In praise of limpets

In his poem in Mollusc World 21, Peter Dance did not show much liking for the humble limpet. On the contrary, I rather like them and there is plenty of good educational mileage in Patella vulgata on marine field courses. This is particularly so when one focuses on how they live and what they do as living animals. However, Peter will find some allies amongst algologists (seaweed experts) as the field card for The Big Seaweed Search (http://www.nhm.ac.uk/seaweeds) gives limpets as enemies of seaweeds and the survey also asks for counts of limpets.

In wild weather limpets have to tolerate rough seas and the broad foot sole with its suction action together with the wide base of the conical shell enable them to hold on to the rocks. Only a sudden movement can dislodge them. The steepness of the cone is also an indicator of the degree of exposure with the steepest cones being found on exposed rough shores and on causeways with strong currents: those of sheltered shores are flatter, as are limpets generally in the almost tideless Mediterranean.

At low tide they hunker down and sit through the hours of dryness and hot sun and can you blame them for inactivity at times such as this (figure 1). Losing body water is dangerous for a mollusc. Limpets have homing instincts and usually return to the same spot for low tide. When attached for some hours to the same spot, carbon dioxide from the tissues dissolves in seawater to become a weak acid and this erodes the oval of rock where they rest, especially if it is limestone. The edge of the shell also etches into the rock as they nestle down to their resting place (figure 2). It is a common student project to paint numbers on the limpets and the same number of the limpet to check how far they may have moved.

Once covered by water and also while the rock is still wet when the sea has receded, limpets crawl around and feed by rasping micro-organisms and fine seaweed off the rock. The tough teeth on the scratchy tongue or radula often leave tooth-marks on the rock where it has been feeding (figure 3). Feeding grounds are sometimes seen as contrasting grey areas of rock surrounded by rock with a green veneer. And when the meal has been digested, out comes the waste in little pellets.

When limpets crawl in very small rock pools you can see the pair of tentacles on the head with eyes at the base and also round the edge, when under water in a pool, there are numerous small tentacles. These are extra surfaces that help the limpet to absorb oxygen from the seawater to augment the true gill. Look out for some of these features when you are next on a rocky shore.

_{figure 1: Limpets on the rocks exposed at low tide. Where limpets are numerous, seaweeds do not stand a chance.}