Nick Baker’s Hidden Britain
The slipper limpet’s survival strategies
K ick your way along the strandline of any UK coast south of Spurn Head or Cardigan Bay and you’ll almost certainly find a peculiar shell (it’s sometimes recorded in Scotland, too). An elongated ovoid, a slipper limpet shell is hunched in profile and often covered in algae and other growths, but wipe it clean with your fingers and you’ll reveal a pink-white colour that’s decorated with darker dots and dashes. Flip over the shell and you’ll see a pearly white shelf that closes off half of the cavity underneath.
The slipper limpet has become so numerous that, on some southern beaches, it seems there are more shells than shingle. Its omnipresence means the species is often overlooked but this sea snail has many a tale to tell.
The first is an ecological one – Crepidula fornicata doesn’t really belong here. It is native to the USA’s eastern seaboard, assumed to have arrived on these shores attached to American oysters or the bottom of ships, or as larvae in ballast water. Our modern population arrived here in the 1890s. By the 1970s, in places such as the Solent, there was an abundance of the species. While a snail out of context is bad news, the means by which it became so competitive is quite fascinating.
Pop a slipper limpet in a clear-sided vessel and you can admire its muscular foot – a rarely seen view of this common animal. The ability of these limpets to attach to almost any hard surface means they can quickly take over scallop, mussel and oyster beds, readily competing for food and space. There is also indication that, in heavily infested areas, they multiply so effectively that they alter the structure of the seabed, physically and chemically, which impacts on whole ecosystems, including fish-spawning grounds.
Continue to peer at the underside of the limpet and you may also notice a delicate curtain of striations – these are its gills. It is a filter feeder, not a grazer like other related gastropod molluscs. When the animal is hunkered down, water is drawn in under the edge of the shell, over the gills in the mantle cavity and out the other side – all powered by the beating of microscopic hairs called cilia. Food particles in the water are trapped in strands of fine mucus and rolled slowly toward a gutter-like food canal. By this process, the snail simply sits in place and sucks nutrients out of the water.
It doesn’t even have to move to reproduce. Have a look at a stack of these snails and you’ll notice the one on the bottom is bigger – this is a female. Those at the top of the stack are males and those in between are undecided or in the process of changing – influenced by chemicals the female releases.
Slipper limpets are sequential hermaphrodites and, unlike other molluscs, don’t simply cast their eggs and sperm into the water. Instead, the male reaches under the female’s shell with his extending penis to fertilise around 1,000 eggs, which are then brooded in her shell before being released as free-swimming larvae that either settle on top of another stack or start their own.
It’s all very efficient and, with a lack of their natural predators, you’re likely to continue finding these shells on UK beaches.
Male slipper limpets can be seen piggybacking on top of the larger females.