Podial sensory receptors and the induction of metamorphosis in echinoids

Larvae of Strongylocentrotus droebachiensis (Müller), Lytechinus pictus (Verrill), and Lytechinus variegatus (Leske) which are competent to metamorphose display what appears to be substratum-testing behavior prior to metamorphosis. Larvae cease swimming, partially evert the adult rudiment, and walk about examining the substratum with their five primary podia. Larvae eithe r metamorphose or withdraw their podia and resume swimming to settle again elsewhere. Scanning and transmission electron microscopic examinations of the primary podia revealed sensory receptor cells on the rim and on a conical projection at the center of the podial sucker. Each sensory cell has a single short cilium on its apical surface a axonal process at its base which contributes to the basiepithelial nerve plexus. Mature adults of the same species also have comparable sensory structures on their tube feet suckers. It is suggested that the sensory receptors on the primary podia of setting larvae, although they are not specialized larval structures, may be involved in the perception of tactil e stimuli which have been previously demonstrated to be involved in the induction of metamorphosis.