They have no significant economic or cultural importance to humans. ♦
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Aldemaro Romero, PhD
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(Cusk-eels and relatives)
Class Actinopterygii Order Ophidiiformes Number of families 4 or 5
Photo: A pearlfish (Carapus bermudensis) about to enter a sea cucumber. (Photo by Chesher/ Photo Researchers, Inc. Reproduced by permission.)
Photo: A pearlfish (Carapus bermudensis) about to enter a sea cucumber. (Photo by Chesher/ Photo Researchers, Inc. Reproduced by permission.)
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 unde-scribed 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 tax-onomic limits of the order Ophidiiformes remain uncertain and require further research.
The cusk-eels and their allies are classified in the superorder Paracanthopterygii, a large and morphologically diverse assemblage that includes the freshwater trout-perch and its relatives (Percopsiformes), the codfish and its relatives (Gad-iformes), 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 Paracan-thopterygii 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 para-canthopterygian orders remain speculative, however.
Extensive morphological analysis has not produced a satisfactory hypothesis of the evolutionary history of the ophidi-iform 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, aphy-onids and bythitids are sister taxa, each possessing uniquely specialized morphological characters (termed "synapomor-phies"). Ichthyologists have been unable to specify synapo-morphies 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 supramax-illary 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 in-tromittent 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 echiodon-tines) 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 Lu-cifugia 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. Hy-
droacoustic surveys often show large assemblages of vocal cusk-eels. Ophidiiform fishes generally have a close association with the bottom, hovering near holes, ledges, and dropoffs 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.
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 cara-pids 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, vex-illifer 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 Bros-mophysis (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. Aphy-onid 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.
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. bla-codes reach 33,000 tons (30,000 metric tonnes) annually. Bro-tula barbata is a food source of growing importance to the peoples of several west African countries.
1. Key brotula (Ogilbia cayorum); 2. Pearlfish (Carapus bermudensis); 3. Band cusk-eel (Ophidion holbrooki). (Illustration by Patricia Ferrer) Grzimek's Animal Life Encyclopedia 19
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