Desiccation of intertidal limpets: Effects of shell size,fit to substratum,and shape

For undisturbed, field populations of the northeast Pacific, intertidal limpet Collisella pelta (Rathke), water loss was found to be proportional to apertural circumference rather than mantle cavity surface area. This contrasts with previous laboratory measurements for other limpet species and emphasizes the importance of the fit of shell to substratum for non-homing limpets. This close fit maintained a meniscus of water between the edge of the shell and the substratum, thereby reducing evaporative surface area and increasing the advantage of large size for reducing the rate of water loss. Shell-raising behavior eliminated the meniscus, increasing the rate of desiccation, and water loss appeared to become proportional to mantle cavity surface area. Indirect evidence suggests that C. pelta may utilize shell-raising behavior for evaporative cooling under thermally stressful conditions in the field.Both shell size and shape affect the ratio of water stores (proportional to internal shell volume) to evaporative surface area (proportional to apertural circumference) and per cent water loss is, potentially, a function of this ratio. Shape, however, (when defined as volume/circumference) exhibits an average allometric increase with increasing size (volume) for C. pelta, as well as for three other sympatric limpet species: C. Paradigitalis (Fritchman), C. Digitalis (Rathke), and Notoacmea scutum (Rathke). An independent measure of shape is, therefore, required to separate shape effects on desiccation from size effects; this measure was obtained by rearranging the allometry equation. In contrast to significant size effects, no measurable effect of shape on desiccation was detected. Variation in limpet shell shape may be partially or wholly maintained by factors other than an adaptational response to desiccation.