pleuronectiform

▪ fish order

Introduction

also called  flatfish,  

      any member of the order Pleuronectiformes, which includes about 600 species of bony, oval-shaped, flattened fishes such as the flounder, halibut, and turbot. The pleuronectiforms are unique among fishes in being asymmetrical. They are strongly compressed, with both eyes on one side, while other fishes and vertebrates in general are bilaterally symmetrical. The asymmetry is believed to have evolved from a generalized, symmetrical percoid (sea bass) body pattern in a fish that habitually rested on its side. Larval flatfishes have an eye on each side of the head, but during a period of rapid body change (metamorphosis) one eye migrates to the other side of the head, after which the larvae settle to the bottom. Osteological changes resulting from the eye migration are responsible for the asymmetry in the flatfish skull.

General features

      Flatfishes of the family Pleuronectidae are commercially important (commercial fishing) in northern waters, and members of other families are taken in limited quantities. Some Bothidae and Soleidae (soles (sole)) are exploited in tropical and temperate waters, but no other flatfishes are utilized to the extent that Pleuronectidae are.

      Flatfishes are primarily found in temperate and tropical seas, with some species extending northward into the Arctic. Sizes range from about 100 millimetres (four inches) to the large Atlantic halibut, which attains a length of more than two metres (nearly seven feet) and a weight of about 325 kilograms (716 pounds). Most species are marine, but some spend all or part of their life in freshwater. Flatfishes are found in depths up to 1,000 metres (3,300 feet), but most occur on the continental shelf in less than 200 metres of water.

Natural history

Reproduction

      Flatfishes generally spawn offshore, but some spawn in estuaries. Fecundity is high, females generally releasing at least several hundred thousand eggs (large female halibut have between 2,500,000 and 3,000,000 eggs). The eggs are small and float freely (pelagic) or sink to the botton (demersal), with or without oil globules. Newly hatched larvae are 1.5 to three millimetres long (approximately ¹/₁₆ to ¹/₈ inch). Active feeding begins shortly after hatching, and mortality of newly hatched larvae is extremely high. Larvae drift with currents (planktonic) until metamorphosis, or shortly afterward, and then settle to the bottom to assume their adult bottom-living (benthic (benthos)) existence. Swimming is accomplished by undulating movements of body and fins, rather than by sculling with the caudal fin as in most other fish.

Feeding behaviour

      Flatfishes lie on the bottom, generally covered by sand or mud, with only their eyes protruding. The eyes can be raised or lowered and moved independently. Flounders feed primarily on crustaceans, other bottom invertebrates, and small fish. When feeding they remain motionless until their prey ventures too close and then literally leap off the bottom in pursuit. Flatfishes in turn fall prey to a variety of large fish and cetaceans (whales, porpoises, etc.), but the primary predator of flatfishes is man.

Form and function

      Many species display sexual dimorphism, with the male having one or several of the following characteristics: elongated pectoral-fin rays, wider interorbital bones, spines on the head, tentacles on the eyes, more elaborate pigmentation. Flounders have a long dorsal fin extending from the head to caudal (tail) fin and an anal fin extending from vent (anus) to caudal fin in most species. Pectoral fins are present on all larval flatfishes but are lost or reduced in adults of the families Soleidae and Cynoglossidae. Caudal-fin rays and their supporting structures are variable. Scales are ctenoid (rough edged) or cycloid (smooth). Dentition is variable and corresponds to feeding habits of the species. Active predators have large, well-developed teeth in both jaws, whereas those living primarily in mud and feeding on bottom invertebrates have teeth only on the lower jaw of the blind side. Sexes are easily distinguished because the ovaries extend posteriorly from the body cavity beneath the skin and a thin layer of muscle immediately above the muscles of the anal-fin rays. Testes cannot be seen without dissection. The stomach and intestines curl within the body cavity to form a loop.

      The main feature of metamorphosis is the migration of the eye around or through the head. This is accomplished as a movement either over the middorsal ridge or through the head, in a depression between the supraorbital bars (over the eye) and ventral edge of the dorsal fin. The supraorbital bars extend forward from the cranium to the ethmoid region of the skull (the area in front of the eye), gradually shifting ventrally and coming to lie next to one another. As the eye migration begins, the dorsal edge of the supraorbital bar is reabsorbed to make room for the eye moving through the head. The supraorbital bars ossify and become the interorbital bone after the eye has completed its migration. The blind- (bottom-) side frontal bone shifts to the ocular (upper) side and forms a portion of the optic-capsule floor for the upper eye. Torsion (twisting) of the frontals, ethmoid, and mouthparts is the essential feature of the flatfish skull.

      Normal pigmentation on adult flatfishes consists of a coloured ocular side and an unpigmented (white) blind side. The ocular side is variable in pigment pattern and intensity. Flatfishes can mimic their background by assuming a similar coloration. Partial or complete albinism is known in some species, but a more common colour variation is ambicoloration (coloration on both sides). Ambicoloration can be partial or complete and is often associated with incomplete migration of the eye (in which the migrating eye stops on middorsal ridge) and a hooked appearance, caused by the unattached origin of the dorsal fin. Reversal (eyes and pigmentation on the side that normally is unpigmented) is fairly frequent in some species but quite rare in others.

Classification

      Flatfishes are divisible into three suborders and seven families.

Order Pleuronectiformes (Heterosomata of some authors)

 Allied to Perciformes but asymmetrical, compressed, both eyes on one side of head; pelvic bones attached directly to cleithrum. Swim bladder absent in adults. Fossil records for this group of fish are limited, extending from Paleocene to Recent, about 65,000,000 years.

      Suborder Psettodoidei

 The least specialized (most primitive) flatfish. Spines present in dorsal, anal, and pelvic fins; dorsal fin not extending onto head; eyes on either right (dextral) or left (sinistral) side; maxillary (upper-jaw) bone with well-developed supplemental bone; vertebrae 24–25 (10 precaudal, 14–15 caudal).

      Family Psettodidae (primitive flatfishes)

 Same characters as given for the suborder. Two species, one from Indo-Pacific and one from Africa; attain a length of about 0.6 m (2 ft).

      Suborder Pleuronectoidei

 No spines in fins (one spine in pelvic fin of Citharidae); dorsal fin extending forward onto head; usually no supplemental bone on maxillary (may be present or absent in Citharidae); vertebrae 27–70 (generally numbering 34 or more); preopercular margin free; lower jaw prominent; nostrils asymmetrical (that on blind side being near edge of head).

      Family Citharidae

 Eyes either dextral or sinistral; anus on ocular side; gill membranes widely separated; dorsal- and anal-fin rays not shortened posteriorly. Five monotypic genera found in the Indo-Pacific and Mediterranean and off Africa and Japan; attain a length to about 30 cm (about 1 ft).

      Family Scophthalmidae

 Eyes sinistral; anus on blind side; gill membrane widely separated; dorsal- and anal-fin rays shortened posteriorly; pelvic-fin bases long (both extending forward onto the urohyal). Four genera and 10 species found in North Atlatic and Mediterranean Sea; attain lengths to about 1 m (3 ft 3 in.) and weights to about 23 kg (approximately 50 lb).

      Family Bothidae (left-eyed flounders)

 Eyes sinistral; anus generally far up on blind side; gill membranes connected; dorsal- and anal-fin rays shortened posteriorly; pelvic-fin bases on ocular side short, on blind side may be short or long, 6-fin rays in all but 1 species. Thirty genera with about 200 species, found primarily in the tropical and temperate seas of the world; generally small.

      Family Pleuronectidae (right-eyed flounders)

 Eyes dextral; anus on blind side, commonly on or near midline; gill membranes connected; dorsal- and anal-fin rays shortened posteriorly; pelvic-fin bases of ocular side short or long, on blind side short, 3–13 pelvic-fin rays. Forty-three genera with about 100 species, found primarily in northern and Arctic seas, but some occur in tropical and temperate seas.

      Suborder Soleoidei

 Same as Pleuronectoidei, but preopercular margin not free (covered by skin and scales); lower jaw not prominent; nostrils nearly symmetrical, that on blind side not near edge of head.

      Family Soleidae (soles)

 Eyes small, dextral; mouth curved downward; sensory or tactile papilla on head (primarily on blind side); caudal fin with numerous rays. Over 100 species found in tropic and temperate seas; many true soles are small and found along the shore, somes species inhabiting freshwater.

      Family Cynoglossidae (tongue soles)

 Eyes small, sinistral; head large; mouth curved downward, head lacking sensory or tactile papilla; dorsal and anal fins confluent with caudal fin; caudal fin with only about 12 fin rays; generally small, slender fishes, found in tropical seas.

Elmer J. Gutherz

Additional Reading

J.T. Cunningham, A Treatise on the Common Sole, Solea vulgaris, Considered Both as an Organism and as a Commodity (1890), a monograph on the morphology, development, life history, and economics of this species, including a taxonomic review; C.L. Hubbs, “Phylogenetic Position of the Citharidae, a Family of Flatfish,” Misc. Publs. Mus. Zool. Univ. Mich., no. 63, pp. 5–38 (1945), presents a definition of the family Citharidae and phylogenetic relationships of the flatfish families; H.M. Kyle, “Flat-fishes (Heterosomata),” Rep. Danish Exped. Medit. 2:1–150 (1913), a major treatise on the development of some flatfishes in the families Bothidae, Soleidae, and Cynoglossidae (illustrations of larvae included when known); and “The Asymmetry, Metamorphosis and Origin of Flatfishes,” Phil. Trans. R. Soc., Series B, 211:75–129 (1921), a major paper on the origins of pleuronectiform metamorphosis; J.R. Norman, A Systematic Monograph of the Flatfishes (Heterosomata), vol. 1, Psettodidae, Bothidae, Pleuronectidae (1934, reprinted 1966), a classic on the taxonomy of flatfishes, the only paper dealing with these families on a worldwide basis; B. Bennet Rae, The Lemon Sole (1978).

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