Thermal Stress in a High Shore Intertidal Environment: Morphological and Behavioural Adaptations of the Gastropod Littorina africana

Abstract

Thermal stress, coupled with desiccation, is one of the major physical hazards to be overcome by intertidal organisms. Biogeographic trends are generally temperature mediated (eg. Brown and Jarman, 1978; Bolton, 1985) and, although intertidal organisms generally live well within their physiological limits (Wolcott, 1973; Underwood, 1979), zonation effects are often considered to be linked to the increasing range of temperatures experienced higher up the shore (eg. Sandison, 1967; Sterling, 1982). In cold climates thermal stress is related to problems of very low temperatures and even freezing (resistance to freezing of intertidal invertebrates is reviewed by Murphy, 1983) but in warm temperate, tropical and sub-tropical areas thermal stress takes the form of potential overheating (es. Garrity, 1984). Members of the genus Littorina occupy the highest reaches of the littoral fringe along the coast of southern Africa and are only wet by wave splash or at high spring tides. Consequently they may spend weeks when, especially during the summer, they are subject to high day time temperatures without access to water which could be used for evaporative cooling.