Lancetfish are large oceanic predatory fish in the genus Alepisaurus (“Scaleless lizard”), the only genus in the family Alepisauridae.
They grow up to 6.56 ft (2 m) long. Very little is known about their biology, even though they are widely distributed in all oceans, except the polar seas. Specimens have been recorded as far north as Greenland. They are often caught as by-catch for vessels long-lining for tuna.
The scientific name is from Greek a- meaning “without”, lepis meaning “scale”, and sauros meaning “lizard”.
Lancetfish possess a long and very high dorsal fin, soft-rayed from end to end, with the presence of an adipose fin behind it. The dorsal fin has 41 to 44 rays, and occupies the greater length of the back, is rounded in outline, about twice as high as the fish is deep, and can be depressed in a groove along the back. The body is slender, laterally flattened, deepest at the gill covers, and tapers back to a slender caudal peduncle. The mouth is wide, gaping to the back of the eye, and each jaw has two or three large fangs, besides smaller teeth. The caudal fin is very deeply forked; its upper lobe is prolonged as a long filament, and most seem to lose this when captured. The anal fin originates under the last dorsal ray, and is deeply concave in outline. The ventral fins are about halfway between the anal and the tip of the snout, while the pectoral fins are considerably longer than the body is deep and are situated very low down on the sides. There are no scales and the fins are exceedingly fragile.
There are no commercial fisheries for Lancetfish. Their flesh is watery and gelatinous and, although edible would prove difficult to utilize. They are caught as bycatch by tuna fisheries and are seen as a pest, taking bait intended for more valuable species. Anecdotal evidence suggesting that they have an aphrodisiac effect is likely to be an urban myth.
Lancetfish have large mouths and sharp teeth, indicating a predatory mode of life. Their watery muscle is not suited to fast swimming and long pursuit, so it is likely that they are ambush predators, using their narrow body profile and silvery coloration to conceal their presence, then using their large dorsal fin to generate large acceleration, and large mouth and teeth to engulf prey before it can escape. That said, stomach content studies have revealed that they feed mainly upon planktonic crustaceans and tunicates, as well as other fish. They have also been shown to be cannibalistic. They are preyed upon by Opah, sharks, Albacore, Yellowfin tuna, and Fur seals.
The reproductive system of Lancetfish is something of a mystery. The gonads of adolescents have been shown to be hermaphroditic, although there is no evidence of functional hermaphroditism in adults. They are oviparous.