Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Kenneth M. Brown

Abstract

I examined how wave action and predators affect the feeding of the intertidal snail, Stramonita haemastoma. In laboratory wave simulators, the numbers of oysters eaten and tissue mass consumed were reduced compared to controls, but prey size selection was not affected. Handling times and profits (g dry tissue/handling time) of snails feeding on oysters did not differ between snails in wave simulators and controls. Over both treatments, small oysters reduced snail handling times but provided profits similar to that of large oysters. In contrast, field experiments showed that wave exposure was correlated with an increase in the number of oysters eaten, but not total tissue consumption. Electivity indices suggest that snails at exposed sites chose smaller prey. Thus, under laboratory conditions with continuous wave disturbance, feeding rates were reduced, but under field conditions snails maintained tissue consumption rates at exposed sites similar to that at protected sites and possibly lowered risk of dislodgement by switching to smaller prey with shorter handling times. The affect of predator presence on S. haemastoma foraging was examined using the Gulf stone crab, Menippe adina as a predator. In small-scale laboratory experiments, snail feeding rates were reduced by chemical or acoustical cues from stone crabs. However, small and large snail feeding rates were not differentially affected by crabs, even though small snails were more susceptible to predation. In laboratory experiments with free-ranging predatory crabs, snail feeding and growth were reduced because snails spent more time in refuges. Neither small nor large snails altered their feeding rates when exposed to crabs, but small snails spent more time in refuges. Small individuals may thus devote more time to feeding when exposed to crabs, perhaps to more rapidly reach a size refuge from predation. However, feeding by snails was reduced in field experiments only where crabs had direct access to snails. There was no measurable effect of chemical or auditory cues. Thus, reduced feeding in the field may occur only when predator abundances are high enough to cause frequent direct contact between predator and prey.

Pages

94

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