detailed studies of the diet or feeding behavior of this species. Juvenile and adult Key brotulas probably eat small crustaceans and are preyed upon by larger fishes and wading birds. Embryonic nutrition is remarkable. To supplement energy reserves in yolk sacs, embryos of Key brotula "suckle" on fluid-filled bulbs of ovigerous tissue. This highly unusual developmental strategy may be an evolutionary response to the poor feeding evironment of larvae after hatching.
Males and females have delicate, interlocking copulatory organs. Genital morphological features have been described in detail, but mating has never been observed. The copulatory apparatus of the male is partly calcified and is thought to be derived from the first anal ray. The penis rests on a pedestal surrounded by muscular pseudoclaspers and covered by a fleshy urogenital hood. The urogenital pore of the female is similarly protected. Most bythitids have the capability of sperm storage in the oviduct or ovary. Fertilization is internal, and embryos grow rapidly. Late embryos are well developed, with pigmented eyes and fully developed and functional digestive system, musculature, gills, and caudal fin. Newborn young are about 0.5 in (12 mm) in length, and brood size is about 14 individuals.
Not listed by the IUCN.
significance to humans
This species is rarely observed and is not fished commercially. It has limited appeal as an aquarium species, since it rarely shows itself to the viewer.
Lefroyia bermundensis Jones, 1874, Bermuda. The holotype was named in honor of Lefroy, a former governor of Bermuda. The species is now valid as Carapus bermudensis (Jones) following several taxonomic revisions. Numerous names are probably referable to C. bermudensis, including C. recifensis, C. chavesi, and Fierasfer dubius. C. bermudensis may be related most closely to C. acus, its eastern Atlantic relative. The species-level relationships of the genus, however, are poorly known.
other common names
Carapus bermudensis is long, slender, and eel-like, with a large head and relatively large eyes. It is translucent, with silvery bands along the flanks, black internal pigment visible along the vertebral column, a silver cheek patch, and large pigment blotches along the bases of the dorsal and anal fins and head. The anal fin origin is anterior to the dorsal fin origin. There are 13–18 anal rays anterior to the first dorsal ray. This number varies among pearlfish species and is useful in identification. There are no pelvic fins, and the caudal fin usually is absent. The pectoral fin has 17–20 rays. The teeth on the upper jaw are small, and some are heart-shaped. The teeth on the lower jaw are larger and conical. The air bladder is separated into two parts by an internal constriction under vertebrae 11 and 12. This feature of the internal anatomy is characteristic of all species in the genus Carapus, and the position of the constriction relative to the vertebrae allows for separation of species.
Distributed in shallow waters along the shores of the western Atlantic, Bermuda, and the Caribbean Sea south to Brazil. Its larvae sometimes are collected far north and east of this range in plankton samples taken by research cruises.
All species of the genus Carapus have obligatory commensal relationships with sea cucumbers (Holothuria), starfishes (Asterioidea), or sea squirts (Ascidiacea). Many species exhibit host specificity. Carapus bermudensis has been collected in the body cavity of nine holothurian species in the genera Actinopyga, Isostichopus, Thone, Astichopus, Holothuria, and Theelothuria. These host species generally reside in shallow waters, to about 98.4 ft (30 m) on sandy bottoms or grass beds in tropical and subtropical lagoons near reefs. In one study in the Bahamas, pearlfishes were found in relatively few restricted areas, although more than 1,000 sea cucumbers were surveyed.
Resides within the body of its host during daylight and is believed to exit at night to forage and perhaps spawn. This strategy limits the probability of predation by larger fishes. This species has been observed in aquaria as it rapidly enters its primary host, Actinopyga agassizi. The species first locates the anal opening of the sea cucumber with its snout, presumably through olfaction. As the fish holds its head in the proper position at the anal opening, the body curves and the tip of the tail tracks along the mid-lateral line until it reaches the anus. Once the tail tip is aligned and pointed into the opening, the fish abruptly turns, forcing its way tail first into the host by body undulations. There are no observations of living pearlfishes in the wild and little data on its habits and behaviors.
feeding ecology and diet
Some inquiline pearlfishes are parasitic, dining on the internal organs of the invertebrates they occupy. This species of pearlfish is not parasitic and feeds outside the holothurian host, probably at night. There have been no detailed studies of the food habits of this species, but gut contents of individuals are mostly crustacean invertebrates, such as amphipods, small shrimps, crabs, and mysids. Rarely, a pearlfish is found in the stomach of larger, predaceous fishes.
Spawning of pearlfishes has not been observed, and carapid reproductive behavior is poorly known. Some investigators have identified eggs collected in plankton samples by subsequent incubation in the laboratory. There also are a few reports of pearlfish species spawning in aquaria. In these cases, the scientists did not observe spawning directly but found eggs in tanks after periods of darkness. The eggs of pearlfishes are ellipsoid, usually containing an oil droplet and deposited into a jellylike, mucous matrix that floats at the surface. The egg mass has been described as oval, spherical, or somewhat flattened. Eggs hatch in one to two days. Early larvae are easily identified, since they possess a vexillum that first appears as a small protuberance but rapidly grows in length. Older pearlfish larvae have a long pigmented and ornamented vexillum that often is damaged in collection. Pearlfish larvae are extremely elongate, reaching about 7.1 in (180 mm) in length, and possess a distinct small ring of melanophores on the snout. Larvae are remarkable, in that they undergo two separate growth phases: the first as vexillifer larvae that become very elongate and the second as tenuis larvae that shrink to about half their original length.
Not listed by the IUCN.
significance to humans
This species is rarely observed and is not fished commercially.
Ophidium holbrooki Putnam, 1874, Key West, Florida, United States.
other common names
Variations in squamation (scale pattern), counts of fin rays, vertebrae, and other skeletal features, and body coloration are
often useful in identifying ophidiid fishes. The band cusk-eel lacks scales on the top of the head and has 66–69 total vertebrae, 117–132 dorsal rays, 977–109 anal rays, and 19–21 pectoral rays. The dorsal and anal fins are continuous with the caudal fins. The pelvic fins, each consisting of two rays, are located far forward on the chin. The head and the body are tan in color, with no mottled patterns, blotches, or bands of pigment. The dorsal and anal fins are edged in black. This species is larger, attaining about 11.8 in (30 cm) in total length, and somewhat deeper bodied than other cusk-eels (genera Ophidion, Lepophidium, Otophidion, and Parophidion) found in its home range.
Along the Atlantic coast of the United States from North Carolina south to the Gulf of Mexico and extending along the coast of Brazil to Lagoa dos Patos. The species is reported to be absent from the Bahamas.
Little is known of the specific habitats of the band cusk-eel. It has been collected with research and commercial trawls on soft muddy to sandy bottoms from near shore to about 246 ft (75 m).
There have been no studies of the behavior of this species. Ophidiid fishes are bottom dwellers, and many reside in burrows dug into soft mud and sand. Observations by sumersibles suggest that some ophidiid species are nocturnal and abundant in some areas. Many, perhaps all, cusk-eels produce sound, and recent acoustic surveys have found large and vocal assemblages of ophidiid fishes in some areas. Sound production most likely is related to spawning behavior.
feeding ecology and diet
There have been no studies of the feeding habits of the band cusk-eel. Ophidiid fishes consume benthic invertebrates, primarily small crustaceans (shrimps, amphipods, mysids, and crabs) and worms. Small fishes, such as anchovies, gobies, and tonguefish, also are consumed. In turn, the band cusk-eel is preyed upon by larger fishes, especially dogfish, skates, conger eels, and flounders.
Unlike bythytids, male band cusk-eels do not have a copulatory organ, and fertilization occurs externally. The eggs and larvae have not been described, and the early life stages of this and most cusk-eel species are unknown.
Not listed by the IUCN.
significance to humans
This species is landed as by-catch in trawl fisheries for shrimps and bottom fishes and may appear in fish markets in some countries of South America. It is relatively small and has limited value in commercial markets, although its flesh is considered good.
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John E. Olney, PhD