The induction of metamorphosis by environmentally derived cues is reviewed in barnacles, molluscs, hydroids, echinoids, and ascidians in the context of the neurological and behavioral model of stimulus and response. The model proposes that cues associated with preferred juvenile or adult habitats are the stimuli. Stimuli are received by receptors that communicate with the effectors of metamorphosis, larval and adult tissues. The response is a combination of morphogenetic, histolytic, and histogenic processes. Receptors in all five taxa are assumed to be superficial sensory receptors, though there is no direct evidence for their involvement in the perception of cues. Although the induction of metamorphosis by environmental cues in all five taxa fits well within a stimulus–response model, there is currently only circumstantial evidence for neural or endocrine control of metamorphosis.
A microbial perspective on the life-history evolution of marine invertebrate larvae: If, where and when to feed