Nick Baker’s hidden Britain
Blue-rayed limpet
Beachcombing is one of life’s great pleasures – you never quite know what you might find. One creature always worth looking out for is the diminutive blue-rayed limpet, Patella pellucida. Very different to its more familiar conical cousins in rockpools, it lacks spherical symmetry and is more delicate, with an almost translucent shell reminiscent of a little fingernail in size and shape.
Like its relatives, the bluerayed limpet is also an algae grazer, but it specialises in the big stuff: kelp. For most of the tidal cycle, this impressive seaweed and its limpet predator are submerged, so beyond reach of rockpoolers. However, kelp may be cast ashore after a storm, and then you can find this beautiful mollusc among the tangled creases and root-like holdfasts, or haptera.
Triggered by the shorter days of autumn, the limpet underwent a vertical migration from the kelp frond down to the holdfast, to minimise the chances of being torn from the habitat on which it depends. Under normal circumstances, this is where it would have spent the winter, waiting until the fresh growth of spring.
The shells of most individuals are bedecked with broken parallel streaks of blue that radiate along the axis of their shell, like stitches. This blue isn’t just blue, it’s an eyesearing, electric, dazzling blue. I’ve often wondered why these animals appear so bright.
Well, it is all about the physics of light, and is similar to the phenomena exhibited by the feathers of parrots and the wings of some butterflies and beetles. The blue isn’t a pigment as such, but something known as structural colour.
In the blue-rayed limpet, things get even cleverer. The reflective nano-architecture is not on the shell’s surface, but 30 microns (0.03mm) below, buried in the layers of calcium carbonate. Here, there is a zone of parallel, equally spaced layers with a uniform zig-zag pattern. Light passing through the transparent shell hits this, and because of the angle of the reflective plates, only blue and green wavelengths of light bounce back. The layer acts as a selective mirror.
The brightness is further enhanced by another specialised nano structure in the layers beneath, made up of many random spherical particles that catch all the other wavelengths of light passing through the shell. This creates darkness that contrasts with the reflected light. The end result is a super-vibrant blue that pops out at you when you’re staring at a kelp holdfast.
But why have such glowing features? As limpets don’t have eyes to speak of, it’s not about sending signals to others of their species. Instead, it has been suggested that the limpet mimics a toxic predatory sea slug, and thus is avoided by potential predators. But for that defence to be effective, the animal being mimicked has to be at least as common as the pretender, which is probably not the case.
That leaves the possibility that it is a form of interference camouflage. Roll the limpet shell around in your fingers and you’ll discover that the intensity of the blue rays changes. In the sun-dappled, ever-shifting forest of kelp fronds, this play of light may be enough to detract from the limpet’s outline. Or it may become part of the twinkling of the fronds, or resemble other iridescent encrusting algae also found in these habitats.