Lancetfishes are large oceanic predatory ray-finned fishes in the genus Alepisaurus ("scaleless lizard") in the monogeneric family Alepisauridae.

The caudal fin is very deeply forked; its upper lobe is prolonged as a long filament, although most lancetfishes seem to lose this when captured.

Their watery muscle is not suited to fast swimming and long pursuit, so they likely are ambush predators, using their narrow body profile and silvery coloration to conceal their presence.

Data from longline fisheries in the central and western Pacific indicates shows an apparent increases in lancetfish bycatch.

This increase is thought by researchers to reflect a faunal regime shift as other commercially target species, like yellowfin and bigeye tuna populations are reduced through fishing.

Hook and line capture of lancetfish from surf zones is not unheard of and dietary surveys suggest at least some feeding occurs within inshore waters.

[8] The large size, wide depth distribution, and opportunistic diet of lancetfish have lent them to the study of other pelagic biodiversity because their voraciousness can be used to survey smaller organisms throughout the deep-sea that are difficult to capture by other means.

[7][10][9][11] In addition to a high degree of cannibalism and consumption of gelatinous foods, lancetfishes have also been documented with plastic refuse in their stomachs in the tropical north Pacific.

One particularly bizarre example of this affinity for surface waters comes from a gut survey of lancetfish in the Indian Ocean where a large amount (24.1%) of floating macroalgae was documented in the stomachs of exclusively adult (>100 cm) individuals.

[12] The voracious appetite, low degree of prey selectivity, broad depth distribution, slow rate of digestion and ease of sampling via longline bycatch make lancetfishes useful platforms by which to study the greater ecology of deep mid-water fauna.