The serotonin receptor 5-HTR modulates larval settlement and metamorphosis in Manila clam Ruditapes philippinarum

The Manila clam ( Ruditapes philippinarum ), as one of the most economically significant shellfish species in China, faces major challenges in seed production due to the high mortality during larval settlement and metamorphosis. 5-Hydroxytryptamine (5-HT) has been identified as a crucial neurotransmitter involved in regulating larval settlement and metamorphosis in molluscs. However, the underlying signaling mechanisms of its receptor (5-HTR) in molluscs remain poorly understood. In this study, the full-length cDNA sequence of 5-HT1R gene (1343 bp) is successfully cloned, encoding a protein of 393 amino acids. Bioinformatics analysis results reveal that the encoded protein has the typical characteristics of G protein-coupled receptors, including seven transmembrane domains and a molecular weight of 45.61 kDa. Quantitative real-time PCR (qPCR) results show that 5-HT1R expression in gill tissue of adult clams is significantly higher than that in other tissues, followed by adductor muscle and foot tissues. Whole-mount in situ hybridization illustrates its dynamic expression pattern throughout larval development, especially during larval settlement and metamorphosis. Prior to larval settlement and metamorphosis, 5-HT1R is predominantly localized in the apical sensory organ (ASO) of the larva, while the positive signals are progressively diminished during metamorphosis and eventually disappear after metamorphosis. Moreover, chemical induction experiments of larval settlement and metamorphosis using 5-HT agonists and antagonists have proved that 1 μM 5-HT treatment significantly enhances larval settlement and metamorphosis (60.9 %), whereas antagonists (clozapine) markedly suppress this process. These evidences indicate that 5-HT1R exerts the crucial regulatory function during larval settlement and metamorphosis of R. philippinarum . This study not only provides a theoretical foundation for elucidating the molecular mechanism underlying larval settlement and metamorphosis, but also offers potential targets for further investigation of 5-HT signaling pathway in bivalves. • The 5-HT1R gene was cloned in R. philippinarum , with bioinformatics analysis confirming its identity as a typical GPCR. • WISH revealed specific 5-HT1R expression in the larval apical sensory organ. • 5-HT significantly promoted (60.9 %) while its antagonist suppresses larval settlement and metamorphosis.