Class Actinopterygii Order Perciformes Suborder Zoarcoidei Number of families 9
Photo: An ocean pout (Zoarces americanus) in the Gulf of Maine. (Photo by Andrew J. Martinez/ Photo Researchers, Inc. Reproduced by permission.)
Evolution and systematics
The zoarcoids, except for a few cryptic tidepool species, are a relatively obscure group of fishes of unverified affinity. On the basis of overall shape, fin structures, and position, and their ecological preference for living on sea bottoms in cold water, the most primitive family of zoarcoids, the ronquils, (Bathymasteridae) are thought to be close to the Antarctic "cods" or icefishes (suborder Notothenioidei). The zoarcoids and notothenioids share several important anatomical features, such as having only a single pair of nostrils, females with one ovary, lacking a gas bladder, and in having many similar skull characteristics. But they differ in other skull features and in the head sensory canal system slightly.
The time of appearance of the zoarcoids is unknown. Only the ear stones (otoliths) of an Upper Pliocene (3.2 to 1.9 million years ago) eelpout from southern California are known for the entire suborder's fossil record, and that species, Ly-codes pacificus, is extant. If the zoarcoids are indeed related to the notothenioids, then we might construct an evolutionary scenario like this: In their 1966 classification of teleost fishes, Humphry Greenwood and colleagues noted that by the Eocene (57 to 35 million years ago), or possibly earlier (the view now taken by more evidence), marine inshore fish faunas had become composed of the groups that make up today's faunas. Sea floor spreading along tectonic plate boundaries separated Antarctica from Australia at the close of the Eocene, and for about the next 15 million years Antarctica drifted to its present position. As the Drake Passage deepened a circumpolar current, the Antarctic Polar Front, developed lying between latitudes 50° and 60° south, isolating Antarctica climatically. This spurred the creation of its ice cap and frigid sea temperatures. The notothenioids subsequently diversified rapidly here. This implies that prior to the Eocene, an an cestral group common to both zoarcoids and notothenioids split into two subgroups that developed physiological adaptations to the cold. One of these radiated throughout the Northern Hemisphere (zoarcoids) and the other across the cold Southern Hemisphere (notothenioids and the more primitive sandperches). Later re-invasions also took place, such as by deep-sea eelpouts (Zoarcidae) into the Southern Hemisphere and sandperches (Pinguipedidae) into the tropics.
As of 2002, science recognizes about 325 species of zoarcoids in about 100 genera and nine families, although further taxonomic work is needed and new species are discovered almost every year. Three families, the quillfish (Ptilichthyidae), the prowfish (Zaproridae), and the graveldiver (Scytalinidae) have only one species. Four of the familes are very small, with 6-7 ronquils (Bathymasteridae), five wolffishes (Anarhichadi-dae), four wrymouths (Cryptacanthodidae) and 14 gunnels (Pholidae). Thus only two families are sizeable, with 20% of all zoarcoids in the family Stichaeidae (pricklebacks) and 70% in the Zoarcidae (eelpouts).
An analysis of the relationships within Zoarcoidei has never been published. The ronquils were seen to be the most primitive family in having no advanced features common among the other families. The gunnels and graveldiver share a common, elongate shape and have lost the pleural ribs. Pricklebacks, gunnels, and the graveldiver have characteristic elongated crania not shared by the other families except for a few advanced eelpouts. However, many of the primitive pricklebacks, particularly the subfamily Stichaeinae, have many similarities to ronquils. The prowfish and wolffishes have similar shapes, large body size, fin structures, and features of the skeleton except for the skull and may be just generally primitive zoarcoids that split off the main lineage early in the group's evolution at two different stages. Finally, the bizarre quillfish is so highly modified that its relationships are obscure. It is an extremely elongated fish (over 230 vertebrae) with many modifications of the head and fin structures and may represent another early offshoot of the main lineage that became highly modified rapidly, unlike the prowfish and wolffishes.
Two basic body shapes exist among zoarcoids. Primitively, a somewhat elongated, torpedo-like shape characterizes ron-quils, some pricklebacks, wolffishes, and the prowfish; however, most pricklebacks and the wolffish Anarrhichthys ocellatus have attained a much elongated eel-like shape. All the eelpouts, gunnels, wrymouths, graveldiver, and the quillfish are eel-like in shape. Some eelpouts, especially the genus Lycenchelys and the quillfish are extremely elongated.
Most zoarcoids are small fishes reaching lengths of less than 15.8 in (40 cm). Some eelpouts attain 23.6 in (60 cm), and the largest of these, Zoarces americanus, reaches 46.5 in (118 cm). Other "giants" among the zoarcoids are the prowfish, reaching just over 3.3 ft (1 meter), and the wolffishes Anarhichas spp., reaching 47-57 in (120-145 cm), and Anarrhichthys ocellatus, the largest zoarcoid, which reaches 80 in (203 cm).
Deep-living, bottom-dwelling zoarcoids are rather somber in color, exhibiting more or less uniform shades of gray, brown, black, or purplish. Shallow-dwelling species can be more colorful, such as the prickleback genus Chirolophis, with shades of red, orange, and yellow, the ronquil genus Rath-bunella, with its beautiful yellow and blue anal fin, and most of the gunnels, which show shades of green and brown to red reflecting the colors of the seaweeds among which they live. The dwarf wrymouth, Cryptacanthodes aleutensis, is uniformly red, and some North Pacific eelpouts, such as Andriashevia aptera and Puzanovia rubra, have also evolved this red to pinkish coloration. Sexual differences in coloration either do not exist or are usually subtle, with spawning males having dark anal fins, as in some gymneline eelpouts. However, the male prickleback, Opisthocentrus ocellatus, is normally drab grayish with dark markings but turns red with darker variegations in the spawning season.
Ranges of the various zoarcoid groups can be confusing, especially in attempting to understand the pricklebacks and eelpouts; thus it is best to examine distributions on a family basis.
Bathymasteridae (ronquils): Central Sea of Japan, around the North Pacific rim to northern Baja California, Mexico.
Zoarcidae (eelpouts): In the Pacific from the northern Yellow Sea, east around the Pacific rim to the tip of South America and across the Scotia Arc to Antarctica. Four species are known from deep water in the tropical western Pacific. Distributed throughout the Arctic, especially the genera Gym-nelus and Lycodes (Lycodes frigidus has been photographed in the Chukchi Sea at 78°28'N at a depth of 1.7 mi (2,653 m)
and is probably widespread under the polar ice cap in suitable habitats). They live in the Atlantic from the Arctic south along the eastern United States to the Gulf of Mexico and Caribbean Sea; a few species are known from the Mid-Atlantic Ridge at hydrothermal vents. They also live in the eastern Atlantic from northern Europe, along the west African coast to South Africa. A few species of the deep-water genus Pachy-cara are known from the northern Indian Ocean abyss and may be widespread there. A few species of the mesopelagic genera Lycodapus and Melanostigma are circumpolar in sub-antarctic waters but do not reach coastal Antarctica, which has an endemic eelpout fauna.
Ptilichthyidae (quillfish): Hokkaido Island, Japan, around the North Pacific rim to central Oregon, United States.
Zaproridae (prowfish): Hokkaido Island, Japan, around the North Pacific rim to San Miguel Island, California.
Anarhichadidae (wolffishes): In the Pacific from Hokkaido, Japan, to the eastern Bering Sea (Anarhichas lupus) and the eastern Bering Sea to off southern California (Anarrhichthys ocellatus). In the Atlantic (three species of Anarhichas) from Massachusetts to southern Greenland and east to the Kara Sea. The American wolffish, Anarhichas lupus, reached as far south as New Jersey in the nineteenth century.
Cryptacanthodidae (wrymouths): In the Pacific throughout the Sea of Japan north to Sakhalin Island, Russia (Cryptacanthodes bergi); northern California to the Bering Sea (C. aleutensis and C. giganteus). In the Atlantic from New Jersey to Labrador (C. maculatus).
Stichaeidae (pricklebacks): In the Pacific from the northern Yellow Sea around the North Pacific rim to Baja California, Mexico. In the Atlantic from Massachusetts to Iceland, northern Norway, Britain, and throughout the Baltic Sea. A few species enter the Canadian Arctic; Lumpenus medius is circumpolar and even reaches the Sea of Japan.
Pholidae (gunnels): In the Pacific from the Yellow Sea around the North Pacific rim to Guadalupe Island, Baja California, Mexico. In the northwestern Atlantic from Delaware to Hudson Strait and the northeastern Atlantic from southern France to the Kara Sea including Iceland and Jan Mayen Island.
Scytalinidae (graveldiver): Central California (San Luis Obispo County) north to the Aleutian Islands, Alaska.
The zoarcoids inhabit a wide variety of ecological niches and some have adapted to rather extreme habitats. At one end of this spectrum several gunnels, pricklebacks, and the grav-eldiver are found above the high tide line in rock pools with broad daily variation in temperature and salinity. Graveldivers also can be found burrowing in sand or gravel beaches above the surf wash. Most gunnels and pricklebacks are found in rocky areas with growths of seaweeds or invertebrate colonies, where they keep well hidden by day. In the opposite extreme many eelpouts are abyssal, or found on the bottom of the
world's ocean basins, at depths of more than 4,375 yd (4,000 m). The deepest catch of a zoarcoid is that for a specimen of the eelpout Lycenchelys antarctica, trawled at 5,818 yd (5,320 m) in the Peru-Chile Trench by Russian researchers. Most eelpouts inhabit continental slopes of the North Pacific and North Atlantic, where bottom temperatures are below 44.6°F (7°C). Two eelpout genera, Lycodapus and Melanostigma, have adapted to a deep-water, free-swimming habit (in the mesopelagic zone), but have been photographed near the bottom. The ronquils are generally found in rocky reefs from inshore to about 301 yd (275 m). Nowhere very abundant, they are mostly seen by scuba divers in calm waters either in the rocks or over sandy patches. The wrymouths are mud bur-rowers and may be more active at night than in the day. The three Pacific wrymouths are found only in moderately deep water 33-383 yd (30-350 m), but the Atlantic species may be found in intertidal areas. The Atlantic wolffishes (genus Anarhichas) occur in rocky areas, never over muddy bottoms, generally on the outer shelf 109-328 yd (100-300 m), although the American species, A. lupus, has been seen in tide-pools. The maximum depth is around 503 yd (460 m), although the three Atlantic species generally occur shallower to the north of their ranges. The Pacific wolffishes, Anarhichas orientalis and Anarrhichthys ocellatus, also occur in rocky reefs usually not deeper than 164 yd (150 m). All the wolffishes seem to lurk about in crevices and under overhangs when not foraging, sometimes in more open areas. The prowfish mostly occurs over soft bottoms of the outer shelf, but juveniles have been collected as shallow as 11 yd (10 m). One prowfish was caught in a trawl at 738 yd (675 m). Juvenile prowfish often occur in midwater where they may linger. The adult quill-fish's habitat is probably free-swimming just above the bottom in outer shelf waters by day then migrating into surface waters at night to feed. One researcher speculated that it may burrow in mud, but without direct evidence.
Most zoarcoids are cryptic and solitary throughout their lives, but may congregate in shelters or around food sources temporarily. Some deep-sea eelpouts have been photographed gathering around mammal carcasses and baited traps to feed on the amphipods that devour the carcass or bait. Some gunnels are known to share rock crevices for shelter. Social organization is not well documented in these hard-to-observe fishes. What behavioral patterns are known exist mostly as notes in the scientific literature from aquarium or scuba observations focusing mainly on reproduction. Nest guardianship has been noted for a few ronquils, pricklebacks, gunnels, eelpouts, and the wolf-eel, and this is discussed in the next section. During the winter in higher latitudes intertidal zoar-coids migrate into deeper water to avoid freezing, but populations are very localized and no great migrations occur. Thus, population movements are solely through larval dispersal by nearshore currents. Territoriality, as far as known, is probably weak among established adults of nearshore zoarcoids, since many share close spaces, but nothing is known of this in the deeper-living, cryptic species.
The vast majority of zoarcoids are grazing predators of small invertebrates such as worms, crustaceans, molluscs, and echinoderms. Many switch prey items by season or as they age, but a significant number are dietary specialists, a situation often localized for a particular species. No in-depth study of the biology of the ronquils has been undertaken. Postlarvae (0.2-0.4 in [5-11 mm] in length) of Ronquilus jordani in southern British Columbia had been feeding near the surface on crustaceans (Cladocera, Copepoda, Cirrepedia larvae), clam larvae, and polychaete worms. The stripedfin ronquil, Rathbunella hy-poplecta, is known to feed on the bottom primarily on crustaceans, but fish in one sample had fed exclusively on sea slugs (nudibranchs). A nest-guarding male R. hypoplecta had cannibalized part of his brood. Adult ronquils, then, are probably all opportunistic bottom grazers. The diets and feeding habits of a few eelpouts have been studied. Bottom-living species rely on crustaceans, particularly amphipods, but also eat smaller fishes, sea snails, clams, other kinds of crustaceans and, to a lesser extent, brittle stars and polychaete worms. The diets of the mesopelagic eelpouts Lycodapus mandibularis and Melanos-tigma pammelas off California diversify with increasing size but mainly consist of a few kinds of crustaceans. Young prickle-backs of several species in the water column off British Columbia feed on copepods and clam larvae. As they grow and settle into bottom habitats, a dietary shift occurs and most adult pricklebacks then rely on polychaetes, amphipods, sea snails, nudibranchs, various kinds of algae, bryozoans, shrimps, crabs, and sponges. Although this appears to be a rather diverse diet when several species are examined from different areas, many of the xiphisterine pricklebacks are herbivores, eating only a few kinds of red and green algae. Atlantic prick-lebacks have been recorded with similar diets: amphipods, copepods, ostracods, brittle stars, clams, sea cucumbers (Holothuria) and polychaetes. Gunnels, living among rocks and seaweeds with pricklebacks, feed on the same prey. The Atlantic butterfish, Pholis gunnellus, eats various mollusks,
especially sea snails, crustaceans, and polychaetes, and has been known to raid other fishes' nests to consume their eggs. In the Canadian Pacific, Pholis ornata and Pholis laeta are known to switch diets with age. Both mostly rely on harpacticoid cope-pods when young, but large fish shift mostly to caprellid am-phipods (P. laeta) or clam siphons (P. ornata). Young wrymouths in the Canadian Pacific also rely on copepods when feeding in midwater. The diets of adults are not well recorded, but food recorded for the Atlantic wrymouth consists of various crustaceans, mollusks, and fish. The wolffishes all have canine teeth in the front of their jaws and viselike molars in the back, dental adaptations for dealing with hard-shelled prey. The Atlantic wolffishes prey on mollusks like whelks, cockles, clams, and mussels as well as large crabs, hermit crabs, starfish, and sea urchins, the latter rather formidable as food. The Pacific wolf-eel, Anarrhichthys ocellatus, eats mostly commercial crabs (Cancer magister) and other hard-shelled invertebrates. Fish rarely enter the diets of wolffishes. Not much is known of the food habits of the prowfish, graveldiver, or quillfish. Young prowfish are free-swimming and are known to associate with jellyfish. Remnants of amphipods that also associate with jellyfish, as well as jelly fragments, have been found in stomachs of large prowfish. Thus they may utilize various jel-lyfishes as a food source, at least in part, throughout their life. Graveldivers are burrowers and probably hunt tiny interstitial invertebrates. The quillfish, with its small, upturned mouth and free-swimming habits, probably preys on a variety of small planktonic crustaceans and mollusc larvae.
Because nearshore zoarcoids are generally small, cryptic fishes and offshore species usually live beyond scuba depths, almost all observations on the reproductive biology of these fishes come from aquarium observations on a few nearshore species. A few zoarcoids captured from scientific research vessels have been examined in some detail, but generally little about their reproduction is known.
Direct observations on courtship are few, and fertilization is internal in all known bottom-living species. Males of the elongate zoarcoids (eelpouts, gunnels, pricklebacks, wolf-eel) wrap around receptive females snake-like and fertilization is internal, or eggs are fertilized as they are laid in clusters. The pectoral fins in courting male viviparous eelpouts (Zoarces vi-viparus) turn bright red and males assume a characteristic coiled position with the fins outstretched. When females are receptive males then assume a transverse position under and alongside the female for mating. Pelagic eelpouts like Lyco-dapus may have little formal courtship behavior and probably just pair up during a protracted spawning season and release eggs and milt into the water column. The northern ronquil (Ronquilus jordani) has a rather long courtship with the male displaying to the female with body and fin quivering and fanning. It is generally assumed that most of the nearshore zoar-coids spawn during daylight hours when they can see one another for courting, but the Japanese tidepool gunnel (Pholis nebulosa) courts and spawns from about midnight to dawn.
Egg size and number vary greatly in zoarcoids, owing to the variation in adult body size. Reported sizes range from about 0.1-0.3 in (1-7 mm), but the eggs of the quillfish, grav-eldiver, and prowfish are unknown. Fecundity ranges from less than 20 eggs per female in some primitive eelpouts (Gym-nelinae) to about 50,000 in one wolffish. Eggs can be clear, white, yellowish, or orange with oil droplets usually yellow, orange, or red, and most are adhesive since they are laid in nests. Incubation times range from about two weeks to three months. Yolk sac larvae of bottom-living zoarcoids generally stay near the bottom, but some become planktonic for anywhere from a few seconds to minutes (Zoarces americanus), or up to two years (Anarrhichthys ocellatus).
Nests are generally simple hideaways among most zoar-coids. Ronquils lay loose egg masses on flat surfaces, which are fanned and usually guarded by the male. In the stripedfin ronquil (Rathbunella hypoplecta), there is an aquarium observation of a female laying eggs six times every two weeks for a total of about 10,000 eggs during the three months. A new clutch was laid just days after the most recent spawn hatched, and the male guarded the egg mass. Bottom-living eelpouts excavate a crater in muddy bottoms and coil around a clump of eggs. However, the Atlantic midwater eelpout (Melanos-tigma atlanticum) is known to burrow tunnels into soft muddy bottoms with a male and female spawning, but sometimes several apparently non-spawning individuals may occupy the tunnels to help circulate water in this anoxic environment. Gunnels and pricklebacks lay eggs on seaweeds (Zostera, some algae) or in nests in rock crevices and holes; some gunnels are known to use empty oyster or mussel shells. Wolffishes build nests in rocky shelters or lay their eggs among seaweeds and stones.
Parental care is fairly limited in most zoarcoids that are known for this behavior. The male of the smooth ronquil noted above sometimes ate some of the eggs he was guarding. Guardianship of nests may be limited to fanning with charging displays against intruders (ronquils) or of parents holing up with the eggs with no overt signs of care simply because the nest area provides good cover (gunnels and prick-lebacks).
The spawning season of zoarcoids can be short, just a few months between courtship and egg hatching, or protracted into the better part of a year. Some deep-sea eelpouts spawn only once or twice at the end of their lives, and seasonality may not exist for these. Ronquils and many gunnels and pricklebacks spawn from the spring to summer; other gunnels and prickle-backs are autumn to winter spawners. The North Atlantic wolffishes spawn from spring to winter, and the more northerly populations start earlier in the year than southern populations. The northwestern Atlantic wrymouth (Cryptacanthodes macula-tus) is a winter spawner, but the Japanese wrymouth (Cryptacanthodes bergi) spawns from spring to summer. On the basis of larval captures in scientific collecting nets, quillfish spawn at least in spring but probably have a longer season. Nothing is known of the spawning habits of the prowfish or graveldiver.
Most nearshore zoarcoids are fairly common in their preferred habitats. This includes most ronquils, gunnels, prick-lebacks, shallow-dwelling eelpouts, and wolffishes where they're not fished. Other species are rare, and this is a result of either naturally low population densities or that they are rarely caught or seen, such as the graveldiver, prowfish, and quillfish. No zoarcoids have been considered endangered or threatened, mostly owing to these fishes' usually cryptic habits and limited commercial use.
The zoarcoids are a fairly insignificant group of fishes in human history. No mythology or significant art or literature exists, but it is interesting to note that in the Middle Ages it was commonly held that the European viviparous eelpout (Zoarces viviparus) was said to birth all eels (order Anguilli-formes), owing to its live-bearing mode of reproduction. Significant fisheries do not exist for any zoarcoid today. However, after World War II a minor longline fishery for wolffishes (Anarhichas) developed across the North Atlantic, but catches have declined dramatically since. An unsuccessful attempt was made to develop a fishery for the American ocean pout (Zoarces americanus) after World War II, but natural parasitic infections in the flesh quickly killed the plan. No zoarcoid poses any real danger to humans except wolffishes. Trawl-caught Anarhichas species have been known to snap at fishermen and a large, speared wolf-eel (Anarrhichthys ocellatus), because of its strength and imposing teeth, may inflict serious bites to careless scuba divers.
1. Ocean pout (Zoarces americanus); 2. Saddleback gunnel (Pholis ornata); 3. Wolf-eel (Anarrhichthys ocellatus); 4. Black prickleback (Xiphister atropurpureus); 5. Wrymouth (Cryptacanthodes maculatus). (Illustration by Marguette Dongvillo)
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