Table of Contents
The most distinctive external characters of the lanternfishes are their large eyes (situated close to the tip of the blunt snout), wide mouths gaping back beyond the eye, one soft-rayed dorsal fin, a deeply forked tail, and the presence of a series of luminous organs as conspicuous pale spots along the sides. Some of them have an adipose fin on the back behind the dorsal fin, but others lack this. When present, this fin is so small and fragile that it is apt to be destroyed by the rough treatment the fish receive in the tow net in which they are taken. They most nearly resemble the anchovy (p. 118), the pearlsides (p. 144), and the cyclothone (p. 146) among Gulf of Maine fishes; but they are readily distinguished from the first of these by the presence of luminous organs and by the fact that the snout does not project beyond the mouth; from the second by their much wider mouths; and from the third by their much larger eyes.[page 142]
They are among the most numerous fishes on the high seas, where they live at a considerable depth by day but often rise to the surface at night. Only two species of the group, representing as many genera (Diaphus and Myctophum), have been recorded within the Gulf of Maine. But each of these genera includes a considerable number of species that are common along the continental slope abreast of the Gulf, hence are as likely to stray into the latter as are the two that have actually been found there. And this applies equally to various other genera of lanternfishes.
The species of Diaphus and of Myctophum all resemble one another in general appearance, in having a short dorsal fin, with an adipose fin behind it; a deeply forked tail; large eyes; wide, oblique mouth; and numerous luminous organs along the sides; all, too, are blackish-silvery in color. The members of each genus are separable only by differences in the arrangement of the luminous organs. Hence, positive identification of a given specimen calls for the services of a specialist in the group. Should a lanternfish be taken in the Gulf in which the arrangement of luminous organs does not agree precisely with the two described here, we suggest that it be submitted to the U. S. Fish and Wildlife Service to be named.
 Parr (Bull. Bingham Oceanog. Coll., vol. 3, art. 3, 1928), and Tåning (Vidensk. Meddel., Dansk Naturhist. Forening, vol. 86, p. 49, 1928) have recently published critical synopses of the lanternfishes.