Table of Contents
The flatfishes are a very homogenous tribe, so different from all other fishes that no one is likely to mistake any one of them for any other sort of fish. What strikes one first is their flatness; less obvious is the fact that they do not lie on the belly but on one side, right or left. And their skull twists in the course of development so that the eye which was originally on the side that is fated to be underneath, migrates around the head, until both the eyes finally come to lie close together, on the side that is uppermost as the fish lies on bottom. But the mouth retains its original position more nearly, so that it is often described as opening sidewise. The larval flounder swims on edge like any other fish; the migration of the eye takes place shortly before the fry take to the bottom.
All of the flatfishes have a single long fin on each edge, one the dorsal and the other the anal; they also have well-developed ventral fins (at least on the eyed side) which are either on the right-hand edge or on the left-hand edge as the fish lies. Most of the Gulf of Maine species also have pectoral fins, one on the upper side as the fish lies on the bottom, the other on the lower side. The ventral fins are in front of the pectorals or in line with them; the abdominal cavity is very short, and some species are armed with a stout anal spine.
Our several flatfishes look much alike; indeed, they are often confused. But it is not difficult to tell one from another, for the distinctive characters are rather precise, even if not obvious at first glance. Huntsman has published a very useful key to the eastern Canadian species, which is expanded here to cover the Gulf of Maine.
|KEY TO GULF OF MAINE FLATFISHES|
|1.||Eyes on the left-hand side, and guts at left-hand edge, as the fish lies on bottom||2|
|Eyes on the right-hand side, and guts at right-hand edge||5|
|2.||The lateral line is straight||—||Citharichthys arctifrons, p. 294|
|The lateral line is arched over the pectoral fin||3|
|3.||The two ventrals are not alike, the left (upper) being continuous with the anal fin, the right (lower) separate from it||—||Sand flounder, p. 290|
|The two ventral fins are alike||4|
|4.||The upper side is marked with four large oblong black eye-spots: there are fewer than 82 rays in the long right-hand (dorsal) fin||—||Four-spotted flounder, p. 270|
|The upper side is marked with many small spots; there are more than 84 rays in the long right-hand (dorsal) fin||—||Summer flounder, p. 267|
|5.||There is a well-developed pectoral fin on the eyed side||6|
|There are no pectoral fins||—||Hog choker, p. 296|
|6.||Mouth large, gaping back as far as the eye; jaws and teeth nearly equally developed on both sides||7|
|Mouth small, not gaping back as far as the eye; the jaws are nearly straight on the upper side, but curved on the lower side||9|
|7.||Margin of tail fin rounded||—||American dab, or plaice, p. 259|
|Margin of tail fin slightly concave, with angular corners||8|
|8.||Lateral line arched close behind the gill opening||—||Halibut, p. 249|
|Lateral line nearly straight||—||Greenland halibut, p. 258|
|9.||Lower side of head with large open mucous pits; 100 or more rays in the long left-hand (dorsal) fin||—||Witch, p. 285|
|Lower side of head lacks open mucous pits; fewer than 90 rays in the long left-hand (dorsal) fin||10|
|10.||Lateral line arched behind the gill opening||—||Yellow-tail, p. 271|
|Lateral line nearly straight||11|
|11.||Top of the head between the eyes rough with scales||—||Winter flounder (including the Georges Bank flounder) p. 276|
|Top of the head between the eyes naked and smooth||—||Smooth flounder, p. 283|
 In all the flounders of this type so far recorded from the Gulf of Maine both of the pectoral fins are well developed. Should one be taken with no pectoral fin on the blind side it would probably be the deep-water Monolene sessilicauda.
 Our Eastern Flat Fishes, Canadian Fisherman, vol. 5, No. 6, 1918, pp. 788-790.