Pleuronectiformes

(Flatfishes)

Class Actinopterygii

Order Pleuronectiformes

Number of families Approximately 13

Evolution and systematics

The flatfish body plan, with its spectacular morphological specializations, has had a long and successful presence among marine teleost fish assemblages dating back at least to the Tertiary, more than 50 million years ago (mya). The oldest flatfish fossils are otoliths dating from early Eocene times (53–57 mya). Eobothus minimus, a representative of the bothoid lineage with uncertain affinities within the group, is the oldest known skeleton representative of the Pleuronectiformes, dating at least to the Lutetian (some 45 mya) in the Eocene. The oldest soles, Eobuglossus eocenicus and Turahbuglossus cuvillieri, both known from single specimens from the Upper Lutetian (Eocene) of Egypt, are early flatfish fossils that appear nearly identical to skeletons of recent soleids. The earliest bothid and pleuronectid fossils also are surprisingly "modern-looking" species dating to the Eocene. The appearance of representatives of different flatfish families in fossil deposits dating to about the same time period indicates that diversification of many of the major lineages of flatfishes took place in the distant past, earlier than 45 mya. The nearly simultaneous appearance of flatfish fossils representing different lineages and encompassing nearly all of the structural features and diversity of the order also may indicate that diversification of these lineages occurred suddenly.

It also is evident from these early fossils that anatomical specializations of flatfishes, including asymmetry of the skull, supracranial extension of the dorsal fin, and modifications of the caudal skeleton, occurred earlier than the period to which these fossil flatfishes belong. When flatfishes evolved and how rapidly they diversified are unresolved questions. Flatfish fossils are unknown from true freshwater sediments, which may indicate that the ancestor of this group was a marine fish. Because fossil flatfishes are relatively rare, our knowledge concerning their evolutionary history is still very incomplete.

Flatfishes have unique morphological specializations related to their asymmetry. Although earlier hypotheses proposed that flatfishes share a common ancestor with some as yet unidentified perciform group of symmetrical fishes, the origin and sister group of flatfishes are unknown. Interrelationships among flatfishes are not well resolved, and work continues toward understanding the evolutionary relationships of these interesting fishes. The order Pleuronectiformes is monophyletic, based on the presence of three derived characters: the ontogenetic migration of one of the eyes; the anterior position of the dorsal fin origin (overlapping the cranium); and the presence of a recessus orbitalis, a muscular, sac-like evagination in the membranous wall of the orbit that can be filled with fluid, causing protrusion of the eyes to a higher position above the surface of the head (and above the bottom when the fish is buried).

Many groups of flatfishes that traditionally were recognized as families and subfamilies do not seem to represent monophyletic groups. The lack of detailed phylogenetic studies for several pleuronectoid groups hinders understanding of the interrelationships of flatfishes even at the family level. Ongoing research using both morphological and molecular approaches is expected to provide interesting results on such interrelationships of pleuronectiform taxa. Changes in our understanding of higher relationships among these fishes can be expected as additional information is discovered.

Two major lineages of flatfishes are recognized: the Psettoidei, made up of the family Psettodidae, and the Pleuronectoidei, containing all remaining flatfish groups. The Psettodids, or spiny-flounders, are a basal group of flatfishes hypothesized to be the sister group of the Pleuronectoidei. This suborder has one family with two species of Psettodes. These are relatively large flatfishes that do not feature strong

morphological asymmetries, as found in other flatfishes; there are both dextral and sinistral individuals in populations. These fishes are recognized easily by the posterior location of the dorsal fin, which does not advance onto the cranium anterior to the eyes; by spines in the dorsal and anal fins; by large mouths with specialized teeth; and by nearly rounded bodies without the obvious bilateral asymmetry in the lateral musculature that is evident in other flatfishes.

The Pleuronectoidei contains all of the more familiar flatfishes. At present, 13 families of pleuronectoid flatfishes are recognized, with Tephrinectes also representing a distinct lineage of uncertain status within the order. Phylogenetic relationships of some families and subfamilies and the monophyly of others (e.g., Paralichthyidae) are uncertain and in need of further study. Family groups within this suborder are the Citharidae, Scophthalmidae, Bothidae, Paralichthyidae, Pleuronectidae, Paralichthodidae, Poecilopsettidae, Rhombosoleidae, Achiropsettidae, Samaridae, Achiridae, Soleidae, and Cynoglossidae.

Physical characteristics

Flatfishes are deep-bodied, laterally compressed fishes that are easily and immediately recognizable anatomically, in that juveniles and adults (post-metamorphic individuals) have both eyes on the same side of the head. All flatfishes begin life as pelagic, bilaterally symmetrical fishes with an eye on each side of the head. During larval development, however, flatfishes undergo a spectacular ontogenetic metamorphosis, during which one eye migrates from one side of the head to the other, so that both eyes come to be present on the same side of the head. Depending upon the species, either the right or the left eye migrates. In relatively few species, eye migration is indeterminate,

but in most species eye migration is genetically fixed. The eyes may or may not come to lie in close proximity to each other when eye migration is completed.

Further deviations from a bilaterally symmetrical body plan occur in various external and internal structures, including placement of nostrils in the head, differential development of osteological features (especially bones in the anterior head skeleton), differences in jaw shape and dentition on either side of the body, degree of development of lateral body musculature, lateral line development and scale type on different sides of the body, differential coloration on ocular and blind sides, and differences in paired fin development on ocular and blind sides of the body. As a group, flatfishes are unique among fishes in their asymmetry, and they are noteworthy in that only they, among vertebrates, deviate so radically from a bilaterally symmetrical body plan.

Body shapes vary widely, ranging from nearly round, oval, and rhomboid to elongate and sometimes tapering to a sharp point. They may be either thick-bodied or thin-bodied, with or without a well-defined caudal peduncle. Flatfishes span a size range of about three orders of magnitude, from diminutive species, such as tonguefishes (Symphurus), which are sexually mature at a standard length (SL) of 0.98–1.6 in (2.5–4.0 cm), to giant species of halibuts (Hippoglossus stenolepis and H. hippoglossus), which reach nearly 6.6 ft (2 m) in total length and may weigh well over 661 lb (300 kg). Average total lengths of adults of most flatfish species are about 11.8 in (30 cm) or less.

Some flatfishes possess remarkable abilities to change the color and color patterns of their ocular surfaces to match the colors and patterns of the backgrounds on which they lie. Flatfishes typically exhibit distinct asymmetrical differences in pigmentation, with the ocular side of the head and body uniformly

whitish to dark brown or black, upon which there may be additional markings, such as ocelli, spots, crossbanding (complete and incomplete), or longitudinal or wavy stripes. Ocelli, spots, and crossbands may be fixed in number and position and may be useful for identification of some species. In species with strong asymmetrical coloration, the blind side of the head and body is conspicuously paler than the ocular side, typically uniformly whitish to pale yellowish. Flatfishes without strong asymmetrical pigmentation usually have blind sides that are darkly pigmented, sometimes as intensely pigmented as the ocular side; in others, although the blind sides are distinctly pigmented, they are less so than the ocular surface.

Except for the spiny flounders (Psettodidae), flatfishes typically lack spines in their fins. All of the fin rays are soft. Flatfishes have a single, long dorsal fin, whose origin is located in an anterior position overlapping the cranium. The single anal fin is also long and extends along most of the ventral side of the body from a point just behind the anus nearly to, or sometimes connecting to, the caudal fin. Most flatfishes typically have paired pectoral and pelvic fins, but in some groups these fins are reduced or lost. In addition, most flatfishes have a lateral line, at least on the ocular side, and most also have a lateral line on the blind side. Adult flatfishes also lack a swim bladder (though it is present in larvae).

Distribution

Species of flatfishes have nearly global occurrence in marine habitats, ranging from Arctic and boreal marine waters to Antarctic and southern waters. They are distributed broadly throughout the world's temperate marine zones and are especially speciose in marine habitats in tropical regions. Within all regions, flatfishes are most diverse where extensive continental shelves with complex habitats are located in shallow water. The widest diversity of flatfish species occurs in the Indo-West Pacific region.

Habitat

Flatfishes occur nearly globally in marine habitats and occupy diverse bathymetric environments, ranging from shal low-water to deep-water habitats to about 6,560 ft (2,000 m). Relatively few species inhabit freshwater environments. The greatest diversity of flatfishes, about 74% of the known species, is found in habitats ranging from near shore to depths of about 328 ft (100 m) on the continental shelf.

The majority of flatfishes occur in shallow marine waters, in coastal areas and estuaries, and on the inner continental shelf, where there are soft-sediment bottom types. Flatfishes also occur on a variety of sediments on the outer continental shelf and upper continental slope. Flatfishes can be found on intertidal flats following the tide line to deeper habitats. In tropical waters, flatfishes inhabit shallow mangrove estuaries and adjacent mud flats, sea grass beds, and mud bottoms along the coast. Within reef-associated habitats, which are distributed widely across tropical oceans, flatfishes are found on reef flats, on back-reef slope areas and in lagoons associated with reefs, and around coral outcrops, as well as on sandy substrates interspersed around reef spurs. Flatfishes inhabit various sediments, including silt, mud, sand, and sand-shell mixtures, with some species also occurring on rocky or pebbly bottoms.

Behavior

Flatfishes generally lie on the bottom on their blind side. They can be found either on top of the sediments or partially buried under a fine layer of sand or silt with only their eyes protruding above the sediments. Many flatfishes are stationary for long periods of time. When swimming above the bottom, they use a "pleuronectiform" swimming mode, in which waves of muscle contraction are passed along the body, beginning in the anterior region and continuing posteriorly. Most species can utilize a more rapid escape response, where the caudal fin is brought into play, creating a powerful and speedy swimming response.

Surprisingly little is known concerning the social organization of most flatfish species. Flatfishes are non-schooling species; many occur as solitary individuals, but a few and perhaps many individuals may congregate in a general area. Males of some species may display aggressive behavior to one another during the mating season. The majority of flatfish species are diurnally active. Some species are active throughout the daytime, whereas others have peak activity at or around sunrise and sunset. Nocturnal activity is a major adaptation evident in the Soleidae, Achiridae, and Cynoglossidae.

Extensive migration patterns have been well documented for some commercially important species of northern temperate flatfishes, such as the plaice, summer flounder, and halibuts. For most flatfishes, in particular the many tropical species, little is known concerning their movements or migrations. Small, reef-associated species probably have limited home ranges and do not engage in any seasonal migrations.

Feeding ecology and diet

Flatfishes are extremely successful in conducting life on or near the bottom, where they function in pivotal ecological roles as both predator and prey. Flatfish diets include such prey as shrimps, decapod and other crustaceans, mollusks, polychaetes, and many other types of small invertebrates, as well as echinoderms, fishes, and cephalopods. Small-mouthed species, especially tonguefishes (Cynoglossidae), achirid soles (Achiridae), and true soles (Soleidae), feed on a broad spectrum of smaller epifaunal and infaunal organisms.

Halibuts, larger species of bothid and paralichthyid flounders, larger pleuronectids, and the larger scophthalmids are active predators that consume fishes, larger and more active crustaceans (shrimps, lobsters, crabs), and cephalopods (squids and octopuses). The halibuts, with their great size and swimming abilities, actively pursue and chase down their prey, whereas other large flatfishes generally are ambush predators that lie on the bottom or partially buried within the sediment, concealed by their camouflage coloration and awaiting unsuspecting prey to approach within striking distance.

All life stages of flatfishes are eaten by predators that include both invertebrates and vertebrates. While in the plankton, eggs and larvae are consumed by jellyfishes, ctenophores, arrow worms, mysid shrimps, and fishes. Young, newly settled flatfishes are attacked and consumed by crabs, shrimps, and fishes. Juvenile and adult flatfishes fall prey to a wide variety of predatory fishes, including cods, hakes, sculpins, rock-fishes, striped bass, other flatfishes (sometimes their own species), monkfish, bluefish, cobia, groupers, moray eels, sea ravens, large skates, stingrays, and various sharks, as well as birds (egrets, herons, cormorants, gulls), seals, and sea lions.

Reproductive biology

The reproductive behavior of most flatfishes is not known. Direct observations of courtship and mating have not been made for the majority of flatfish species. The sexes are separate, and individuals usually do not change sex during their lifetimes. Flatfishes have external fertilization. Where reproductive behavior has been observed, individual males and females may pair up during courtship and spawning. Sometimes the mating pair is joined by other males.

Most flatfishes spawn planktonic eggs that float freely in the water column. Some pleuronectid flatfishes, such as the winter flounder, lay eggs that are demersal and adhesive, such that after the female releases them, they remain on the bottom and stick to each other and to other items. Even among species with demersal, adhesive eggs, flatfishes do not construct nests during spawning, nor do they exhibit any type of parental care. Upon hatching, flatfish larvae are planktonic and usually are found in the water column far above the bottom. Larval stages vary in duration from a few days to a couple of months; the duration of larval stages is influenced greatly by ambient water temperatures. Following eye migration and metamorphosis, young flatfish settle out of the water column and assume a benthic lifestyle, with many species utilizing shallow-water habitats as nursery areas.

A strong seasonality in reproductive period has been noted for most temperate and boreal flatfishes, with most species having one spawning season per year. The timing of spawning seasons within the year varies by species and also by latitude. Some species spawn during periods of seasonally high temperatures, whereas others spawn during wintertime. Some warm temperate

species may have two spawning periods per year; for tropical and subtropical species, spawning periods may extend over several months. Spawning among temperate marine species corresponds to annual productivity cycles that are related to temperatures and photoperiods. In some tropical regions, spawning by some flatfishes also seems to correspond to seasonal monsoons, which influence productivity cycles.

Conservation status

Two pleuronectids are cited by the IUCN—the Atlantic halibut, which is listed as Endangered, and the yellowtail flounder, which is listed as Vulnerable. Overfishing is primarily responsible for reductions in many flatfish populations, especially for large, commercially important species. Throughout the world, stocks of commercially important flatfishes are considered to be fully exploited—for many, even overexploited. Other factors contributing to reductions in populations of flatfishes include habitat destruction and pollution, especially serious situations for flatfishes that utilize estuaries and other coastal habitats, such as sea grass meadows and mangrove forests, as nursery habitats.

Significance to humans

Flatfishes are an important group of food fishes. Medium-size and large species of most families are consumed wherever they are captured, and in some regions even the smallest flatfishes also are sold as food for people. In such regions as the North Atlantic and North Pacific Oceans and also in Southern Hemisphere regions, such as Australia, New Zealand, and South America, flatfish populations are sufficiently large to constitute major fishery resources. Some of the smaller flatfishes, especially those taken as by-catch in shrimp trawl fisheries (tonguefishes, soleids, achirids, bothids, and paralichthyids), are considered to be a nuisance by fishermen because they so firmly entangle themselves in the nets that they cannot easily be shaken out. Clearing the nets after heavy catches of these flatfishes, especially tonguefishes, soleids, and achirids, requires manually extracting fishes from the nets, a time-consuming task.

Species accounts

List of Species

Hogchoker

Trinectes maculatus

family

Achiridae

taxonomy

Pleuronectes maculatus Bloch and Schneider, 1801, Tranquebar, India (in error).

other common names

French: Sole bavoche; German: Amerikanische Seezunge.

physical characteristics

Small, oval, dextral flatfish that have a deep and thick body without a definite caudal peduncle. The dorsal and anal fins are free from the caudal fin, and the right pelvic fin is joined to the anal fin. A relatively small head, with the snout slightly overhanging the small, subterminal mouth. The eyes are small, flat, and separated by a small space. The dorsal fin originates at the tip of the snout. No pectoral fins. The lateral line is straight. The skin is very slimy with mucus. Scales are ctenoid and very rough on both sides of the body. The ocular side is slate-olive to dark brown, with numerous conspicuous, darker transverse crossbands. There is also a longitudinal stripe along the midregion and, occasionally, a number of darker, diffuse blotches scattered over the surface. The blind side is dirty white; some specimens have numerous irregularly rounded spots, varying in both size and number, scattered over the blind side. This species reaches lengths of about 7.9 in (20 cm), with most averaging about 2.4–5.9 in (6–15 cm). They live for about seven years. Females grow larger and live longer than males.

distribution

Western North Atlantic in marine, estuarine, and freshwaters along the Atlantic coast of North America from Maine to the Gulf of Mexico.

habitat

They occur most commonly on mud, sand, or silt bottoms in coastal bays and estuaries with brackish water. In larger estuaries, young (small) fish tend to be found in upper reaches of estuaries, sometimes at considerable distances upstream into freshwater portions of coastal rivers. Fish size generally increases with increasing distances down estuary. The largest hogchokers usually are found in the lower estuary and also on the inner continental shelf to about 82 ft (25 m) and rarely to about 246 ft (75 m). Able to withstand a considerable range of temperatures of about 34–95.2°F (1.1–35.1°C). Euryhaline (able to withstand a range of salinities), ranging in salinity from freshwater to about 50 ppt. Can tolerate low oxygen conditions for periods up to 10 days. Will move out of areas with extremely low oxygen levels.

behavior

Under laboratory conditions, hogchokers were active only during the dark period, with peak activities associated with times of slack tide in the natural habitat. Under continuous dim light, activity peaks coincided with slack tide, and fish were active in the diurnal as well as the nocturnal phase of the cycle.

feeding ecology and diet

Opportunistic, nocturnal feeders that eat a variety of small invertebrate prey, including amphipods, clam siphons, annelid worms, copepods, and small fishes. These fishes tend to macerate their food. Hogchokers are consumed by a variety of predators, including bull sharks, sandbar sharks, smooth dogfish, stingrays, striped bass, weakfish, bluefish, and cobia.

reproductive biology

Mature at two to four years old and at sizes as small as about 2 in (5 cm). Probably a serial spawner. Annual fecundity has not been estimated for this species. Batch fecundity varies with fish size. Small females, about 3.5 in (9 cm), produce about 11,000 eggs, and larger females, 4.3–6.3 in SL (11–16 cm SL), produce from 23,000 to 54,000 eggs. The spawning season is April to October, but eggs have been reported as early as January, and in the southern Gulf of Mexico spawning may occur year-round. Spawning takes place in estuaries between 6 p. m. and 10 p. m., when water temperatures reach 68–77°F (20–25°C). Hogchoker eggs are pelagic in high-salinity waters and demersal in lower-salinity waters. Hatching occurs about one to two days after spawning. Eye migration begins at about 0.2 in (5 mm), 34 days after hatching, and is completed when fish are about 0.7 in (18 mm).

conservation status

Not threatened.

significance to humans

Edible but noncommercial species of no interest to fisheries, owing to their small size. They sometimes are captured and sold to hobbyists in the aquarium fish trade.

Peacock flounder

Bothus lunatus

family

Bothidae

taxonomy

Pleuronectes lunatus Linnaeus, 1758, Bahamas.

other common names

French: Rombou lune; Spanish: Lenguado ocelado.

physical characteristics

Sinistral flatfish with an oval and moderately deep body. Rounded to bluntly pointed caudal fin. The dorsal profile of the snout has a distinct notch above the nostrils, and there is a stout spine on the snout of adult males (a bony knob in females). Eyes are relatively large, with the lower eye distinctly anterior to the upper and with a broad interorbital space that is conspicuously wider in males. The moderately large and oblique mouth extends slightly beyond a vertical line through the anterior margin of the lower eye. Jaws have an irregular double row of small teeth. Ocular side upper pectoral fin rays are conspicuously elongate in males. Scales are ctenoid on the ocular side and cycloid on the blind side. The lateral line has a steep arch above the pectoral fin. Ocular side is grayish brown,

with numerous bright blue rings and rosettes covering the entire ocular side and with two to three large blackish spots on the straight portion of the lateral line. Larger individuals also have dark transverse bands on the ocular side pectoral fin. Maximum lengths to about 17.7 in (45 cm), with most individuals about 13.8 in (35 cm) long.

distribution

Marine coastal waters of the tropical and subtropical western Atlantic, including Bermuda, the Bahamas, Florida, throughout the Caribbean, and south to Fernando de Noronha, Brazil.

habitat

Shallow waters from the shore to about 213 ft (65 m). Found chiefly on sandy bottoms, often within or near coral reefs, and also in sea grass and mangrove habitats.

behavior

Diurnally active. Often observed resting on the sandy bottom, sometimes partially buried in the sand. Occasional specimens are also observed resting on top of small coral reef tops. When swimming, they glide along just above the bottom using wavelike motions. Peacock flounder can change colors rapidly to blend in with the background.

feeding ecology and diet

Visually feeding, ambush predator that eats primarily small fishes but also consumes crustaceans and octopuses. They often lie in wait on sand patches adjacent to reef areas to intercept small fishes undertaking crepuscular migrations between reef and sea grass habitats. Lizard fishes, snappers, groupers, and various sharks and stingrays eat peacock flounders.

reproductive biology

Off Bonaire in December, peacock flounder spawning took place just before sunset, with elaborate spawning behavior observed for mating pairs. Males and females would approach each other with pectoral fins held erect to initiate courtship activity. The male, with its ocular side pectoral fin held erect, first paralleled the female as they swam above the substrate. The male then positioned himself underneath the female; with their snouts touching and the male's body arched backward, the pair began a slow rise (about 15 seconds) of about 6.6 ft (2m) off the bottom, when they simultaneously released their gametes and rapidly returned to the bottom.

conservation status

Probably not threatened, but population status is unknown throughout most of its distribution. Because of its size and food qualities, this species could be susceptible to local overfishing.

significance to humans

Peacock flounder are caught incidentally in artisanal fisheries throughout their range.

California tonguefish

Symphurus atricaudus

family

Cynoglossidae

taxonomy

Aphoristia atricauda Jordan and Gilbert, 1880, San Diego Bay, California.

other common names

French: Langue californienne; Spanish: Lengua de perra.

physical characteristics

Small, sinistral flatfish with the characteristic tonguefish teardrop-shaped body terminating posteriorly in a point without a distinct caudal fin. The small head has a pointed snout. The small eyes are set close together. A small, subterminal mouth

with small teeth best developed on the jaws of the blind side. Dorsal and anal fins are conjoined with the caudal fin, forming one continuous fin around nearly the entire body. Lacks a lateral line on either side of the body, lacks pectoral fins in adults, and has a pelvic fin only on the ocular side. Ocular side is uniformly medium to dark brown, with a series of complete or incomplete darker crossbands and with the posterior fifth of the body much darker than the anterior regions. Blind side is uniformly whitish or yellowish. Reach lengths to about 8.3 in (21 cm), but most are smaller, usually averaging only about 5 in (13 cm). Little is known concerning longevity, growth rates, or population structure of this species.

distribution

Inner continental shelf of the eastern Pacific from Washington to the Pacific side of Baja California Sur and along the western shore of Sonora and Sinaloa, Mexico.

habitat

Sand or mud bottoms at depths ranging from 9.8 to 328 ft (3–100m), with most adults taken between 98.4 and 262.5 ft (30–80 m). Juveniles tend to inhabit shallower waters than do adults.

behavior

Little is known. Probably nocturnally active and also active at other periods of low-light levels. During the daytime it remains partially or totally buried in the sediment, except for the anterior head region.

feeding ecology and diet

Consume a variety of small benthic invertebrates, including harpacticoid copepods, amphipods, ostracods, nematodes, small bivalve mollusks, and polychaetes. Predators of California tonguefish include sharks, electric rays, stingrays, and various bony fishes.

reproductive biology

Little is known. They spawn planktonic eggs from June to September; larvae hatch at about 0.08 in (2 mm). Larvae transform between 0.7 and 1 in (19 and 25 mm) in length and settle to the bottom during late fall and winter. Probably a serial spawner, producing several batches of eggs during a protracted spawning season.

conservation status

Not threatened.

significance to humans

Of little commercial value, owing to its small size.

Pacific sanddab

Citharichthys sordidus

family

Paralichthyidae

taxonomy

Psettichthys sordidus Girard, 1854, San Francisco, California.

other common names

English: Mottled sanddab, soft flounder, melgrim; Spanish: Lenguado arenero del Pacifico, lenguado.

physical characteristics

Medium-sized, sinistral flatfishes with an elongate to oval body; large head with large terminal mouth; and slightly rounded, almost square caudal fin. Eyes are large, with the lower eye in advance of the upper eye and separated from it by a sharp, naked bony ridge. Pectoral fins are large and pointed. Lateral line is nearly straight. Scales are ctenoid on the ocular side of the body and cycloid on the blind side. Ocular side is a dull light brown, mottled with brown or black and sometimes yellow to orange speckles or white spots. Blind side is off-white to tan. Maximum lengths of about 16.1 in (41 cm) and weights up to about 2 lb (0.9 kg), but most are much smaller, only 4.9–5.6 oz (140–160 g). They live to be at least eight years of age.

distribution

Marine waters in the northern Pacific Ocean from the Sea of Japan to the Bering Sea and the Aleutian Islands south to Cape San Lucas, Baja California.

habitat

Adults inhabit gravel, sand, or mud-sand bottoms at 16.4–1,801 ft (5–549 m) but are most abundant at 164–492 ft (50–150 m); they rarely occur below 984 ft (300 m). Juveniles occur at shallower depths than those occupied by adults and sometimes move into tide pools.

behavior

Diurnally active. Spend much of their time on the bottom, although occasionally they are captured at night up in the water column.

feeding ecology and diet

Opportunistic, visually oriented predators that feed principally on pelagic crustaceans, such as euphausiids, shrimps, crab larvae, calanoid copepods and amphipods, and occasionally small fishes and benthic prey, among them, annelid worms and crustaceans. Pacific sanddabs are consumed by a variety of larger predators, including blue sharks and other sharks, stingrays, and halibut, and also have appeared in the diets of Guadalupe fur seals.

reproductive biology

Begin maturing between ages two and three years. Spawning takes place on or near the bottom from July to September off California. Eggs are released independently, are buoyant, and are fertilized outside the female. Females may spawn more than once during the same spawning season.

conservation status

Not threatened.

significance to humans

Excellent food fish. Regarded as a delicacy in California but with less appeal elsewhere.

Summer flounder

Paralichthys dentatus

family

Paralichthyidae

taxonomy

Pleuronectes dentatus Linnaeus, 1766, Carolina, United States.

other common names

English: Fluke; French: Cardeau d'été; Spanish: Falso halibut del Canadá.

physical characteristics

Large, sinistral flatfish with a narrow, relatively elongate and thick body. Prominent head with a large terminal mouth with a wide gape and strong canine teeth on both jaws. A rounded or doubly emarginate caudal fin. Eyes are relatively large, separated, and nearly equal in position. Lateral line is arched above the pectoral fin. Scales are small and cycloid, with secondary squamation. Varies in coloration, as individuals change color to match the background of their habitat. Ocular side coloration ranges from drab olive-green to brown to gray; the blind side usually is white. Individuals captured on white sand are nearly completely white, whereas others on dark sediments can be nearly black. Some individuals have pink, green, orange, or brown coloration on the ocular side. The ocular side is marked variously with irregular spots, often with a series of more or less distinct ocelli that are slightly darker than the background coloration, with the most posterior of these arranged in a double triangle (one above and one below the lateral line). Males grow to about 24 in (61 cm) in length, with a weight of 5.7 lb (2.6 kg), and females grow to about 37 in (94 cm) in length, with a weight of 29.5 lb (13.4 kg). Most adults are 15.7–22 in (40–56 cm) in length and weigh between 2.2 and 5.1 lb (1.0–2.3 kg).

distribution

Western North Atlantic in estuarine and continental shelf waters of eastern North America from Nova Scotia to Florida.

habitat

Inshore waters, including estuaries and even freshwater (juveniles), bays, harbors, and the inner continental shelf. Summer flounder prefer sandy or muddy bottoms, where they often are found in sand patches near and within eelgrass beds or around pier pilings. Sensitive to low oxygen concentrations and will move out of hypoxic areas (less than 3 ppm oxygen). Concentrated inshore during warmer periods of the year, with smaller fish found in very shallow water. Larger fish in the northern part of the range occur farther offshore, usually at depths of 230–509 ft (70–155 m) and deeper, even during the summer. Estuaries are important nursery areas for this species. Young summer flounder can withstand a wide range of temperatures and salinity levels and are well adapted for estuarine life. Juveniles remain in estuaries until their second year of life in the southern part of their range, but to the north they move just outside them during winter. Many young fish return to the same estuary during their second summer.

behavior

Spend most of their lives on or close to the bottom. Occur most often on top of the sediment and do not ever bury deeply. Juveniles may be active at night, but adults appear to be most active during the daytime. Adults undertake strong seasonal inshore-offshore movements, especially in northern regions of the species' geographic range. Adults and juveniles are concentrated in shallow coastal and estuarine waters during the warmer months of the year and remain offshore in deeper waters (to about 492 ft, or 150 m) in the fall and winter, presumably to escape cold winter temperatures. Seasonal movements of summer flounder are complicated and are affected by fish size, location, and stock. Some may spend the winter in deeper bays and channels, and older fish may remain permanently on the outer shelf.

feeding ecology and diet

Diurnally active, opportunistic ambush predators. They feed while on the bottom but also rise off the bottom in pursuit of smaller fishes. The primary food of summer flounder is bony fishes, but cephalopods also are important prey of fish larger than 12.2 in (31 cm). Crustaceans, especially mysids and decapods, are important prey for smaller fishes (those fish less than 8.3 in, or 21 cm). Juveniles (3.9–7.9 in, or 10–20 cm) consume mysids, fishes, amphipods, and crabs. Feeding is most active during warmer periods and slows in winter. Spiny dogfish, blue sharks, skates, codfish goosefish, sea robins, bluefish, and winter flounder prey on summer flounder during various stages of their life history.

reproductive biology

Male and female summer flounder mature at about 9.8 in (25 cm) and 11 in (28 cm), respectively, corresponding to ages two and a half years for females and two years for males. Many fish may reach maturity at one year. Spawning occurs on or near the bottom, where temperatures range from 53.6 to 66.2°F (12–19°C), and usually takes place during the autumn migration to offshore wintering grounds on the outer continental shelf. Large females (more than 26.8 in, or 68 cm) may produce in excess of four million eggs during a spawning season. Females are serial spawners, continuously producing egg batches that are shed over a period of several months (September to March). Larvae are transported to coastal areas during winter and early spring by prevailing water currents. Post-larvae and young juveniles are found in or near the mouths of estuaries. Metamorphosis is completed within bays and estuaries, where young fish settle to the bottom.

conservation status

Not threatened. Exploitation of summer flounder by commercial and recreational fishers resulted in stock reductions throughout the species' range during the latter quarter of the past century. Fishery management plans have been developed to conserve and rebuild stocks by limiting commercial and recreational catches through size and season restrictions.

significance to humans

Summer flounder support the most important commercial and recreational fishery for flatfishes along the Atlantic coast of the United States. This highly prized game fish has strong fighting qualities and is excellent table fare.

Pacific halibut

Hippoglossus stenolepis

family

Pleuronectidae

taxonomy

Hippoglossus stenolepis Schmidt, 1904, Aniva Bay, south Sakhalin Island, Sea of Okhotsk.

other common names

English: Northern halibut, right halibut; French: Flétan du Pacifique; Spanish: Fletán del Pacifico.

physical characteristics

One of the largest species of flatfishes and among the largest of bony fishes. Typically dextral flatfishes, with a thick, sturdy, elongate, and diamond-shaped body and a highly compressed caudal peduncle with a crescent-shaped caudal fin that often is indented near the edges. The head is large, with a large terminal mouth featuring a wide gape. Nearly symmetrical jaws containing two rows of prominent, conical teeth on the upper jaw and a single row of conical teeth on the lower jaw. The eyes are large, and the upper eye is slightly in advance of the lower one. The lateral line has a high arch above the pectoral fin. Small cycloid scales cover both sides of the body. Ocular side coloration is greenish brown to dark brown or black, marbled with lighter blotches; the blind side usually is white to milky white, sometimes also with blotches. Females reach lengths up to 8 ft, 9 in (267 cm), and weights of about 498 lb (226 kg); males are about 4 ft, 7 in (140 cm) and 220 lb (100 kg). Females grow considerably faster and typically live longer than do males. Almost all halibut larger than 100 lb (45.5 kg) are females. Halibut first become available to the offshore fishery at about five to seven years of age. The oldest recorded age for a male is 55 years, and the oldest recorded age for a female is 42 years.

distribution

Marine waters of the eastern and western North Pacific Ocean. In the west, they are found from the Sea of Japan and Okhotsk Sea north to the Gulf of Anadyr and Chukchi Sea and throughout the Bering Sea. In the eastern Pacific, this species ranges from the Gulf of Alaska southward to about Santa Barbara, California, and, rarely, southward to Point Chamalu, northern Baja California.

habitat

Occurs on a variety of bottom types at depths from about 19.7 to 3,609 ft (6–1,100 m) but most commonly found between 180 and 1,385 ft (55–422 m). In summer they are found between 92 and 902 ft (28–275 m) and sometimes shallower, whereas most halibut occur in deeper waters during winter. They have a preferred temperature range of 37.4–46.4°F (3–8°C).

behavior

Diurnally active fishes found most often on the bottom. They often rise off the bottom into the water column and may at times even come close to the surface when pursuing prey. Seasonal movements of adults are associated with winter reproduction offshore and summer feeding inshore. Adult halibut move seasonally from deep water to the edge of the continental shelf and then to shallower banks and coastal waters during the summer; they move back to deep water in the winter. Migrations may be extensive—the longest migration on record was that of a fish tagged near Atka Island in the Aleutian Islands, which was recaptured at Coos Bay, Oregon, a distance of 2,500 mi (4023 km).

feeding ecology and diet

Large, powerful, opportunistic, visual feeder that consumes a wide variety of prey, including fishes, crabs, clams, squids, and other invertebrates. Small halibut eat a variety of benthic prey items and small fishes, with prey size increasing with fish length. Larger halibut consume almost anything they can catch, with fishes constituting a major portion of their diet. They also feed on squid, octopus, and diverse benthic and nektonic fishes, including cods, pollock, rockfishes, sculpins, other flatfishes, and occasionally smaller halibuts. Halibut are eaten by marine mammals, perhaps some sharks, and other halibuts, but because of their large size, they are rarely found as prey for other fish species.

reproductive biology

This species spawns in deep water, 902–1,352 ft (275–412 m), at the edge of the continental shelf during winter (November to March). The Gulf of Alaska is an important spawning area. On average, females mature at 12 years of age (range, eight to 16 years), and males mature by about age eight. The number of eggs produced increases with the size of the female. Large females (those more than 250 lb, or 113.4 kg) can produce as many as two million to four million eggs annually. The eggs are buoyant and fertilized externally. Larvae hatch in about 15 days, depending on water temperature; they remain pelagic for four to five months after spawning. Eggs may be encountered anywhere between 131 and 3,068 ft (40–935 m) but are concentrated between 328 and 656 ft (100–200 m). Newly hatched larvae (0.3–0.6 in, or 8–15 mm) usually are found deeper than 656 ft (200 m). Eye migration begins at a length of about 0.7 in (18 mm). By 1.2 in (30 mm) the young fish resemble adults. With growth, young fish rise in the water and are found predominately at about 328 ft (100 m) by three to five months of age. They are transported great distances and move shoreward with currents, where they settle to the bottom at about six to seven months. After about two years, juveniles begin to move into deeper water. Fishes ages two to four years occur primarily at 361 ft (110 m) or shallower, but some at this size also have been taken at depths of 597 ft (182 m) and occasionally deeper.

conservation status

Not threatened. Exploitation has resulted in stock reductions of this species throughout its range. Commercial and recreational halibut fisheries are highly regulated, with size and seasonal limitations employed to attempt to keep stocks from diminishing further or disappearing altogether.

significance to humans

Largest and most commercially important flatfish in the North Pacific Ocean. Excellent table fare highly prized by consumers. A commercial fishery for halibut has existed for longer than 100 years.

Plaice

Pleuronectes platessa

family

Pleuronectidae

taxonomy

Pleuronectes platessa Linnaeus, 1758, European seas.

other common names

English: European plaice; French: Plie d'Europe, carrelet; German: Scholle; Spanish: Solla europea.

physical characteristics

Dextral flatfish with a deep, oval body; a relatively large head; large eyes; a small mouth; and a series of four to seven bony knobs on the head along a curved line from the eyes back to the lateral line. Teeth are best developed on the jaws of the blind side. Strong, molariform pharyngeal teeth are present on the gill arches. Lateral line is curved slightly above the pectoral fin. Scales are cycloid on both sides of the body. Distinctive ocular side coloration, consisting of a uniformly brown background with brilliant red or orange spots. Blind side usually is

uniformly white. The species can reach lengths to about 39.4 in (100 cm) and weights up to 7.9 lb (3.6 kg), but most adults average only about 13.8–19.7 in (35–50 cm) and weights of about 2.2 lb (1 kg). Individuals can reach at least 30 years of age, but most are much younger. Females grow faster and live longer than males, which rarely live longer than 10–12 years.

distribution

Northeastern Atlantic Ocean in marine and sometimes estuarine waters from the White and Barents Seas southward to the North Sea, including the British Isles and western Baltic Sea; off Iceland; occasionally off Greenland; and along the European coast from Germany and Denmark south to Spain and Portugal and in the western Mediterranean Sea.

habitat

Usually found on the inner continental shelf from shallow waters to about 656 ft (200 m) but most abundant in 33–164 ft (10–50 m); usually occur in waters of 35.6–59°F (2–15°C). Most commonly found on sandy sediments but also occur on mud or gravel bottoms. Newly settled plaice recruit to inshore waters typically between 9.8 and 88.6 ft (3–27 m), and sometimes juveniles are found in sandy intertidal pools. Plaice can tolerate reduced salinity levels but do not usually penetrate estuaries to any great degree and are not typically found in freshwater reaches within estuaries. During their first year, young plaice generally are found in shallow waters. By their second year, they begin to move to deeper waters. Older and larger plaice tend to live deeper than smaller and younger plaice.

behavior

A diurnally active, benthic fish that lies partially buried when possible. Plaice remain stationary for long periods of time, lying partially buried in the sediment. They are often active at night, especially in shallow water, and have been reported to exhibit homing behavior, at least in near-shore environments. Where there are tidal currents, the plaice orientates itself by pointing into the current and retains its position by pressing its dorsal and anal fins against the bottom. Many individuals congregate in the same general area. Plaice, including larger individuals, sometimes move on rising tides into intertidal areas to forage, retreating with the receding tide.

feeding ecology and diet

Opportunistic, visual predators that feed mainly during daylight hours. After metamorphosis, juvenile plaice consume small polychaete worms and harpacticoid copepods, but with increasing size their diet broadens to include a wider variety of prey, such as small crustaceans, amphipods, cumaceans, and small mollusks. Larger plaice consume a greater proportion of thin-shelled bivalve mollusks, especially the siphons of burrowing species (which they nip off using the teeth of their blind-side jaws), as well as gastropod mollusks, shrimps, small crabs, and various polychaete worms. Feeding activity varies with season (temperature), with higher feeding rates occurring during warmer periods than during wintertime. The plaice takes its food in a nearly horizontal position, with its head raised slightly off the bottom. Predators that consume plaice include sculpins, lumpfish, spiny dogfish, weaver fish, seals, and cormorants. Shrimps and ctenophores prey on plaice in the early-life stages.

reproductive biology

Maturity schedules vary, depending upon the area where the fish live, their food supply, and ambient temperatures. Female plaice reach sexual maturity between three and seven years of age (11.8–15.7 in, or 30–40 cm) and males at two to six years of age (7.9–11.8 in, or 20–30 cm) in the North Sea. Plaice spawn throughout their range, usually on well-defined spawning grounds. The spawning season varies with latitude and location but usually occurs in the early months of the year throughout its range (December to March in the North Sea, February to March off Denmark, and March to April off Iceland), when water temperatures are about 42.8°F (6°C). Mature fish may undertake extensive migrations from feeding grounds to discrete spawning grounds. The extent of migration depends upon the individual stocks. Spawning grounds generally are located at depths of 66–131 ft (20–40 m). Males and females may pair up and swim one above the other during spawning. Plaice do not guard a nest but rather scatter eggs, which may number up to 50,000 during a spawning event. Eggs are planktonic at first, gradually sinking as development proceeds. Hatching occurs in 18–21 days, depending on temperature. The larval stage lasts between four and six weeks, after which the fish metamorphose at about 0.4–0.7 in (10–17 mm) in length.

conservation status

Not threatened, although stock sizes have declined over time as a result of overfishing, habitat destruction, and pollution.

significance to humans

One of the most familiar flatfishes in northern European waters, highly desired owing to its size, abundance, and edible qualities. It is the single most important commercial flatfish to the fisheries of Europe. The species also is targeted by a large recreational fishery. Plaice are considered a potential species for aquaculture and are kept as an aquarium species.

Winter flounder

Pseudopleuronectes americanus

family

Pleuronectidae

taxonomy

Pleuronectes americanus Walbaum, 1792, New York.

other common names

English: Blackback, Georges Bank flounder, lemon sole, rough flounder; French: Limande-plie rouge; Spanish: Solla roja.

physical characteristics

Medium-sized, dextral flatfish with an oval and thick body with a relatively wide caudal peduncle and a broadly rounded caudal fin. The head is relatively small, with a small terminal mouth with a small gape and thick, fleshy lips. Jaws on the blind side are equipped with a series of incisor-like teeth, whereas the jaws on the ocular side are toothless or nearly so. The lateral line is nearly straight, with only a slight arch above the pectoral fin. Ocular side scales are ctenoid, whereas blind side scales are ctenoid in males and cycloid in juveniles and females. Winter flounder vary in color, depending on the bottom where they live. Larger specimens are dark muddy brown or reddish brown, olive-green, or slate-colored to almost black. On the ocular side, coloration varies from uniformly pigmented to patterned with definite flecks, spots, and darker blotches of differing hues, depending on the bottom type. The blind side usually is uniformly white and translucent with a bluish tinge toward the body margins and sometimes with yellow on the caudal peduncle. Winter flounder can attain a length of 24.8–26.4 in (63–67 cm) and weights to about 7.9 lb (3.6 kg). Winter flounder are relatively long lived, reaching a maximum age of about 15 years. After about age five, females begin to grow faster than males; they also live longer than males.

distribution

Western North Atlantic in estuarine and marine waters along the Atlantic coast of North America from Labrador south to Georgia and also on the offshore banks. Most abundant between the Gulf of Saint Lawrence and Chesapeake Bay.

habitat

Brackish waters of tidal rivers, estuaries, and river mouths to areas on the inner continental shelf. Larger and older fish tend to inhabit deeper waters than do younger, smaller fish. Typical inshore habitats consist of muddy sand, especially where the sand is broken by patches of eelgrass; clean sand; clay; and pebbly and gravelly ground. Offshore, winter flounder usually are found on hard bottom. They can survive a wide range of temperatures, from nearly the freezing point of saltwater to about 68–69.8°F (20–21°C). Their blood serum contains an antifreeze protein that helps protect them against freezing.

behavior

Diurnally active, with activity beginning at sunrise. At night they lie flat, with heads resting on the bottom and eye turrets retracted. On muddy bottoms, winter flounder usually lie buried, all but the eyes, working themselves down into the mud soon after settling on the bottom. Fish living on tidal flats typically remain motionless during low tides but actively forage during high tides. Can change color to match background surroundings, ranging from whitish on white backgrounds to dark brown or nearly black on dark sediments. Local conditions in inshore waters appear to determine inshore distribution patterns, whereas offshore movements seem to be associated with extreme summer and winter conditions. In general, in summer months adult winter flounder stay in the shallow shore zone when the water temperature is not excessive and food availability is adequate. If these conditions are not met, they may move into deeper channels or offshore or may take evasive action. During winter in the southern parts of their range, they remain or move into shallow water to spawn, whereas in northern regions they remain inshore in protected areas and move offshore in exposed areas to avoid turbulence and drifting pack ice. Winter flounder in deeper waters, such as Georges Bank, remain there year-round. Winter flounder bury in sediments when water temperatures are below 32°F (0°C) and ice crystals are present in the water. They are active at water temperatures up to 71.6°F (22°C), beyond which they become inactive. They are sensitive to low levels (3 ppm) of dissolved oxygen.

feeding ecology and diet

Visual predators. Consume a wide variety of small invertebrates and, rarely, small fishes, such as sand lance. Principal food includes polychaetes, anthozoans, and amphipods but also shrimps, small crabs and other crustaceans, ascidians, holothurians, squids, bivalve and gastropod mollusks, and sometimes fish eggs. They often bite off clam siphons that protrude from the sand. While feeding, a winter flounder lies with its head raised off the bottom with its anterior body braced vertically against the bottom. Eye turrets are extended, and the eyes move independently of each other. After sighting its prey, the fish remains stationary, pointed toward the target, and then lunges forward and downward to seize its prey. Winter flounder are eaten by codfish, spiny dogfish, goosefish, hakes, winter skate, smooth dogfish, striped bass, bluefish, sea raven, seals, ospreys, gulls, blue herons, and cormorants.

reproductive biology

In the fall, as gonads ripen, adult winter flounder remain in or move into shallow water to spawn. They spawn in winter in southern locations and in early spring (January to May) in more northern locations. Spawning in inshore waters occurs nearly at the seasonal lowest temperatures, which range from 31.1 to 35.6°F (0.5–2.0°C), depending on latitude. Spawning on Georges Bank happens at temperatures ranging from about 37.9 to 41.9°F (3.3–5.5°C). Winter flounder spawn on sandy bottom and algal mats, often in shallow water. Spawning in estuaries occurs in areas with salinity levels as low as 11.4 ppt. Males mature at age two and females at age three off New York, when fish are 7.9–9.8 in (20–25 cm) in total length. Fish in more northern areas mature later—age three years and four months for males and three years and six months for females north of Cape Cod and age six for males and seven for females in Newfoundland. Maturity seems to be a function of size rather than age. On Georges Bank, the mean age at maturity is just under two years for both sexes.

Females produce, on average, 500,000 eggs, but larger fish can produce up to 3.5 million eggs. Winter flounder migrate into shallow water or estuaries and coastal ponds to spawn, and tagging studies show that most return repeatedly to the same spawning grounds. They are batch spawners. Females in captivity spawned up to 40 times and males up to 147 times. Males initiated all observed spawning events, which occurred throughout the night but primarily between sunset and midnight. Spawning by one pair frequently elicited sudden convergence and spawning by secondary males. Strict pair spawning is uncommon. Male and female activity patterns were almost entirely nocturnal during the reproductive season but became increasingly diurnal during the post-spawning season. Eggs are fertilized outside the body and sink to the bottom, where they stick together in clusters. Incubation takes place over 15–18 days at 37–37.9°F (2.8–3.3°C). Young larvae hatch at about 0.12–0.14 in (3.0–3.5 mm). Metamorphosis is complete when larvae are only 0.31–0.35 in (8–9 mm) long. Winter flounder exhibit little movement from areas where they settle, unless seasonal temperatures become extreme.

conservation status

Not threatened, although their numbers have diminished substantially since the 1970s, and the stocks throughout most, if not all, of the species' range are considered to be overexploited. Overfishing, pollution, and habitat destruction all contribute to reductions in stock sizes of this species. Current fishery management plans are to restore stock sizes to former levels. As of 2002 little success was evident in these attempts.

significance to humans

Highly desirable commercial and recreational species. Winter flounder were the most frequently captured flatfish taken by recreational fishers along the eastern coast of the United States until about 1970. Since then, landings of fish in both commercial and recreational fisheries have declined substantially.

Windowpane flounder

Scophthalmus aquosus

family

Scophthalmidae

taxonomy

Pleuronectes aquosus Mitchill, 1815, New York, United States.

other common names

English: Sand dab; spotted flounder, brill; French: Turbot de sable.

physical characteristics

Medium-sized, sinistral flatfish characterized by a deep, nearly round, thin, and almost translucent body with a rounded caudal fin. The head is small with a relatively large mouth having a wide gape and with a projecting lower jaw that has a knob on its ventral surface. Small teeth are present on both jaws. The first 10–12 fin rays of the dorsal fin are free from the fin membrane along the distal half and branched toward their tips, forming a conspicuous fringe. Eyes are large, separated, and nearly equal in position on the head. Pelvic fin bases are elongated and slightly asymmetrical. Scales are cycloid and smooth to the touch. The lateral line is strongly arched above the pectoral fin. The ocular side is rather translucent greenish olive or slightly reddish brown or pale slate-brown, mottled with darker and paler irregular markings and usually dotted with many small, irregularly shaped brown spots and sometimes also with white spots that vary in size. Blind side is generally whitish, occasionally with some irregular darker blotches. Can reach sizes to about 15.7 in (40 cm) and weights to about 2.2 lb (1 kg), but adults typically average only 9.8–11.8 in (25–30 cm). Can live to be 15–18 years of age, but most individuals are 11 years old or younger.

distribution

Western North Atlantic in lower estuarine and marine waters on the inner continental shelf of eastern North America from the Gulf of Saint Lawrence to Florida. Most abundant from Georges Bank to Chesapeake Bay.

habitat

Shallow, inshore waters from the high tide line down to about 656 ft (200 m), with the greatest numbers occurring at depths of less than 180 ft (55 m). They occur most often on sandy bottoms but also can be found on softer and muddier sediments.

behavior

Diurnally active. Often lie on or within sandy sediments. Young windowpane flounder settle in shallow water inshore and tend to move into deeper offshore waters as they grow. Adults may undertake movements along the coast for considerable distances (80 mi, or 129 km, in three months) or even move across open water.

feeding ecology and diet

Visually oriented, ambush predators that forage on a variety of actively swimming prey, particularly mysids, various fishes, and decapod crustaceans, especially shrimp. They also eat chaetognaths, squids, mollusks, ascidians, polychaetes isopods, amphipods, euphausiids, and salps. Windowpane flounders, in turn, are eaten by various sharks, skates, stingrays, codfish, cobia, bluefish, and other windowpane flounders.

reproductive biology

Windowpane flounder of both sexes mature at about the same size, 8.3–8.7 in (21–22 cm) between ages three and four years, with males sometimes maturing at age two. Spawning occurs from February to November, with peak spawning from May through October. There is a strong correlation between water temperatures and spawning. They spawn in the evening or at night on or near the bottom at temperatures from 42.8 to 69.8°F (6–21°C), with optimal spawning temperatures of 60.8–66.2°F (16–19°C) in the Mid-Atlantic Bight and 55.4–60.8°F (13–16°C) on Georges Bank. Eggs are transparent, have an oil globule, and are buoyant. Larvae hatch at 0.07–0.09 in (1.8–2.3 mm), usually at eight days post-spawning in temperatures of 50–55.4°F (10–13°C). Eye migration begins when they reach a length of about 0.26 in (6.5 mm), proceeds very rapidly, and typically is completed by 0.39–0.51 (10–13 mm) total length.

conservation status

Not threatened.

significance to humans

Not commercially important and not targeted directly by commercial fisheries. They often are taken as by-catch during trawl fishing.

Common sole

Solea solea

family

Soleidae

taxonomy

Pleuronectes solea Linnaeus, 1758, European ocean.

other common names

English: Dover sole; French: Sole commune; German: Seezunge; Spanish: Lenguado commúne.

physical characteristics

Dextral flatfish with an elongate but rather thick body. The dorsal fin extends to the anterior part of the head, often to a point equal to a horizontal line drawn through the upper eye. Has a smoothly rounded head, projecting snout, and a small, subterminal mouth with small teeth and with the preoperculum covered by skin. On the blind side, head and snout are covered with close-set, whitish sensory papillae. Moderately large pectoral fins. On the ocular side this fin has a distinct elliptical black patch (not surrounded by white ring) on its upper extremity. On the blind side it is only slightly smaller than the ocular side counterpart. Last rays of the dorsal and anal fins are joined to the base of the caudal fin by a distinct membrane, and the last rays of the dorsal and anal fins overlap the bases of the caudal fin rays. Ocular side is uniformly dark brown or grayish brown, with numerous darker irregular blotches. Dorsal and anal fins are edged in white. Blind side is creamy white. Lengths reach about 27.6 in (70 cm) and weights about 6.6 lb (3 kg), but most fish are 11.8–15.7 in (30–40 cm) in length. They attain ages up to seven to eight years and perhaps older.

distribution

Northeastern Atlantic in estuarine and marine waters from off Norway and the western Baltic Sea and commonly off Ireland, England, and Scotland (uncommon northwest of Scotland). Also commonly along the European coast in the southern North Sea, the Mediterranean Sea, and the eastern Black Sea and southward to Senegal.

habitat

Occurs on soft sandy or muddy bottoms over a wide range of salinity levels and depths, ranging from shallow estuarine waters of 3.3 ft (1 m) or less down to about 656 ft (200 m). Estuaries are important nursery areas for newly settled fish and juveniles. Young-of-the-year soles appear in surf zone waters on shallow sandy beaches during summertime and can be taken even in shallow tide pools during this time. They occur at temperatures from 46.4 to 75.2°F (8–24°C). Adults and larger juveniles undertake seasonal migrations between shallower waters on the inner shelf (in warmer periods) to deeper areas (230–427 ft, or 70–130 m) on the outer shelf in winter. Larger fish generally are found in deeper waters than are smaller fish. Seasonal inshore migrations during springtime are complicated, because larger fish, besides moving inshore, also move toward definite spawning grounds. Young fish move into inshore waters earlier, usually during spring, whereas larger fishes move into shallower waters by late spring or early summer.

behavior

Benthic fishes that live somewhat solitary lives. They spend much of the daylight hours partially buried in sediments or lying on top of them. In overcast conditions or in turbid waters, such as in estuaries, they are more active during daylight hours. Generally, they are most active at nighttime, when they sometimes are found off the bottom and up in the water column. They are found pelagically during migrations.

feeding ecology and diet

Opportunistic, nocturnal feeders that rely on chemosensory and tactile information to locate their prey. Some feeding by adults and juveniles also takes place during daylight hours when fish are active. In some estuaries, feeding activity showed a strong relationship to the tidal cycle. Juveniles may use intertidal areas as feeding grounds during flood tides. They consume a diverse array of mostly benthic prey, including amphipods, polychaetes, oligochaetes, small bivalve mollusks and siphons of bivalve mollusks, gastropod mollusks, mysids and crangonid shrimps, brittle starfish, and, to a lesser extent, small benthic fishes, such as sand eels (Ammodytes) and gobies. Diets often vary between habitats and in terms of size of fish, with prey sizes generally increasing with increasing fish size. Soles feed on small quantities of prey very often. Predators of sole include spiny dogfish, hakes, lizardfish, codfish, weaver fish, and cormorants.

reproductive biology

Mature at three to five years of age and at sizes of about 9.1–11.8 in (23–30 cm). Spawn primarily during one well-defined season in the spring and early summer, which varies according to latitude (March to May off England and April to June farther north). Spawning grounds are located in both shallow and deep waters. Spawning takes place between 42.8 and 53.6°F (6–12°C). Females may release up to 100,000 eggs during a spawning event. Eggs are buoyant and pelagic; planktonic larvae hatch when eggs reach a length of about 0.12–0.16 in (3–4 mm). Metamorphosis occurs at about 0.47–0.59 in (12–15 mm), with settlement taking place at 0.59–0.7 in (15–18 mm) in length. Adults undertake migrations to definite spawning grounds located on the inner continental shelf at depths of 131–197 ft (40–60 m). Spawning grounds have been identified in the North Sea, the Irish Sea, and the English Channel.

conservation status

Not threatened. Stock sizes have been reduced owing to overfishing. At present, efforts have focused on limiting fishing pressure and reducing mortalities, with expectations that stock sizes will not decline further.

significance to humans

Abundant and valued food species. The most valuable fishing grounds lie in the southern and central North Sea and the Bay of Biscay. Little recreational fishing occurs, primarily because of this species' nocturnal feeding habits and also because of the difficulty in catching it.

Resources

Books

Able, Kenneth W., and Michael P. Fahay. The First Year in the Life of Estuarine Fishes in the Middle Atlantic Bight. New Brunswick, NJ: Rutgers University Press, 1998.

Bowman, Ray E., Charles E. Stillwell, William L. Michaels, and Marvin D. Grosslein. Food of Northwest Atlantic Fishes and Two Common Species of Squid. NOAA Technical Memorandum NMFS-NE-155. Woods Hole, MA: National Marine Fisheries Service, 2000.

Carpenter, K. E., and V. H. Niem, eds. FAO Species Identification Guide for Fishery Purposes. Vol. 6, The Living Marine Resources of the Western Central Pacific. Part 4, Bony Fishes: (Labridae to Latimeridae), Estuarine Crocodiles, Sea Turtles, Sea Snakes and Marine Mammals. Rome: FAO, 2001.

Chang, Sukwoo, Peter L. Berrien, Donna L. Johnson, and Wallace W. Morse. Essential Fish Habitat Source Document: Windowpane, Scophthalmus aquosus, Life History and Habitat Characteristics. NOAA Technical Memorandum NMFS-NE-137. Woods Hole, MA: National Marine Fisheries Service, 1997.

Chapleau, F., and K. Amaoka. "Flatfishes." In Encyclopedia of Fishes, edited by John R. Paxton and William N. Eschmeyer. 2nd edition. San Diego: Academic Press, 1998.

Collette, Bruce B., and Grace Klein-MacPhee, eds. Bigelow and Schroeder's Fishes of the Gulf of Maine. 3rd edition. Washington, DC: Smithsonian Institution Press, 2002.

Fischer, W., F. Krupp, W. Schneider, C. Sommer, K. E. Carpenter, and V.H. Niem, eds. Guía FAO para la Identificación de Especes para los Fines de la Pesca: Pacífico centro–oriental. Vols. 2 and 3, Vertebrados—Parte 1 and Parte 2. Rome: FAO, 1995.

Hart, J. L. Pacific Fishes of Canada. Bulletin 180. Ottawa: Fisheries Research Board of Canada, 1973.

Heemstra, P. C. "Achiropsettidae, Southern Flounders." In Fishes of the Southern Ocean, edited by O. Gon and P. C. Heemstra. Grahamstown, South Africa: J.L.B. Smith Institute of Ichthyology, 1990.

——. "Family No. 260: Pleuronectidae." In Smiths' Sea Fishes, edited by M. M. Smith and P. C. Heemstra. Johannesburg: Macmillan South Africa Publishers, 1986.

International Pacific Halibut Commission. The Pacific Halibut: Biology, Fishery, and Management. Technical Report no. 40. Seattle: International Pacific Halibut Commission, 1998.

Kramer, D. E., W. H. Barss, B. C. Paust, and B. E. Bracken. Guide to Northeast Pacific Flatfishes: Families Bothidae, Cynoglossidae, and Pleuronectidae. Marine Advisory Bulletin no. 47. Fairbanks: University of Alaska Fairbanks, Alaska Sea Grant College Program, 1995.

Lythgoe, J., and G. Lythgoe. Fishes of the Sea. Garden City, NJ: Anchor Press/Doubleday, 1975.

Mecklenburg, Catherine W., T. Anthony Mecklenburg, and Lyman K. Thorsteinson. Fishes of Alaska. Bethesda, MD: American Fisheries Society, 2002.

Nelson, J. S. Fishes of the World. 3rd edition. New York: John Wiley & Sons, 1994.

Nielsen, J. G. "Pleuronectidae." In Fishes of the North–eastern Atlantic and the Mediterranean, edited by P. J. P. Whitehead, M.-L. Bauchot, J.-C. Hureau, J. Nielsen, and E. Tortonese. Vol. 3. Paris: UNESCO, 1986.

O'Brien, Loretta, Jay Burnett, and Ralph K. Mayo. Maturation of Nineteen Species of Finfish off the Northeast Coast of the U.S., 1985–1990. NOAA Technical Report NMFS-113. Seattle: National Marine Fisheries Service, 1993.

Packer, David B., Sara J. Griesbach, Peter L. Berrien, Christine A. Zetlin, Donna L. Johnson, and Wallace W. Morse. "Essential Fish Habitat Source Document: Summer Flounder, Paralichthys dentatus, Life History and Habitat Characteristics." NOAA Technical Memorandum NMFSNE-151. Woods Hole, MA: National Marine Fisheries Service, 1998.

Rackowski, J. P., and E. K. Pikitch. "Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes I (Pacific Southwest). Pacific and Speckled Sanddabs." Biological Report 82, U. S. Fish and Wildlife Service. Washington, DC: U.S. Department of the Interior, 1989.

Scott, W. B., and M. G. Scott. Atlantic Fishes of Canada. Canadian Bulletin of Fisheries and Aquatic Sciences no. 219. Ottawa.

Schwarzhans, Werner. Piscium Catalogus: Part Otolithi Piscium. Vol. 2, A Comparative Morphological Treatise of Recent and Fossil Otoliths of the Order Pleuronectiformes. München: Verlag Dr. Friedrich Pfiel, 1999.

Wheeler, Alwyne. The Fishes of the British Isles and North–West Europe. East Lansing: Michigan State University Press, 1969.

Periodicals

Bannikov, A. F., and N. N. Parin. "The List of Marine Fishes from Cenozoic (Upper Paleocene–Middle Miocene) Localities in Southern European Russia and Adjacent Countries." Journal of Ichthyology 37, no. 2 (1997): 133–146.

Bengston, David A. "Aquaculture of Summer Flounder (Paralichthys dentatus): Status of Knowledge, Current Research and Future Research Priorities." Aquaculture 176 (1999): 39–49.

Berendzen, P. B., and W. W. Dimmick. "Phylogenetic Relationships of Pleuronectiformes Based on Molecular Evidence." Copeia 2002, no. 3 (2002): 642–652.

Brewster, B. "Eye Migration and Cranial Development During Flatfish Metamorphosis: A Reappraisal (Teleostei: Pleuronectiformes)." Journal of Fish Biology 31 (1987): 805–833.

Cabral, H. N. "Comparative Feeding Ecology of the Sympatric Solea solea and S. senegalensis, Within the Nursery Areas of the Tagus Estuary, Portugal." Journal of Fish Biology 57, no. 6 (2000): 1550–1562.

Chanet, B. "A Cladistic Reappraisal of the Fossil Flatfishes Record Consequences on the Phylogeny of the Pleuronectiformes (Osteichthyes: Teleostei)." Annales de Sciences Naturelles, Zoologie, Paris 18 (1997): 105–117.

——. "Eobuglossus eocenicus (Woodward 1910) from the Upper Lutetian of Egypt, One of the Oldest Soleids [Teleostei, Pleuronectiformi]." Neues Jahrbuch für Paläontologie Monatehefte 7 (1994): 391–398.

Chapleau, F. "Pleuronectiform Relationships: A Cladistic Reassessment." Bulletin of Marine Science 52, no. 1 (1993): 516–540.

Cooper, J. A., and F. Chapleau. "Monophyly and Intrarelationships of the Family Pleuronectidae (Pleuronectiformes), with a Revised Classification." Fishery Bulletin 96, no. 4 (1998): 686–726.

Hoshino, K. "Monophyly of the Citharidae (Pleuronectoidei: Pleuronectiformes: Teleostei) with Considerations on Pleuronectoid Phylogeny." Ichthyological Research 48, no. 4 (2001): 391–404.

Konstantinou, H., and D. C. Shen. "The Social and Reproductive Behavior of the Eyed Flounder, Bothus ocellatus, with Notes on the Spawning of Bothus lunatus and Bothus ellipticus." Environmental Biology of Fishes 44, no. 2 (1995): 311–324.

Litvak, M. K. "The Development of Winter Flounder (Pleuronectes americanus) for Aquaculture in Atlantic Canada: Current Status and Future Prospects." Aquaculture 176 (1999): 55–64.

Pearcy, W. G., and D. Hancock. "Feeding Habits of Dover Sole, Microstomus pacificus; Rex Sole, Glyptocephalus zachirus; Slender Sole, Lyopsetta exilis; and Pacific Sanddab, Citharichthys sordidus, in a Region of Diverse Sediments and Bathymetry off Oregon." Fishery Bulletin 76 (1978): 641–651.

Phelan, B. A., J. P. Manderson, A. W. Stoner, and A. J. Bejda. "Size-Related Shifts in the Habitat Associations of Young-of-the-Year Winter Flounder (Pseudopleuronectes americanus): Field Observations and Laboratory Experiments with Sediments and Prey." Journal of Experimental Marine Biology and Ecology 257 (2001): 297–315.

Ramachandran, V. S., C. W. Tyler, R. L. Gregory, D. Rogers-Ramachandran, S. Duensing, C. Pillsbury, and C. Ramachandran. "Rapid Adaptive Camouflage in Tropical Flounders." Nature (London) 379, no. 6568 (1996): 815–818.

Stoner, A. W., and A. A. Abookire. "Sediment Preferences and Size-Specific Distribution of Young-of-the-Year Pacific Halibut in an Alaska Nursery." Journal of Fish Biology 61 (2002): 540–559.

Terceiro, M. "The Summer Flounder Chronicles: Science, Politics, and Litigation, 1975–2000." Reviews in Fish Biology and Fisheries. 11 (2002): 125–168.

Other

Coughenower, D., and C. Blood. "Flatout Facts About Halibut." 1997. <http://www.uaf.edu/seagrant/Pubs_Videos/pubs/SG-ED-29.pdf>

Thomas A. Munroe, PhD