Assessing whether larvae of the opisthobranch gastropod Phestilla sibogae Bergh become responsive to three chemical cues at the same age

Marine gastropod larvae must typically develop for a time in the plankton before becoming competent to metamorphose in response to natural or artificial chemical cues. In this study we asked whether larvae of the nudibranch Phestilla sibogae Bergh become competent to respond to three chemical cues-natural cue extracted from the coral prey of the adult nudibranch, excess K +, and ethanol-at the same age, and conducted related studies to test hypotheses about the sites at which those three cues act. Larvae became competent to respond to natural inducer before 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 tended to increase the percentage of larvae metamorphosing in response to natural inducer but to inhibit the response to excess K +; precompetent larvae habituated to natural inducer, but not to excess K + or ethanol; larvae that had habituated to natural inducer nevertheless metamorphosed in response to excess K +; natural inducer did not act synergistically in combination with excess K +; sensitivity to ethanol and excess K + increased with larval age, so that older larvae responded to ethanol concentrations that were too low to stimulate the metamorphosis of younger larvae, and responded more quickly to the single excess K + concentration tested. Taken together, the data support the hypothesis that excess K + and ethanol act at sites different from those acted on by the natural coral inducer, or act through different mechanisms. The data suggest that both ethanol and excess K + act internally rather than directly on surface receptors, and show increasing accessibility to those internal sites as larvae age.