Lancetfishes (family Alepisauridae) are one of the largest fishes inhabiting the mesopelagic zone. They are characterised by their incredibly high dorsal fin, slender long body, and long sharp teeth.

Their teeth indicate that they are voracious predators. However, the fish has weak, jelly-like muscles and is therefore not suited for long fast-paced swimming to chase its prey. Scientists therefore hypothesize that the fish is an ambush predator, laying in wait quietly while looking for prey with their large eyes, striking when the time is right.

Close-up of Alepisaurus ferox. Long deepwater fish with sharp teeth found in deep Pacific ocean water. (Photo by Ian Dyball, Adobe Stock)

Resembling a swimming lizard without scales, it can grow to the great length of 2 meters, and can be found swimming in an extreme depth range, from surface waters down to the bathypelagic zone at 2000 meters depth. They can be found in all oceans, except polar regions.

Lancetfish are completely scale-less, its skin appearing smooth and shiny. Their dorsal fin can also be depressed into a groove along their back. (Photo by Allen Shimada, NOAA)

Lancetfish are always on the look-out for food, and are willing to eat almost anything they come across. Thus their diverse diet includes other mesopelagic fish like hatchetfish and barracudinas, but also octopods and squid, gelatinous plankton, amphipods, swimming crabs and bristle worms. However, large individuals are also known to be eager cannibals – eating individuals of their own species, especially when other prey is less available! (1)

Their voraciousness and diverse opportunistic diet, in addition to their slow digestion makes them a relevant a study subject for biodiversity in the deep sea. Species that might not otherwise be easily caught and studied can be found in the stomachs of lancetfishes – in fact some deep-sea fish species have been discovered in this manner! (2, 3)

3D model of a lancetfish hanging vertically in the water column. (From our latest video)

Not a lot is known about their reproductive biology, however they are known to be simultaneous hermaphrodites. This means that they have the reproductive organs of both male and female, being able to produce both eggs and sperm. Thus, they are actually male and female at the same time!

With their great appetite and extreme size, the lancetfish is an exceptional predator of the mesopelagic zone which can aid scientists discover and learn more about the food web of the mysterious and inacessible deep sea.

Are you interested in learning more about the incredible life in the ocean mesopelagic zone? Check out our latest animated video below, and keep an eye out for more Creature Spotlights!