News in Science

Squid change colour in twilight zone

Survival tactics Some species of deep ocean squid and octopus can quickly switch their colour from transparent to inky red to evade predators, researchers have found.

Sarah Zylinski and Sönke Johnsen of Duke University studied two species that live in the mesopelagic zone between 600 and 1000 metres below the surface. At that depth, sometimes called the ocean's "twilight zone", the brighter upper layers of water gradually blend into the dark, lightless depths.

In the upper region of the mesopelagic, many animals are transparent, which makes them less visible to predators that hunt by looking for the silhouettes of prey above them, Zylinski explains.

In the lower regions, many animals are red or black pigmented, which is ideal for hiding from fish that hunt with special organs that function as biological 'searchlights'.

In their new study, published in the journal Current Biology this week, Zylinski and Johnsen say they have found the first evidence that some species can quickly switch between these states.

This ability is probably very useful for animals living in parts of the ocean where light levels vary, and where predators using both techniques are present, Zylinski explains.

"Being able to switch allows the cephalopods to optimise their camouflage to the optical conditions of that moment," she says. "This is why the rapidity of the response is of the essence."

Cephalopod is the formal term for the group of animals that includes squids, octopuses and cuttlefish.

The squid and octopus species change their colouration using chromatophores, elastic sacs of pigment that can be pulled open and closed by surrounding muscles.

"They are under direct neural control, which is why cephalopods can change their appearances so fast," Zylinksi says.

"Survival in the deep sea depends on seeing others without being seen yourself," explain Michael Land and Daniel Colaço Osorio in an accompanying dispatch in the journal.

"The ability to expand chromatophores on a timescale of about a second from dot-like structures to comprehensive body covering is a strategy available to most cephalopods, and this is put to good use here to switch from one form of camouflage to a slightly better one."