UBC Theses and Dissertations
Studies on the defensive role of shell ornamentation in the marine gastropod, Ceratostoma foliatum Danko, Jean Paul
The defensive role of shell ornamentation (varices) in the marine gastropod Ceratostoma foliatum Gmelin was investigated from the standpoints of: (1) physiological costs measured as heart-rate and oxygen consumption of crabs Cancer productus Randall when feeding on Ceratostoma, (2) measurement of force and work at the moment of structural failure in Ceratostoma crushed by an Instron device and by crabs, and (3) observations on behaviour (handling time and success rates) when feeding on Ceratostoma. Ceratostoma snails of two shell types were offered to crabs during trials: (1) varices-intact snails that had untreated shells, and (2) varices-removed snails that had their varices removed by grinding. Varices increased the total costs for crabs feeding on varices-intact snails. Heart-rates did not change significantly, but crabs consumed 1.4 times more oxygen per unit time when feeding on varices-intact snails. Total time required by crabs handling (manipulating and crushing) varices-intact snails increased with shell size with a slope 6=5.0, while handling time for varices-removed snails increased with shell size with a slope b=4.3. Shell-structure of varices-removed snails failed catastrophically soon after the first application of force by an Instron device. Shells of varices-intact snails did not fail after the first application of force; instead, small sublethal fractures occurred, delaying the failure of shell-structure by repeatedly undergoing deformations, and demanding 300% more work before failure. Shells of varices-removed snails failed 69% of the time at their body-whorl, while shells of varices-intact snails failed 80% of the time at their columella. Crabs selected prey randomly when the two prey treatments were presented together, instead of preferentially feeding on the "easier" varices-removed snails. Crabs feeding on varices-intact snails were unsuccessful in 68% of all attempts, while crabs feeding on varices-removed snails were successful in 76% of their attempts. In summary, the crushing of varices-intact snails is more costly for crabs as compared with the crushing of varices-removed snails because crabs increased their energy consumption and required more time to succeed. Unable to recognise the easier prey item, crabs suffered even greater energetic losses due to the extra time associated with a high percentage of unsuccessful attempts when feeding on varices-intact snails.
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