Evaluating the functional involvement of CALM and ATP2B in 5-hydroxytryptamine-induced larval metamorphosis in...

Larval settlement and metamorphosis are critical developmental transitions in the macrofouling organisms, directly impacting population recruitment. Although 5-hydroxytryptamine (5-HT) effectively induces these processes, the downstream molecular mechanisms, particularly the role of intracellular Ca²⁺ homeostasis, in larval settlement and metamorphosis of Mytilus coruscus remain poorly understood. In this study, we investigated the roles of the calmodulin gene (CALM) and the plasma membrane Ca²⁺-ATPase gene (ATP2B), in 5-HT induced larval settlement and metamorphosis. Compared to pediveliger larvae, the expression of both CALM and ATP2B showed significant differences after metamorphosis. In 5-HT exposed larvae, CALM and ATP2B were up-regulated by 41.63% and 33.52%, respectively, whereas a CALM knockdown subsequently down-regulated CALM and up-regulated ATP2B. Knockdown of the CALM gene significantly inhibited the inductive effects of 5-HT on larval settlement and metamorphosis, with a similar reduction by ATP2B knockdown. The 5-HT-induced Ca²⁺ fluctuations were resolved over time in ATP2B-deficient larvae, but persisted in CALM-deficient larvae. Collectively, our findings identify CALM as the key mediator of 5-HT-induced larval metamorphosis by reciprocally regulating ATP2B and Ca²⁺ homeostasis. This knowledge of larval metamorphosis regulation is crucial for explaining macrofouling settlement and for advancing biofouling control technologies by providing new targets.