The molecules that make octopus skin so successful as a dynamic camouflage could provide materials scientists with a new way to make super-reflective materials.
Roger Hanlon and colleagues at the Woods Hole Marine Biological Laboratory have found that the leucophores (reflector cells) in the bottom layer of octopus skin contain a colourless, translucent protein which reflects light. And not just a specific wavelength of light: all wavelengths are reflected, and at every incident angle.
Cephalopod skin exhibits what is called “neurally-controlled polymorphism”. That is, because of highly sophisticated control of the skin by the nervous system, cephalopods have the ability to change from one of many patternings and colorations to another, almost instantaneously. This can be used to generate visual signals to other cephalopods as well as for camouflage.
The skin has a complex structure. One layer of skin, the stratum, contains cells called chromatophores, which are filled with pigmented granules contained within an elastic, filament-like compartment of the cell. Under control of the nervous system, the distribution of the pigmented granules within the compartments can change. When the granules are distributed evenly, the cells appear dark and do not let light pass. When they are clustered near the middle, the cell has a light appearance, and light can pass through it. It is the synchronous control of the millions of chromatophores in a cephalopod’s skin that produces the near-instantaneous changes in colour and patterning.
The distribution of pigment in the stratum determines whether the layer underneath – the dermis – is visible or obscured. Light that passes through the chromatophores is reflected, because the dermis contains the flat, ellipsoid leucophores. These cells have a large nucleus and are covered with between 1,000-2,000 membrane-bound protrusions called leucosomes.
A leucophore from Octopus dofleini, modified from Cloney and Brocco (1983).
The protein discovered by Hanlon and his team forms the clear, colourless granule found inside each leucosome. The granule is approximately 1 micrometre (one-thousandth of a millimetre) in length, and is attached to the cell body by a narrow stalk. The clustering of leucosomes makes the surface of the cells highly reflective. Because the protein reflects light of all wavelengths, the dermis produces a scattered reflection of ambient light, so that the octopus blends in with its surroundings.