An Inducer of Molluscan Metamorphosis Transforms Activity Patterns in a Larval Nervous System

Larvae of the nudibranch molluse Phestilla sibogae metamorphose in response to a small organic compound released into seawater by their adult prey, the scleractinian coral Porites compressa. The transformations that occur during metamorphosis, including loss of the ciliated velum (swimming organ), evacuation 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 should therefore elicit changes within the central nervous system. We used extracellular 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 damicornis, and to non-inductive seawater controls. The activity patterns within the nervous systems of semi-intact larvae changed in response to both types of coral exudates. Changes took place in two size classes of action potentials, one of which is known to be associated with velar ciliary arrests.