Environmental conditions and physiological tolerances of intertidal fauna in relation to shore zonation at Husvik, South Georgia

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Abstract
South Georgia, an extremely isolated island within the Antarctic Convergence, was covered by an extensive icecap until ~10,000 years ago. In consequence the depauperate intertidal fauna is of recent origin and consists almost entirely of brooders or direct developers which probably arrived as a result of rafting. Environmental conditions between the tidemarks are comparable with northern Norway and Greenland, so the absence of mussels and barnacles is due to isolation from the nearest feasible sources of colonization (the Falkland Islands and Magellan), and not to a hostile environment. Intertidal animals (eight species studied) have median upper lethal temperatures that are positively and linearly related to maximum height of distribution on the shore. Thermal niche width (median upper lethal temperature minus median lower lethal temperature) is also positively correlated with maximum height of distribution on the shore with species below mid-tide level having narrow niches in contrast to species above mid-tide level that have wide niches. There is no relationship between freezing resistance and position on the shore. Salinity and desiccation tolerances were also greater in animal species from the upper shore than in those from the lower shore. In the case of those species also studied elsewhere (Lasaea rubra (Mollusca: Bivalvia); Nacella condnna (Mollusca: Gastropoda) and Tigriopus sp. (Crustacea: Copepoda)), evidence is presented to show that no special adaptation is exhibited by South Georgian animals, and that upper zonation limits are controlled primarily by physical tolerance. This is particularly marked in the case of N. condnna which has similar thermal tolerances on the Antarctic Peninsula, on Signy Island and at South Georgia; in consequence it can only inhabit the extreme lower shore at South Georgia, yet penetrates to mid-tide level at Signy Island.