Some Adaptations of Gastropods to Physical Stress on a Tropical Rocky Shore

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Abstract
Activity patterns, microhabitat occurrences, and behaviors during low tides of 10 common gastropods inhabiting rocky intertidal shores of the Pacific coast of Panama were examined. All species were active when wet or washed by rising or falling tides, but were inactive during periods of emergence and subemergence. During daytime low tides, most snails occurred in cervices, on vertical surfaces, or in tidepools; few were found on horizontal or sloping rock. Measurements of substrate temperature, water evaporation, and snail tissue temperature indicated that daytime physical stress at a given tidal level increased along the microhabitat gradient; cervices < tidepools < vertical surfaces < slopes < Horizontal surfaces. In addition to the above mechanisms, six gastropods showed specific behaviors during sunny low tides that apparently further ameliorated stress. The importance of these observed patterns was tested in the field. Experiments included (1) transfers of gastropods from "refuge" microhabitat to adjacent, horizontal or sloping surfaces during low tides; and (2) manipulations of possible thermoregulatory or antidesiccatory behavior. Differences between snails in experimental and control treatments were measured by (1) amount of mass lost, (2) changes in tissue temperature, and (3) relative survival. For the predatory gastropods, one herbivorous snail, and two limpets, microhabitat was vital importance in reducing stress. However, several snails and a limpet had additional behavioral or morphological adaptations that further lessened stress and allowed them to persist over a wider range of microhabitats. A neritid formed nultilayered aggregation, loss extravisceral water at a controlled rate, and reduced tissue temperature by evaporative cooling; two littorinids used mucus to attach or orient shells to lessen heat gain as well as to reduce water loss; and at least one pulmonate limpet maintained a raised posture on its home scar to lessen contact with substrate and allow evaporative cooling. Gastropods were ecologically constrained by physical stress. They were constrained also in time and space by predaceous fish during high tides. Thus intense, complementary, and cyclically occurring biotic and abiotic factors severely limited foraging for the gastropods of this region.