Shell damage and shell repair in the Antarctic limpet Nacella concinna from King George Island

Nacella concinna is the most conspicuous macroinvertebrate in the intertida l of King George Island. An important predator, the Kelp gull Larus dominic anus, feeds on Nacella during spring row tides. The gulls deposit empty Nac ella shells as regurgitates mainly on roosts on coastal rocks. The regurgit ates were found to consist of 40% shell fragments by weight and 60% intact shells. Faeces of Kelp gulls contained much smaller fragments than the regu rgitates. Some of the Nacella, particularly those too large to ingest, are handled in the intertidal. The middens are, therefore, inadequate to study size selection by Kelp gulls: the largest Nacella are underrepresented. Sev enty-five per cent of the intact Nacella shells from the Larus middens show ed one or more shell repairs. Such repairs may be due to unsuccessful attac ks by gulls, but more probably they indicate damage caused by rolling ice b locks and stones in the intertidal and shallow subtidal. A number of living Nacella were found stranded on the beach, detached from the rocks. They sh owed damage along the shell margin and even one Nacella was collected witho ut any shell left. The observed repair frequency of 75% in Nacella was much higher than in other (smaller) intertidal gastropods at Potter Peninsula ( 3-11%, av. 8%). Comparably high frequencies are observed for instance in tr opical intertidal gastropods, where repair is due to heavy unsuccessful cra b predation; however, shell-crushing crabs are absent on King George Island . This indicates that palaeontologists should be cautious in ascribing all shell repairs in fossil shells (particularly from tidal environments) to pr edators. Shell repair in the related Nacella deaurata, collected in a less exposed site at Port Stanley(Falkland Islands), occurred only in 13% of the specimens. Another conspicuous form of shell damage was due to grazing by Nacella on the boring algae living in other Nacella shells. Epigrowth of cr ustose calcareous algae inhibited such grazing, but in the absence of epigr owth deep hollows were scraped in the shells, the parallel scratches by the radula clearly visible, urging Nacella to repair its shell by producing mo re shelly material on the inside. (C) 1999 Elsevier Science B.V. All rights reserved.