ASSESSING WHETHER LARVAE OF THE OPISTHOBRANCH GASTROPOD PHESTILLA-SIBOGAE BERGH BECOME RESPONSIVE TO 3 CHEMICAL CUES AT THE SAME AGE

Marine gastropod larvae must typically develop for a time in the plank ton before becoming competent to metamorphose in response to natural o r artificial chemical cues. In this study we asked whether larvae of t he nudibranch Phestilla sibogae Bergh become competent to respond to t hree chemical cues-natural cue extracted from the coral prey of the ad ult nudibranch, excess K+, and ethanol-at the same age, and conducted related studies to test hypotheses about the sites at which those thre e cues act. Larvae became competent to respond to natural inducer befo re they became responsive to either excess K+ or ethanol. Responses to the three cues also differed as follows: competent larvae took longer to respond to excess K+ or ethanol than to natural cue; agitation ten ded to increase the percentage of larvae metamorphosing in response to natural inducer but to inhibit the response to excess K+; precompeten t larvae habituated to natural inducer, but not to excess K+ or ethano l; larvae that had habituated to natural inducer nevertheless metamorp hosed in response to excess K+; natural inducer did not act synergisti cally in combination with excess K+; sensitivity to ethanol and excess K+ increased with larval age, so that older larvae responded to ethan ol concentrations that were too low to stimulate themetamorphosis of y ounger larvae, and responded more quickly to the single excess K+ conc entration tested. Taken together, the data support the hypothesis that excess K+ and ethanol act at sites different from those acted on by t he natural coral inducer, or act through different mechanisms. The dat a suggest that both ethanol and excess K+ act internally rather than d irectly on surface receptors, and show increasing accessibility to tho se internal sites as larvae age.