Peptidergic Neurons in Barnacles: An Immunohistochemical Study Using Antisera Raised Against Crustacean Neuropeptides

Antisera raised against neuropeptides from decapod crustaceans were used to investigate whether balanomorph barnacles produce peptides analogous to those identified in some decapods. The distribution and structure of immunoreactive neurons was examined in Balanus balanus, Balanus perforatus, and Chirona (Balanus) hameri by whole-mount immunohistochemistry. In these species, no immunoreactivity was observed to antisera against CHH (crustacean hyperglycemic hormone), MIH (molt-inhibiting hormone), or RPCH (red-pigment-concentrating hormone), but neurons immunoreactive for pigment-dispersing hormone (PDH) and crustacean cardioactive peptide (CCAP) were observed. In all three species, PDH immunoreactivity was primarily associated with a pair of large (30-50 {mu}m diam.) anterio-ventral perikarya in the ventral ganglion, projecting prominent axons along the great splanchnic nerves, which branched extensively in the segmental splanchnic nerves, directing several arborizing dendrites to the somatic extensor muscles. Occasionally, three pairs of anterio-dorsal perikarya were observed, which projected fine ipsilateral and contralateral axons along the great splanchnic nerves. A further 12 pairs of perikarya, apparently segmentally arranged, were observed in the thoracic ganglion. Several PDH-immunoreactive perikarya and associated branching plexus were observed in the supra-esophageal ganglion. CCAP immunoreactivity was mainly restricted to the ventral ganglion, where three pairs of perikarya (ca. 30-50 {mu}m diam.) projected contralateral descending axons to the cirri. Occasionally a single pair of immunoreactive neurons were observed in the supra-esophageal ganglia. Although the anatomy of the CCAP-immunoreactive neurons in the ventral ganglion of barnacles might be homologous to conserved neural architectures in higher crustaceans, the anatomy of the PDH-immunoreactive neurons seems unique, and the morphology of the two large neurons in the ventral ganglion suggests a neuromodulatory role for this peptide, possibly associated with somatic extension.