Iridescence in the common limpet (Patella vulgata)

Limpets are familiar seashore animals and school biology (well, mine anyway) tells us about the conical shells that allow them to remain attached and store water in the challengingly energetic and sometimes inconveniently dry conditions of the intertidal zone. However, looking at an empty limpet shell on the Isle of Wight a couple of days ago, I couldn’t help but notice the iridescent colours inside its peak. The white colour is, if I remember rightly, one why of identifying this limpet species, but the iridesence is something generally associated with other molluscs such as ormers, abalones, top shells and of course oysters and their pearls.

Inside an empty limpet shell.

Now, I know that I’ve seen iridescence in all sorts of mollusc species, and am aware that the colours are due to the effect of small-scale structures of a size similar to the wavelengths of light and the resulting interference patterns. What piqued my curiosity was what these structures actually are and why they exist – given that the colours are often hidden away inside shells, it seems that they must be an aesthetically pleasing by-product of something with a function other than the decorative; is it simply the crud-capture associated with pearl formation?

Close-up of limpet iridescence

Nacre or ‘mother-of-pearl’ is a composite material (organic and inorganic) made of thin layers of hexagonal platelets of aragonite (a type of calcium carbonate) separated by sheets of elastic biopolymers such as chitin and silk-like materials. This structure is strong and crack-resistant (more precisely it resists crack propagation) and smooths the inside of the shell. It also helps defend the animal against parasites by encysting them in layers of nacre. So, it is both structural and a capturer of crud, which incidentally  happens to be iridescent due to the size of aragonite platelets. Limpets may not be the most well-known iridescent mollusc, but they deserve their place here (there are other types of layering and shine forms in the molluscs as well); for an electron-microscope close-up, have a look at images from the University of Tokyo’s Kogure Lab here (scroll down to number 6 for limpets).