Pleuronectiformes (Flatfishes)

(vertebrate zoology) The flatfishes, an order of actinopterygian fishes distinguished by the loss of bilateral symmetry.

(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

Resources

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.

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.

Coughenower, D., and C. Blood. "Flatout Facts About Halibut." 1997.

[Article by: Thomas A. Munroe, PhD]

One of the most distinctive orders of actinopterygian fishes, also called Heterosomata, composed of the flatfishes: halibut, plaice, flounders, soles, tonguesoles, and their allies. The striking feature of the group is the loss of bilateral symmetry (see illustration), a characteristic of almost all vertebrates. Like the ancestors of pleuronectiforms, young flatfishes are symmetrical and swim upright; early in life, however, one eye migrates across the top of the skull to lie on the same side as the other eye. This transformation is associated with deformation of the skull bones and nerves, a change in position so that the fish lies on one (the blind) side, partial or complete depigmentation of the blind surface, and sometimes modification and development of asymmetry in paired fins, dentition, squamation, visceral anatomy, and other structures. See also Actinopterygii.

Fourspot flounder (Paralichthys oblongus), of the Pleuronectiformes. 1 In. = 2.5 cm. (After G. B. Goode, Fishery Industries of the United States, sect. 1, 1884)