Squid and octopus switch on camouflage
Scientists have discovered how two marine creatures are able to rapidly "switch" their colours - from transparent to reddish brown.
The species, an octopus and a squid, use their adaptable camouflage to cope with changing light conditions in the deep ocean.
The creatures' skins respond light that deep-sea predators produce to illuminate their prey.
The findings are reported in the journal Current Biology.
Sarah Zylinski and Sonke Johnsen from Duke University in North Carolina, US, carried out the research. They say this switchable camouflage allows the animals to hide more effectively in their uniquely gloomy marine environment.
When sunlight diffuses evenly through the water, it passes through transparent animals too, rendering them almost invisible. But, as Dr Zylinski explained, "transparent tissues are actually quite visible when you shine a light directly on them".
And this is exactly what many deep-ocean predators do.
Prof Michael Land, a biologist from the UK's University of Sussex explained that by a depth of 600m, sunlight fizzles out, and hiding becomes much trickier for prey animals. This is the depth at which the octopus Japetella heathi and the squid Onychoteuthis banksii live.
Prof Land told BBC Nature: "[At that depth], you have all these nasty fish that are trying to illuminate you, so it's best to be a dark colour."
These "nasty" predatory fish are equipped with light-producing organs that function as biological headlamps.
To cope with this, the two creatures the scientists examined have evolved a clever way to hide.
Having already seen the two creatures in their two different colour states, Dr Zylinski and Dr Johnsen wanted find out how they switched between the two.
End Quote Sarah Zylinski Duke University
The really striking thing was the speed of their response”
To do this, they had to examine the animals more closely, so they set out to capture them from deep-ocean trenches in the Pacific.
With special nets that held the animals in the cold water from the deep, the team managed to bring the two species on board their research vessel. To test the animals' camouflage, the scientists simply shone a blue light onto them and watched their reactions.
"The really striking thing was the speed of their response," said Dr Zylinski. "We shone a light on them and they would immediately switch from transparent to pigmented."
The animals' skins contain light-sensitive cells called chromatophores, which contain pigments. When these cells detected the blue light of a bioluminescent predator, they immediately expanded, "dyeing" the animal a deep brown colour.
Dr Zylinski said the this dramatic colour change showed just how important camouflage was "in a habitat where there is nowhere to hide".
Neither transparency nor pigmentation is a complete solution to the hunting strategies used by predators in the deep ocean, she explained.
"By switching between these two forms, these cephalopods are able to optimise their camouflage in response to the optical conditions at that moment in time."
Dr Zylinski said studying camouflage gave a wonderful insight into how animals perceive their world very differently from humans.