SIZE, SHAPE AND SHELL MORPHOLOGY IN THE ANTARCTIC LIMPET NACELLA-CONCINNA AT SIGNY ISLAND, SOUTH ORKNEY ISLANDS

Comparison of the size, shape and shell morphology in littoral and sub-littoral morphs of the Antarctic limpet Nacella concinna reveal differences in shell morphology which are enhanced by structural anomalies within the shells of the two types. Infestation of sub-littoral shells by the conchocelis phase of an endolithic alga significantly affects shell density and total chlorophyll levels in the two shell morphs. The surface sculpture of sub-littoral shells is characterised by a series of grooves, the configuration of which closely resembles that of the radular teeth in N. concinna. Limpets utilise the available food supply within the shell matrix of other limpets by grazing the shell material. Epibiotic growth of calcareous algae prevent erosion and preserve underlying shell layers. In severe cases, where protection is lacking, intraspecific shell grazing may remove parts of the shell exposing the internal tissues. The Dominican Gull, Larus dominicanus, is a major shore predator of both shell morphs. Gull middens contain both shell types but are dominated by the more accessible littoral shells. Comparison of living populations and midden assemblages indicates that size and shape selection of prey occurs, with pear-shaped limpets between 21 mm and 29 mm in length being taken preferentially. Apparent differences in shell form are induced by physical, biological and behavioural influences. Littoral animals are robust in nature, resist avian predation and are not extensively grazed whereas those of the sub-littoral are not subject to the same degree of predatory attention but suffer a gradual depletion of their shallower shell form through a combination of algal infection and intraspecific shell grazing.