Nick Baker’s hidden Britain
The mysterious moon snail
Visiting a sandy beach at this time of year, you may like to reflect on something that might be going on below your feet. Lurking behind the veil of wet sand, a killer is on the prowl. A driller-killer, no less.
You’ll know it is there, because combing the strandline often delivers a pocketful of cockles and other clam shells with a neat hole close to where the hinge would have been, in a thickened region of the shell called the umbo. The hole is a precision job – rounded, smooth and chamfered, with the size (up to 3mm) relating to that of the perpetrator.
The story surrounding this hole – a feature much loved by children, as it enables the shells to be easily strung together in a necklace – is a rather dark one. Through this tiny, most perfect of perforations, the shell’s rightful resident was paralysed, killed, liquidised and supped up. And the animal that did this dastardly deed? It’s called the moon snail, Euspira catena, and it looks about as innocuous as a snail can possibly be.
Sometimes you’ll find a moon snail’s shell on the same strandline as its dinner discards. Smooth, rounded, shiny, almost pink… it looks pretty ‘cute’. Decorated with bands and wiggly chevrons of slightly darker pigment, it is almost a caricature of a snail, something Disney himself might have drawn.
When alive, the moon snail looks, at first sight, like a snail riding a pale pink magic carpet. This is its foot, which partly wraps around the shell, smoothing out any resistance when burrowing through sand and sediment. It also forms a skirtlike blanket as the animal slinks over the surface.
The snail detects waste products wafted into the water and wet sand by its prey. It follows the scent plume to its victim, then envelops the hapless mollusc, whose only defence is a shell. But even with this calcium-carbonate fortress clamped shut, the moon snail is not deterred. It now applies the dagger hidden beneath its lubricious, fleshy cloak, in the form of a proboscis-like protuberance.
So, the physical onslaught begins. The snail’s radula, an organ equipped with multiple teeth, starts to grind away at a specific spot on the victim’s shell. Since these teeth are made from the same materials as the shell that the snail is trying to grind through, this process needs chemical assistance. It is provided by glands on the underside of the foot and by an accessory boring organ, which lubricates the grinding surface with a cocktail of hydrochloric acid and enzymes.
Perseverance pays
This double action takes time. Grinding through the shells of thicker bivalve molluscs can take a couple of days. Once the moon snail has broken through the physical barrier, digestive fluids are dribbled into the cavity. The clam is turned to chowder inside its own house.
A question remains. Why go to all that effort, when other shell-boring, predatory molluscs can do a much quicker job by nibbling through the thinner parts of the shell? Perhaps it’s a way of masking and controlling the scene. If you open up a shell, you also invite in the competition – crabs and shrimps may steal the meal. This way, the moon snail has everything to itself.