By Paul Rincon BBC News Online science staff

Sleeper sharks can get caught in trawler nets Enlarge Image

Sleeper sharks even appear to target the biggest species of large squid - the colossal squid, which is about double the size of the shark.

The huge sperm whale was previously the only animal thought to rely on giant and colossal squid for food.

Details of the study are featured in the journal Deep Sea Research.

When French marine biologists opened the stomachs of 36 sleeper sharks accidentally killed by trawlers, they collected the remains of at least 49 colossal squid and eight giant squid.

The sharks feed on giant squid of different sizes from juveniles until adults Yves Cherel, Centre d'Etudes Biologiques de Chize

The research was conducted by Yves Cherel, of the Centre d'Etudes Biologiques de Chize in Villiers-en-Bois, France, and Guy Duhamel of the Museum National d'Histoire Naturelle, Paris.

The sleeper sharks were caught in the Southern Ocean in waters off the Kerguelen island archipelago.

Giant squid (Architeuthis dux) and their even bigger relatives colossal squid (Mesonychoteuthis hamiltoni) are amongst the most mysterious and fascinating creatures in the ocean.

Mystery creatures

Virtually nothing is known of their behaviour, because they have never been observed in their natural habitat.

These latter day sea monsters, which belong to the family of animals known as cephalopods, live in pitch darkness at an ocean depth of up to 600 metres - which is about 350 m deeper than the average operating depth of a submarine.

"We think both the sharks and the giant squid live at the same depths," Dr Cherel told BBC News Online.

"The sharks feed on giant squid of different sizes from juveniles until adults."

Most of what we know about the biology of these large squid comes from dead specimens caught in trawler nets or washed up on beaches.

The giant squid grows up to 12 m (39 feet) in length, though most of this is made up of its two long front tentacles. The colossal squid may grow up to 14 m (46 feet) or more. Sleeper sharks can grow up to 7.3 m (24 feet).

Since large squid tend to dwarf sleeper sharks, the researchers are at a loss to explain how the sharks catch such big prey.

What's more, the sharks appear to devour even bigger squid on average than those eaten by sperm whales.

Beak attack

But the scientists remain open to the possibility that the sharks scavenge dead carcasses.

Giant squid live in the water column up to 600 m deep

"Sleeper sharks are known as scavengers and also as predators. They can feed on the living or the dead. But the juvenile giant squid are probably caught live," said Cherel.

Dr Cherel said fishermen did not report signs of scarring on the sharks from being scratched by the beaks of large squid.

It is reasonably common to find deep scratches on the skin of sperm whales from fighting with squid.

The enormous cephalopods are very occasionally reported in the diet of sharks from the North Atlantic and waters off South Africa and eastern Australia.

But the sleeper shark is the only shark known to deliberately seek out these jumbo-size squid.

The French biologists also found an unknown species of large octopus in one of the sharks' stomachs.