Flounder (Pleuronectes flesus), a common species of flat-fish, of wide distribution in the shallow waters round Britain and other north temperate countries. It is the Scotch 'flinke,' and the Swedish 'flundra,' and differs but a little from the plaice and dab, two of the commonest neighbour-species. Like other flat-fishes, the flounder is asymmetrical, and swims or rests on one side, almost always the left, the eye of which is in early youth brought round to the upturned surface. It measures about a foot in length, and about a third as much between the dorsal and the ventral edge, without including the fringing fins. The colour of the eye-bearing surface varies in sympathetic adaptation to that of the sandy or muddy bottom on which the fish lives. The upper side is olive-green or brownish, sometimes with yellow spots; the blind side is yellowish-white, with small, dark points. The mouth-aperture is narrow; the teeth are conical, and most developed on the blind side; the dorsal fin begins at the eye; the scales are minute and smooth, but rough tubercles occur on the side of the head and along the bases of the dorsal and anal fins.
Of the two dozen related species, the Plaice (P. platessa), the Dab (P. limanda), the Smear-dab (P. microcephalus), and the Craig-fluke (P. cynoglossus) are the commonest. In the soles (Solea) the dorsal fin begins in front of the eye, and the upper eye is slightly anterior to the lower.
The flounders spawn at the mouths of rivers, and the young are hatched in May. The fish often leave the sea and go up the rivers, ascending the Avon to near Bath, and the Moselle as far as Treves. They are even able to thrive in ponds. Sea-birds prey largely upon them, breaking the bones to make swallowing possible. Though inferior to the plaice, the flounder is an esteemed food-fish. See DAB, FLAT-FISH, PLAICE, SOLE.