Flat-fish

Flat-fish (Pleuronectidae), an important family of marine bony fishes, in the order Anacanthini, with the body compressed from side to side, and maximally unsymmetrical. The turbot (Rhombus maximus), the halibut (Hippoglossus vulgaris), the flounder, plaice, and other members of the genus Pleuronectes, and the soles (Solea) are well-known examples, valuable as food-fishes. There are about three dozen genera and two hundred species. They occur in all seas, and some thrive in fresh water. The adults keep to the bottom, especially on sandy coasts, and come nearer shore in the spring spawning season and during summer. They are all carnivorous. Many of them alter the colour of their exposed upper side in sympathetic and automatic adjustment to the nature of the ground on which they lie concealed. They are all destitute of a swim-bladder, but the most remarkable feature is the asymmetry of the skull, eyes, and musculature. Unlike the gristly skates, which are flattened from above downwards, the Pleuronectidae are flattened sideways, and swim or rest on one side, either right or left. The under side is virtually uncoloured, the upper side is often markedly pigmented, and bears both the eyes. The young forus, however, are striking contrasts to the adults. They are almost transparent, occur in the open sea, are quite symmetrical, with an eye as usual on either side, and swim vertically. With increasing size the normal structure and habit are lost, the fishes become asymmetrical, and keep to the bottom. While the skull is still gristly and plastic it is twisted, and the lower eye comes to lie beside its fellow on the upper surface. The exact mechanism is not yet clear. See FISHES, FLOUNDER, PLAICE, SOLE, TURBOT, &c.