Sexual differences in serotonin distribution and induction of synchronous larval release by serotonin in the freshwater mussel Hyriopsis bialatus

Abstract

We have established an in vitro glochidial culture of the freshwater mussel, Hyriopsis (Hyriopsis) bialatus, with the potential of restoring natural populations. However, to develop a successful in vitro culture process the synchronous release of glochidia needs to be achieved. In this study we investigated the distribution of serotonin and its effect on inducing synchronous parturition. Using confocal microscopy on whole-mounted tissues, we show that serotonin is highly expressed in the peripheral neuron somata and their axonal processes in the central neuropils of the visceral ganglion. Serotonergic nerve fibres were also detected along the branchial nerve innervating the demibranchs. A hierarchical ramification from the branchial cord into nerve rootlets and subsequently into the small nerve fibres within the demibranch epithelium was evident throughout this organ. In both visceral ganglia and demibranchs, immunoreactivity of serotonin was relatively higher in female than male tissues, suggesting a role of serotonin in female reproductive functions. Administration of serotonin to the demibranchs, the sites of larval settlement, stimulated synchronous parturition in a dosedependent manner. A single-dose injection of serotonin yielded a large number of released larvae within 1–2 h, with no adverse effect on the quality of cultured larvae.