Immunohistochemical FMRFamide-like distribution in the cyprid of the barnacle Balanus amphitrite

The presence and distribution of FMRFamide-like peptides (FLPs) in the cyprid of the barnacle Balanus amphitrite were investigated by immunohistochemical methods. FLPs immunoreactive (IR) neuronal cell bodies were detected in both central and peripheral nervous system. In the brain three contralateral neuron somata were immunodetected and numerous IR nerve fibers in the neuropil area and optic lobes were observed. An intense immunostaining was also observed in the frontal filament complex: frontal filament tracts leaving the optic lobes and projecting towards the compound eyes, the swollen nerve endings in the frontal filament vesicles and the thin nerve endings in the external frontal filament. Thin IR nerve fibers were also present in the cement glands. Two pairs of neuronal cell bodies were instead immunodetected in the posterior ganglion; some of their axons seem to project to the cirri. FLPs IR neuronal cell bodies were also localized in the wall of the dilated mid-gut and in the narrow hindgut; their processes surround the gut wall and allow gut neurons to synapse each others. Our data demonstrated the presence of FLPs in the barnacle cyprid; their distribution allows us to hypothesise for these peptides an integrator role in the central nervous system, neuromuscular functions for thoracic limbs and intestinal movements and a neurosecretory role, exerted by the frontal filament complex, in the settlement and molting.