Nick Baker’s Hid­den Bri­tain

BBC Wildlife Magazine - - Contents - NICK BAKER Re­veals a fas­ci­nat­ing world of wildlife that we of­ten over­look. NICK BAKER is a nat­u­ral­ist, au­thor and TV pre­sen­ter.

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.

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