For animals living in the deep sea, it seems that there is no such thing as all-occasion camouflage. Under diffuse light conditions, it's generally best to be transparent. But in the deeper ocean, where predators are equipped with special organs that function as searchlights, it's much better to go in darker colors. That's according to a report published online on November 10 in Current Biology, a Cell Press publication.
Sarah Zylinski and Sönke Johnsen of Duke University show that two species of cephalopodâ€"the octopus Japetella heathi and the squid Onychoteuthis banksiiâ€"enjoy the best of both worlds. They rely on special cells called chromatophores to rapidly switch from transparency to a dull red color as optical conditions around them change.
"The boundary between environments where one or the other strategy would be most useful is neither sharp nor fixed, changing with factors such as time of day, cloud cover, and turbidity," the researchers wrote. "Being able to switch between strategies in response to specific threats or changing optical conditions would be highly advantageous to an animal seeking to survive in this unique environment."
The discovery was made in shipboard experiments of animals obtained from ocean trawls in the Peru-Chile Trench and Sea of Cortez, with light responses captured on video. The researchers say the study represents the first time to their knowledge that anyone has carried out such ecologically relevant behavioral studies of chromatophore use in cephalopods.
"Survival in the deep sea depends on seeing others without being seen yourself," explain Michael Land and Daniel Colaço Osorio in an accompanying Dispatch. "There is a problem with the transparency strategy that does not seem immediately obvious. Biological tissues, even if unpigmented, vary slightly in refractive index, which means that the interfaces between them, and the interface with the water outside, produce slight reflections…The ability to expand chromatophores on a time scale 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."