Deep-Sea Squid Go From Transparent to Dark as Fast as a Kindle
12th November 2011
The cephalopods’ abilities are a defence mechanism, apparently arising from the fact that they operate on the depth boundary between two main kinds of deep-sea predators who might fancy them for lunch.
Many of the hungry enemies of J heathi and O banksii spot prey by seeing its silhouette against the light waters above, illuminated by the sky. To avoid these, staying transparent is best. Those parts of the cephalopods which can’t be fully see-through – for instance their guts, filled with food – are reflective, as that’s the next best thing.
Deeper down, however, many ocean hunters use bioluminescent lighting of their own to illuminate prey. The transparent condition, used against these, would be a bad idea as the predators would easily detect the cephalopods’ reflective innards. Thus, when J heathi and O banksii notice that they are being illuminated strongly by certain wavelengths of light – blue is favoured by ocean creatures as it penetrates water better, as any diver will know – they swiftly turn themselves dark.