Small boulders at the top of this beach are often drilled with smooth-sided holes, as wide as my smallest finger, and some rattle when they are picked up and shaken. The sources of the sound, almost hidden in the tunnels, are paired, empty shells of dead wrinkled rock borers, Hiatella arctica.
These bivalve molluscs, growing to about an inch long, should have the safest homes in their lower shore habitat. Once they’ve survived a perilous infancy as planktonic larvae, they settle on the surface of rocks and, by dint of secreting a weak acid and constantly fidgeting, use their toughened, ridged shells to bore into soft limestone. Some boulders become the geological equivalent of Emmental cheese, riddled with holes.
Once inside, Hiatella continues to grow, circulating water through short red siphons across its gill surface, where it filters out its diet of plankton. Few small intertidal animals are as secure from predation by fish and seabirds. But, entombed in solid rock, it soon grows too large to escape through its entry hole. Only those borers that live long enough to tunnel through and out of the other side are likely to escape their sarcophagus.
I’ve sometimes wondered why so many of these drilled rocks end up at the top of the beach, but today a possible answer dawned on me. An autumn gale had washed up heaps of wracks and kelps. These marine algae grip rocks with great tenacity, because their holdfasts grow into the microporous structure of the stone, and some, stranded high on the beach, were still clasping boulders that had once anchored them to the seabed. Some of these had been excavated by rock borers; I could see the live molluscs deep inside their tunnels.
Those borers must have been the engineers of their own misfortune, drilling into the rocks until they were no longer dense enough to withstand the drag of the luxuriant summer growth of seaweed fronds, churned by waves in the rising tide. The borers avoided carnivorous predators, but it was the chance settling of a seaweed spore on their home in spring, combined with their own relentless excavations, that had sealed their fate.
