Localization of serotonin in neuronal ganglia of the freshwater pearl mussel, Hyriopsis (Hyriopsis) bialata

We investigated the histological features of neuronal ganglia and the localization of serotonin in the unionid mussel Hyriopsis (Hyriopsis) bialata in Thailand. The long-term goal of this study is to apply endocrine manipulation to enhance the synchronous release of larvae for in vitro culture of this freshwater mussel and to increase larval survival during settlement. Unlike the case in other bivalves, the cerebropleural ganglia in H. bialata were completely fused to form a single pear-shaped structure, and the pedal ganglia were situated within the visceral mass rather than in the foot. In all three ganglia of this mussel, the neuronal cell bodies were clustered at the periphery of the ganglia, and the nerve fibres were bundled in the central region. Of the three types of cells present in the ganglia, the large neurons were the only ones to stain with paraldehyde fuchsin, indicating a neurosecretory function. These same neurons were also intensely reactive with anti-serotonin, demonstrating their dual function. An intense staining of anti-serotonin was also observed in the nerve fibres and termini, particularly in the visceral ganglia. This suggests that serotonin may be conveyed from the large neurons within the visceral ganglion via an axonal route to reach the target reproductive organs, one of which is the demibranch, to stimulate larval release in this mussel.