Biologists on Wednesday said they had unlocked secrets about the vampire squid, a mysterious creature that feeds on the decaying dead in the unlit depths.

The squid is so weird that it is known as a “phylogenetic relic.” It has the honour of occupying a taxononomic category all of its own, combining features of octopuses and squids in a unique evolutionary formula that has survived for millions of years.

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As Vampyroteuthis infernalis — the “Squid from Hell” — it is the only species in the Order Vampyromorpha, where it was placed in 1903.

The 13-centimetre (five-inch) cephalopod lives in temperate and tropical oceans, inhabiting waters at depths between roughly 600 and 900 metres (2,000-3,000 feet), a niche habitat where at the lowest levels there is just enough oxygen to support life.

It uses huge 2.5-centimetre (one-inch) eyes to detect the slightest gleam of movement, and deploys dark-blue bioluminescence to cloak its jelly-like body from predators below when it drifts at higher depths.

Writing in the journal Proceedings of the Royal Society B, a pair of scientists in California report on 30 years of chance encounters with vampire squids by robot submarine explorers, laborary experiments and dissections.

Unlike its latter-day cousins, the vampire squad does not tuck into living prey, they say.

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Instead, examination of the squid’s digestive tract, faeces and regurgitations suggest it is a “detritivore” — it tucks into the corpses (or what remains of them) of larvae, crustaceans and zooplankton that sink gently to the ocean floor.

A bigger puzzle, though, is a feature of the vampire squid’s mouth.

It opens up like a black umbrella, comprising a web that encompasses eight octopus-like arms, studded with suckers and finger-like spines called cirri.

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It also has a second pair of arms called retractile filaments that can reach out to lengths that are far bigger than that of the squid itself, and can then be withdrawn into pockets within the web.

Unique to the vampire squid, these sticky filaments were long thought to be sensors to detect living prey and predators.

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But the evidence says they are used to reach out and snare morsels of food, say Hendrik Joving and Bruce Robison at the Monterey Bay Aquarium Research Institute.

The scraps are then glued together into a little ball using mucus from secretory tissue in the suckers, and then transported to the squid’s jaws by the cirri.

“Vampyroteuthis’ feeding behaviour is unlike any other cephalopod,” says the study with more than a touch of admiration.

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“(It) reveals a unique adaptation that allows these animals to spend most of their life at depths where oxygen concentrations are very low, but where predators are few and typical cephalopod food is scarce.”