The D P Wilson Lecture. Research on settlement and metamorphosis of marine invertebrate larvae: Past, present and future

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Abstract
In 1952, D P Wilson remarked that invertebrate larvae, “... are indeed not helpless, for they are endowed with the power of choice,...”; at settlement. Subsequent efforts have greatly enhanced understanding of the conditions under which larvae metamorphose, altering the conceptual view of this process from “larval choice”; to “developmental induction,”; and aligning studies of invertebrate metamorphosis with those of modern developmental biology. While the precise chemical nature of most metamorphic inducers evades us, elucidation of the phenomena of reception and transduction appears imminent for many larvae. The evidence for receptor cells and molecules specific for inducer substances is convincing for larvae in several phyla, and a generalized picture of stimulation of electrically excitable cells has emerged; larvae from at least eight invertebrate phyla metamorphose in response to augmented K + . To be determined is whether genes specifying receptor proteins for metamorphic inducers share homology and whether metamorphic signals are transduced by the same intracellular pathways across invertebrate phyla. Metamorphic competence can be a developmentally unique condition where non‐growing, non‐feeding larvae are primed to metamorphose when signaled; a genetic explanation for competence merits intense exploration. The acquisition for marine invertebrate larvae of knowledge comparable to that for the genetic basis of timing of photic responses and pupal eclosion in Drosophila will provide material for exciting research in the years ahead. Many species (e.g. the polychaete Hydroides elegans) offer excellent models for laboratory genetic studies of invertebrate settlement and metamorphosis.