An inducer of molluscan metamorphosis transforms activity patterns in a larval nervous system

Larvae of the nudibranch mollusc Phestilla sibogae metamorphose in response to a small organic compound released into seawater by their adult prey, th e scleractinian coral Porites compressa. The transformations that occur dur ing metamorphosis, including loss of the ciliated velum (swimming organ), e vacuation of the shell, and bodily elongation, are thought to be controlled by a combination of neuronal and neuroendocrine activities. Activation of peripheral chemosensory neurons by the metamorphosis-inducing compound shou ld therefore elicit changes within the central nervous system. We used extr acellular recording techniques in an attempt to detect responses of neurons within the larval central ganglia to seawater conditioned by P. compressa, to seawater conditioned by the weakly inductive coral Pocillopora damicorn is, and to non-inductive seawater controls. The activity patterns within th e nervous systems of semi-intact larvae changed in response to both types o f coral exudates. Changes took place in two size classes of action potentia ls, one of which is known to be associated with velar ciliary arrests.