The vampire squid (Vampyroteuthis infernalis, lit. "vampire squid from Hell") is a small, deep-sea cephalopod found throughout the temperate and tropical oceans of the world.
Unique retractile sensory filaments justify the vampire squid's placement in its own order: Vampyromorphida (formerly Vampyromorpha), which shares similarities with both squid and octopus. As a phylogenetic relict it is the only known surviving member of its order, first described and originally classified as an octopus in 1903 by German teuthologist Carl Chun, but later assigned to a new order together with several extinct taxa.
The vampire squid is able to reach a maximum total length of around 30 cm (1 ft). Its 15-cm (6-in) gelatinous body varies in color between velvety jet-black and pale reddish, depending on location and lighting conditions. A webbing of skin connects its eight arms, each lined with rows of fleshy spines or cirri; the inside of this "cloak" is black. Only the distal half (farthest from the body) of the arms have suckers. Its limpid, globular eyes, which appear red or blue, depending on lighting, are proportionately the largest in the animal kingdom at 2.5 cm (1 in) in diameter. The animal's dark color, cloak-like webbing, and red eyes are what gave the vampire squid its name.
The vampire squid is an extreme example of a deep-sea cephalopod, thought to reside at aphotic (lightless) depths from 600–900 metres (2,000–3,000 feet) or more. Within this region of the world's oceans is a discrete habitat known as the oxygen minimum zone (OMZ). Within the OMZ, oxygen saturation is too low to support aerobic metabolism in most higher organisms. Nonetheless, the vampire squid is able to live and breathe normally in the OMZ at oxygen saturations as low as 3%; a feat no other cephalopod, and few other animals, can claim.
What behavioural data known have been gleaned from ephemeral encounters with ROVs; animals are often damaged during capture, and survive for no more than about two months in aquaria. An artificial environment makes reliable observation of nondefensive behaviour difficult.
With their long velar filaments deployed, vampire squid have been observed drifting along in the deep, black ocean currents. If the filaments contact an entity, or if vibrations impinge upon them, the animals investigate with rapid acrobatic movements. They are capable of swimming at speeds equivalent to two body lengths per second, with an acceleration time of five seconds. However, their weak muscles limit stamina considerably.