Detection of APGWamide-like immunoreactivity in the sea scallop, Placopecten magellanicus

In pulmonate gastropods, the peptide, Ala-Pro-Gly-Trp-NH2 (APGWamide), appears to be located nearly exclusively in the neural circuitry controlling the male reproductive organs. This neuropeptide and related neuropeptides are also present and apparently bioactive in bivalve molluscs, although their physiological role in these latter animals is unknown. The present report uses immunohistochemistry to examine the distribution of APGWamide and/or related peptides within the tissues of the deep sea scallop, Placopecten magellanicus. Much of the APGWamide-like immunoreactivity (APGWa-LIR) was detected in the central nervous system (CNS) of both juvenile and adult scallops, where it was concentrated in the cerebral, pedal and parietovisceral ganglia, particularly in the cortex of ganglionic cells and their axons which extend into the central neuropilar region. APGWa-LIR was also detected in the nerves ramifying from these ganglia. In addition, strong APGWa-LIR was localized in what appeared to be axonal terminals within peripheral tissues including the striated adductor muscle, foot, gills, labial palps, lips, tentacles and gonads of the juvenile scallops. The presence of APGWa-LIR was also confirmed in the gonads of adults of both sexes. Cursory examinations of the CNS of the mussel, Mytilus edulis, and the oyster, Crassostrea virginica, revealed the presence of APGWa-LIR in cell bodies and processes in these bivalve species as well. It is concluded that APGWamide and/or related peptides are probably important neurotransmitters and/or neuromodulators of several central and peripheral functions in P. magellanicus and other bivalves. Future work must focus on the possible roles for APGWamide in the physiological processes of these and other bivalve species.