AN ANALYSIS OF THE DEFENSIVE MECHANISMS OBSERVED IN THE ANEMONEANTHOPLEURA ELEGANTISSIMAIN RESPONSE TO ITS NUDIBRANCH PREDATORAEOLIDIA PAPILLOSA

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Abstract
1. The defensive mechanisms shown by the west coast, intertidal sea anemone, Anthopleura elegantissima, in response to its nudibranch predator Aeolidia papillosa are identified and evaluated in the context of the environment where A. elegantissima occurs. The defensive mechanisms include intertidal distribution, clone formation, alarm response, bulging of the column, crawling and releasing from the substrate. 2. A. papillosa are primarily located at the periphery of clones so that anemones in the interior of the clone have a refuge from predation. Assuming that cloning is an adaptation for space competition in A. elegantissima, then the additional advantage derived as a defensive mechanism should increase selection for clone formation. 3. A. papillosa was less able than A. elegantissima to withstand desiccation from exposure at low tide. This suggests that the intertidal distribution of A. elegantissima is a defensive adaptation which reduces the threat of predation by A. papillosa at least during the warmer months of the year. 4. The bulging of the column at the site of contact was found to be a localized response of several hours duration. The mucus of A. papillosa stimulated the response and the receptors were found to be situated in the column. Mucus from the coelenterate-eating aeolid nudibranch, Hermissenda crassicornis, also initiated the response while neither the mucus from the sponge eating dorid, Anisodoris nobilis, nor control swabs dipped in sea water caused bulging. 5. None of the defensive mechanism directly protects an anemone from attack by A. papillosa. The defensive mechanisms all interact to minimize damage to the clone until the predator is removed by desiccation and or wave action. This strategy is most effective during the 2-to 3-month period between when the veliger metamorphoses and when the nudibranch reaches sexual maturity. 6. This defensive strategy of killing the young nudibranch before it reproduces may negatively influence prey selection by the veliger stage. The evolution of the prey preference hierarchy of the veliger stage should be based on the criterion of survival to sexual maturity. Evidence for the effectiveness of this defensive strategy is that Aeolidia papillosa is primarily associated with the subtidal anemone, Metridium senile, one of the least preferred prey of adult nudibranchs.