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It’s a squid-eat-squid world down there. Remotely controlled submersibles have filmed two deep-sea species of squid gulping down members of their own species.

Several species of squid, including giant squid, are known to eat each other.

But we have mainly gleaned this from their stomach contents, and it wasn’t clear if this was normal behaviour, or something they did when they have been caught in a net, says at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany.

“Animals get stressed in a net,” he says. “They probably start doing something called ‘net feeding’ – they feed on anything that is close by.”

Hoving and his colleagues sent down subs to observe two species of the Gonatus genus of squid in an underwater canyon of Monterey Bay, off the coast of California. The squid span much of the water column, from shallow waters to more than 2000 metres down.

Caught cannibalising

The subs frequently saw both species eating their own kind, especially Gonatus onyx. “Up to 40 per cent of the animals examined were feeding on animals of the same species,” says Hoving. “That was quite high – that was surprising.”

The subs filmed the squid doing this in their native habitat, so it could be ordinary behaviour for them, Hoving says.

The squid grow quickly and breed only once, needing lots of food to fuel their metabolism, says Hoving. When other food is scarce, cannibalism could be a good way to get food, he says.

There are other benefits, too. “It eliminates one competitor for food,” he says.

“Clearly cannibalism is important,” says at the University of South Florida in St Petersburg. “They’re doing it on a semi-regular basis or you wouldn’t see it from a submersible.”

Don’t eat too many

But it wouldn’t make sense for the species to overindulge in itself, Seibel says. “A species can’t exist feeding only on itself.”

Using subs with high-powered lights to observe the creatures could have led to the amount of cannibalism being overestimated, Seibel says. It’s possible that squid clutching large prey items – such as their kin – are less able to swim away from the vehicle, meaning that they’d be counted more often than squid eating lighter meals, he says.

To get the most accurate picture of the elusive animals’ behaviour, you have to combine observations from subs with other methods such as netting, he adds. In general, these squid aren’t picky eaters, says Seibel. “They eat just about whatever they can get their tentacles on.”