THE CUTTLEFISH MYSTERY

The opening of the article on cuttlefish said, “Evolution’s a bitch.” It is if you start out with an outside armoured shell during the Jurassic Era which decides to become an internal skeleton leaving you with no protection against toothy predators.

The cuttlefish developed camouflage using chromatophores, sacs of red, yellow or brown pigment in the skin. Cuttlefish can turn from completely invisible, to visible and back in about 2 seconds and not just by changing colour. They can also change the texture of their skin by using bundles of tiny muscles to go from smooth to spiky. Underneath the chromatophores are leucophores, which reflect light across a wide wave length range, and iridophores which combine platelets of reflectin (protein) with layers of cytoplasm to produce iridescent reflections the same way a butterfly wing does. This optical effect shifts the light towards blue and green wavelengths but the cuttlefish combines this with the chromatophores to make a wider range of shimmering colour.

This is not only for disguise but also to send signals to other cuttlefish. In experiments the cuttlefish would either disguise itself or run depending on the type of predator it was facing. The signals it sent were for their own group rather than trying to engage the predator. Scientists were surprised at the ability to discriminate between species and to apply different tactics in dealing with any threat.

Another trick is to use polarised light to communicate with other cuttlefish. Sunlight is polarised when it hits water, some of its electric and magnetic fields are filtered out so the fields oscillate in only one plane. Cuttlefish can see these patterns and send polarised signals from their iridophores through the chromatophores while remaining camouflaged. In this way they can warn others without attracting attention. They also use postures and movement, such as liting their arms and waving them when hiding in seagrass.

They also use “kinetic patterning”, making a pattern of dark waves move across the body which disorients a pursuing predator. The moving pattern makes it difficult to process visual information such as, how fast the cuttlefish is moving, which way it’s going or how big it is. They can even camouflage at night using an encoding gene, in the skin, for opsin which is a light sensitive molecule.

One of the unanswered questions about cuttlefish is how they match their backgrounds so effectively when evidence suggests they are colour-blind, only seeing in shades of green. (Don’t ask, I haven’t a clue how scientists would do that experiment)

Enjoy this post, it's taken me three days to get it here. I'd like to shove a telephone pole right up Sol's all too large salary package.