HADFIELD, M.G.: Metamorphic competence is a major adaptive convergence in marine invertebrate larvae

Larvae from diverse and unrelated marine-invertebrate phyla are able to rapidly respond to environmental cues to settlement and to undergo very rapid metamorphogenesis because they share the developmental trait of metamorphic competence. This developmental state, characteristic of larvae as diverse as those of cnidarian planulae, molluscan veligers, and barnacle cyprids, is one in which nearly all requisite juvenile characters are present in the larva prior to settlement. Thus metamorphosis, in response to more or less specific environmental cues (inducers), is mainly restricted to loss of larva-specific organs and physiological processes. Competent larvae of two “model marine invertebrates” studied in the author’s laboratory, the serpulid polychaete Hydroides elegans and the nudibranch Phestilla sibogae, complete metamorphosis in about 12 and 20 hrs, respectively. Furthermore, little or no de novo gene action appears to be required for the settlement and metamorphosis response in these species. Contrasting greatly with the slow, hormonally regulated metamorphic transitions of vertebrates and insects, rapid metamorphosis in marine invertebrate larvae is conjectured to have arisen in diverse phylogenetic clades as a response to common environmental pressures that favor extremely fast transition from larval locomotory and feeding modes, adapted to life in the plankton, to a different set of such modes, adapted to life on the sea bottom.