Chemical induction of settlement and metamorphosis in larvae of Manila clam Ruditapes philippinarum

Larval settlement and metamorphosis is a critical biological process governing the recruitment and population dynamics of bivalves, with profound implications for aquaculture sustainability. This study systematically assessed the effects of neurochemical inducers (Serotonin [5-HT], l-DOPA, acetylcholine, choline) and inorganic ions (K^⁺, Ca^²⁺, NH₄^⁺, Mg^²⁺) on larval settlement and metamorphosis in Manila clam Ruditapes philippinarum. Laboratory bioassays demonstrated that 5-HT acted as a potent settlement inducer, eliciting >65% settlement and metamorphosis at concentrations ranging from 10⁻² to 1 μM within 6-24 h. However, it exhibited acute toxicity at 10² μM, resulting in 100% mortality at 72 h. In contrast, l-DOPA showed transient efficacy, achieving 66.3% settlement and metamorphosis at 10 μM within 6 h but lack of induction effects at 48 h. Among ions, K^⁺ (30 mM) and Ca^²⁺ (30-50 mM) significantly enhanced larval settlement and metamorphosis, with the highest induction rates of 41% and 51% at 24 h, respectively, but the elevated concentrations resulted in increased larval mortality (≥ 37.7%). Ammonium chloride (which is usually present in both ionized and more toxic unionized forms, partly dependent on pH) displayed strong toxicity, causing 100% mortality at >20 mM concentrations within 48 h, while Mg²⁺ had the negligible effects on both settlement and survival. These findings highlight the inductive effects of chemical cues on larval settlement and metamorphosis in R. philippinarum, providing practical thresholds (e.g., 1 μM 5-HT with ≤24 h exposure) to optimize hatchery-rearing protocols in aquaculture.