Serotonin-like immunoreactivity in the brachiolaria larvae of Patiriella regularis

Serotonin-like (5-HT-like) immunoreactivity was observed using confocal microscopy in fluorescently labelled brachiolaria larvae of the sea star Patiriella regularis. Prior to this larval stage, the bipinnaria of this species exhibits strong 5-HT-like immunoreactivity in the ciliated bands. As development progressed to the brachiolaria stage, immunoreactivity in the ciliated bands was reduced or absent. In brachiolaria, 5-HT-like immunoreactive cells and processes were conspicuous in the brachial arms, the structures which comprise the brachiolar attachment complex. As the brachia develop, immunoreactive cell bodies were clustered in association with the papillae, raised node-like structures on the surface of the brachia. Immunoreactive cells and axon-like tracts were also observed in the preoral ciliated band adjacent to the post brachia. The 5-HT-like immunoreactivity in the brachiolar complex of P. regularis represents a major change in the distribution of serotonergic-like cells from the ciliated bands of the bipinnaria stage to the attachment complex of the brachiolaria stage. This may indicate a change in the role of serotonin from one involved in feeding to one of a sensory capacity in the larval settlement process.