Settlement requirements of molluscan larvae: New data on chemical and genetic roles

Abstract

Recent studies in the author's laboratory and elsewhere indicate that a high degree of substratum chemical specificity is necessary to induce settlement and metamorphosis in a number of molluscan species. The chemical nature of natural and artificial inducers, as well as the nature of inducer-larval interactions, provides insights into both stimulus and response aspects of the metamorphic process. Larvae of Phestilla sibogae undergo rapid and complete metamorphosis in response to a soluble, coral-produced substance. These larvae undergo slow but complete metamorphosis in response to choline, GABA, and some related compounds; they exhibit partial metamorphosis in the presence of certain catecholamines. Reversible inhibition of metamorphosis (habituation) occurs when larvae are exposed to an inducer prematurely. Analysis of these observations allows the formulation of new models for molluscan metamorphic induction. It may be that a single model will not be appropriate for all marine molluscs. Both age at metamorphic competence and concentration of natural inducer necessary to elicit metamorphosis in competent larvae of Phestilla are known to show wide variance among larvae. Long-term selective inbreeding experiments have failed to indicate obvious genetic components of the observed variances. High variance in age at competence occurs even among the egg masses of a single, highly inbred individual with a single mate.