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Habitat selection by marine snails is profoundly affected by variations in biotic and abiotic factors. In the supralittoral fringe of Caribbean rocky shores, the littorinid Cenchritis muricatus endures a near-terrestrial existence through a combination of active microhabitat choice and, during dry periods, repose. In this study, we sought to compare knobby periwinkle body size, thermal load, water loss, and stress protein expression among common supralittoral microhabitats to determine the physiological consequences of habitat selection. In this study, we show: (1) body temperatures in these snails exhibit daily fluctuations of more than 20°C and regularly exceed 46°C, (2) microhabitats differ in thermal stress over small spatial scales, with snails on black rocks and within crevices experiencing more extreme temperatures than snails on white rocks or grass, (3) water losses of 8.5% of total snail mass do not affect survival during 1 week, and (4) Hsp70, but not Hsp90, expression varies slightly among microhabitats but at a level much lower than physiologically possible. During arousal following hydration, snails exhibited substantially higher levels of Hsp70s than snails on dry substrates in the field. When inactive, Cenchritis appears to utilize a distinctly different physiological state consistent with aestivation metabolism and does not exhibit significant up-regulation of inducible heat shock proteins (Hsps). In summary, studies lacking detailed thermal and hydration history, and relying only upon Hsp levels, may misrepresent the true physiological consequences of microhabitat choice for high-shore tropical gastropods.

Water currents were experimentally manipulated for 3 months in a shallow subtidal sandflat through a series of nine channels with diverging, parallel, or converging walls (7 m long, 1.2 m high). Three treatments were chosen to bracket the ambient flow rates; vertically averaged velocities were reduced by 40%, reduced by 2%, or increased by 65%. Within the center of each channel, six Mercenaria mercenaria [shell length, 36.9±2.2 mm (mean ± 1 SD)] were placed haphazardly in 0.25‐m2 plots. Although water‐column chlorophyll a levels declined near the substratum, concentrations did not differ among channel flow treatments. Mean clam lengths increased 19% during the study, but growth rates did not differ significantly among channel flow treatments. These results suggest that the natural range of flow rates (up to 27 cm s−1), in the absence of local food depletion, does not alone alter growth of this active suspension feeder.