Localization of a SALMFamide Neuropeptide in the Larval Nervous System of the Sand Dollar Dendraster excentricus

Using immunocytochemical methods we describe the localization of serotonin and the SALMFamide peptide, S1 (GFNSALMFamide), during embryonic and larval development of the echinoid Dendraster excentricus. Anti‐SI immunoreactivity first appears in the apical ganglion in late gastrulae at the same time as anti‐serotonin immunoreactivity. Initially, anti‐S1 immunoreactivity is restricted to fibres of the neuropile, but in later feeding stages, cell bodies are also immunoreactive. Anti‐S1 immunoreactivity appears as 2–4 cells in the oral ganglion of early prism stage larvae, whereas anti‐serotonin immunoreactivity does not occur in the oral ganglion until the 8‐arm stage. Anti‐S1 immunoreactivity also occurs in diffuse fibres in the oesophagus and in a single fibre encircling the pyloric sphincter of the gut. A reticular network associated with the apical surface of the epithelial cells of the vestibule of the adult rudiment was anti‐S1 immunoreactive. In double‐labelling experiments, anti‐serotonin and anti‐S1 immunoreactivity co‐localize in the neuropile of the apical ganglion. The distribution of S1, in association with putative sensory cells in the apical and oral ganglia and with muscles of the oesophagus and gut, suggests S1 may have diverse functions in the larval nervous system. The distribution of anti‐S1 immunoreactivity in echinoid embryos and larvae supports the proposal that SALMFamide‐like peptides are widely shared in echinoderms and potentially have a fundamental role in neural function.