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Functional morphology of egg capsules in a marine gastropod Nucella emarginata Rawlings, Timothy Alexander


Egg capsules of the marine whelk Nucella emarqinata were examined with respect to intraspecific variation in capsule morphology and resistance to intertidal predators. Capsules were collected from three intertidal populations separated along a wave-exposure gradient. Wall thickness, dry weight, and protective quality of capsules differed extensively among these populations; however, these capsular properties did not reflect differences in wave-exposure levels. Capsule size was found to increase from wave-exposed to wave-sheltered shores and paralleled differences in snail size among sites. Laboratory experiments, and field tests incorporating predator-exclusion cages, were used to determine the extent of predation on Nucella emarqinata capsules and to identify capsule predators. Capsule cases were regularly eaten by intertidal predators such as shore crabs, Hemiqrapsus spp., and isopods, Idotea wosnesenskii. These invertebrates were responsible for opening a maximum of 32% of capsules present at two study sites. Despite differences in thickness and strength of capsules among populations, no capsules were completely resistant to these predators; however, thick capsule walls did appear to convey some protective advantage. Preliminary laboratory tests indicated that thin-walled capsules were preferentially opened by Idotea wosnesenskii.

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