Cephalopods (Octopuses and Squids), are already a fascinating and complex species. And now it seems that two individual species of cephalopod that occupy the mid ocean depths of 600-1000m have one more remarkable trait to add to their arsenal of survival tools – the ability to control their reflection to avoid being eaten.
Most predators that hunt at such depths do so by spotting the silhouette of their prey against the last remnants of light that penetrate from above, whereas some predators have evolved searchlights mounted to their heads, so called "headlight fish".
If you’re transparent then you have a pretty good chance of not being spotted by a silhouette-spotter, but being transparent won’t save you from the headlight fish.
Instead, Japetella heathi – a bulbous, short-armed, 3" octopus, and Onychoteuthis banksii – a 5" squid – are transparent by default, save for their eyes and guts which cannot be made transparent. This renders them practically invisible when viewed from below against the lighter background that comes from above.
However, when they are hit with the bluish luminescent light that is typical of headlight fish, the cephalopods switch on their chromatophores - skin pigments responsible for colour - to instantly turn themselves red, a colour that doesn’t reflect light in the depths and thus makes them invisible to predators that hunt by sight.
Duke University postdoctoral researcher Sarah Zylinski conducted experiments on board research ships during 2010 and 2011, and of the 15 to 20 different species of cephalopod captured from the deep, only J. heathi and O. banksii responded to the blue-filtered LED light that Zylinski used in her experiments. Zylinski measured the reflectivity and found that the cephalopods reflected twice as much light in their transparent state as they did in their opaque state.
"I went through several things I thought would stimulate behaviours," she says, going on to explain that shallow-water cephalopods (squid, ocotopi and cuttlefish) change their body patterns for a shadow or shape passing overhead, but these deeper water animals don't. Even though the animals could be seen tracking the movements of probes around them, it was only the light that would switch on the pigments.
For more information see the paper: "Mesopelagic Cephalopods Switch Between Transparency and Pigmentation to Optimize Camouflage in the Deep," Sarah Zylinski and Sönke Johnsen. Current Biology 21, Nov. 22, 2011. DOI: 10.1016/j.cub.2011.10.014
Why not take out a subscription to Practical Fishkeeping magazine? See our latest subscription offer.