Neural correlates of settlement in veliger larvae of the gastropod, Crepidula fornicata

Settlement behavior of molluscan veliger larvae prior to metamorphosis requires cessation of swimming, accomplished by arrest of prototrochal cilia on the margin of the velum (the larval swimming organ). Ciliary arrest in larvae of gastropods is mediated by an action potential that occurs synchronously across the velum as a consequence of electrical coupling between the prototrochal ciliated cells. We developed a preparation for extracellular recording of such ciliary arrest spikes from intact swimming and crawling veliger larvae of the caenogastropod Crepidula fornicata, using a fine wire electrode. Ciliary arrest spike rates during bouts of substrate crawling were significantly higher than those recorded during preceding swimming periods in larvae that were competent for metamorphosis, but not in precompetent larvae. Spike rates were similar on clean polystyrene substrates, and on substrates that had been coated with a natural cue for metamorphosis (mucus from conspecific adults). We used immunohistochemical methods to localize neuromodulators that might regulate the function of velar cilia. Labeled terminals for serotonin, FMRFamide, and tyrosine hydroxylase (an enzyme for catecholamine synthesis) were located in positions consistent with modulatory effects on the prototrochal ciliated cells. Prototrochal ciliary arrest spike rates and beat frequencies were measured in isolated velar lobes from competent larvae, which were exposed to serotonin, FMRFamide, and dopamine (105molL1). Serotonin abolished arrest spiking and increased beat frequency; dopamine also increased beat frequency, and FMRFamide depressed it. Competent larvae tested in a small static water column swam to the top of the column when exposed to serotonin, but occupied lower positions than controls when in the presence of dopamine and FMRFamide. The larval nervous system appears to regulate velar functions that are critical for settlement behavior, and is likely to do so by integrating different sensory modalities in an age-dependent manner.