[Jordan and Evermann, 1896-1900, p. 577.]
The presence of an adipose fin between the dorsal and caudal fins, together with luminous organs, distinguishes the pearlsides from all other fishes that occur regularly in the Gulf of Maine. It agrees in both these respects with the lanternfish (p. 143) and with the headlightfish (p. 142), but it has a much smaller mouth and a longer adipose fin than the first of these, and it lacks the large luminous patch on the snout that is so striking a feature of the second. Also, the pearlsides, with its herring-like coloration (p. 88) differs strikingly from the lanternfish, which has a black back overlaid with silver; and probably the headlight fish as well.
The pearlsides is a flat-sided, large-headed little fish, its body (about one-fifth as deep as long, excluding caudal fin) deepest forward of the ventral and dorsal fins; its eye very large; its lower jaw projecting; its mouth oblique; and both its jaws armed with minute teeth. The dorsal fin (about 11 or 12 rays) stands above the space between the ventrals and the anal; the anal is longer than the dorsal. The adipose fin (both of Woods Hole and of Norwegian examples) is low and long, much as it is in the capelin. The caudal fin is broad and slightly forked.
The pearlsides has been described as without scales, but this is not correct, for both Scandinavian and Woods Hole specimens have been found to be clothed with large but extremely thin transparent scales. There is no definite lateral line.
The most interesting and diagnostic feature of the pearlsides is the presence of a series of luminescent dots situated as follows: First, 12 pairs along the belly between the pectoral and the ventral fins, followed by 5 or 6 from the ventral fins to the anal fin, and, after a gap, by 24 or 25 between the center of the anal fin and the base of the caudal fin, all these together form a practically continuous row on each side of the belly from throat to tail. Second, there is a row of larger spots a little higher up on each side, 6 from chin to pectoral fin, and 9 thence backward to the ventrals. Third, there is a group of 6 low down on each side of the cheek and throat; there is likewise a spot in front of the base of each pectoral fin and 2 on the chin.
The pearlsides is colored much like a herring, with dark bluish or greenish back and lustrous silvery-white sides and belly. The luminescent spots are described as black rimmed, their centers as pale blue in life but turning yellow in alcohol; and there is a narrow black band along the base of the anal fin and from there to the base of the caudal, the latter being barred with a similar black band.
Only 1 to 2½ inches long.
The relatives of the pearlsides are oceanic, living in the mid-depths mostly below 150 fathoms, but the pearlsides itself has been found so often in the stomachs of cod and of herring (fish that do not descend to any great depth) that there is no reason to regard it as a "deep-sea" stray, nor has it ever been taken far from land so far as we can learn. It probably spawns in early spring, females with large eggs having been taken in Scottish waters in winter.
The pearlsides (there are several other species closely allied to it) ranges widely in the open Atlantic, occurring at times in shoals on the coasts of Norway and in British waters. It is especially common off the coast of Scotland, but has not been recorded often on the American side of the Atlantic.
The known occurrences of the pearlsides in the Gulf have been few. Storer (1867) records one found alive on the beach at Nahant, Mass., in December, 1837; another taken from the stomach of a cod at [page 145] Provincetown; a third picked up alive there in July, 1865 (pictured by Storer on pl. 25, fig. 5); and five others found on the Provincetown beach soon afterward. We have seen one specimen 41 mm. long taken from the stomach of a cod, on Platts Bank, July 27, 1924; one 43 mm. long, also from a cod's stomach, on Cashes Ledge, August 16, 1928; and four, 32 to 39 mm. long, taken from the stomachs of two pollock that we caught in 20 fathoms, 7 miles southeast of Bakers Island, Mount Desert, Maine, July 24, 1930. It has been found twice at Grand Manan, and specimens were picked up on the beach at Campobello Island at the mouth of the Bay of Fundy in July 1914, while others were taken from the stomach of a pollock caught near by. It has also been recorded twice near Woods Hole.
These locality records are distributed widely enough to show that it is to be expected anywhere in our Gulf. And we suspect that the pearlsides is not as scarce there as the paucity of actual records for it might suggest (in fact, Storer tells us that a Nahant fisherman reported finding them repeatedly in the stomachs of haddock many years ago), but that it keeps out of sight, being an inhabitant of the deeper water layers as its luminescent organs would suggest, coming up to the surface chiefly at night.
 Sumner, Osburn, and Cole, Bull. U. S. Bur. Fish., vol. 31, Pt. 2, 1913, p. 743.
 Smitt, Scandinavian Fishes, vol. 1, 1892, p. 933, pl. 44, fig. 3.
 Goode and Bean (Smithsonian Contrib. Knowl., vol. 30, 1895 p. 96) describe it as "very small," but probably their specimens were damaged.
 This account is based chiefly on Smitt's description and plate, the specimens we have seen being in poor condition.
 Fishes of Mass., 1867, p. 150, as Scopelus humboldtii.
 Cox (Bull. Nat. Hist. Soc. New Brunswick, 14, 1896, append., p. 55) reported one found dead there, on the shore.
 Huntsman (Contrib. Canadian Biol., (1921) 1922, p. 61.)