| This study addresses the role of benthic algae and their metabolites in the substratum-specificity of settlement and metamorphosis on planktonic larvae of the gastropod mollusc, Haliotis refescens. Crustose red algae were shown to induce substratum-specific settlement, attachment and metamorphosis of H. rufescens larvae; other algae were all inactive. Induction is dependent on direct contact of the larvae with the surfaces of any of a number of crustose red algal species of worldwide distribution. The substratum-specificity of larval induction was shown to result from the unique availability of inducing molecules in non-diffusable form at crustose red algal surfaces, apparently as a result of unique surface properties of these algae. Larvae were not induced by surface contact with foliose red macroalgae nor intact cyanobacteria; they were only slightly induced by surfaces of geniculate red algae; however, cell-free extracts of all of these species contained potent inducers associated with macromolecules.;The biological activity of these inducers with Haliotis larvae was mimicked by the mammalian neurotransmitter, (gamma)-aminobutyric acid (GABA). Conversely, it was demonstrated that the LMW inducer from S. platensis mimics the action of GABA in binding specifically to GABA receptors partially purified from mouse brain.;Chromatographic and biochemical analyses of inducers from the crustose red alga, Lithothamnium californicum, the foliose red alga, Porphyra sp. and the cyanobacterium, Spirulina platensis revealed that the inducers could be resolved from proteins as a family of related low molecular weight (LMW) molecules with similar inducing activities and compositions. Results suggest that: (a) LMW inducers are non-covalently linked to proteins; (b) they are polar, containing at least one ionizable group protonated at pH 2; and (c) they are small (ca. 1,000 dalton) peptides containing several unusual amino acids thought to be associated with activity. The most active inducer found was the unhydrolyzed molecule from the recruiting alga, Lithothamnium californicum, purified by reverse-phase HPLC. |
