Induction of larval settlement and metamorphosis of Mercenaria mercenaria using excess Ca2＋and Mg2+alone or in combination

The induction effect of excess Ca 2+ and Mg 2+ alone or in combination on settlement and metamorphosis of Mercenaria mercenaria larvae were studied by initiating ion supplementation at two different developmental stages. Pediveligers were supplemented with a range of excess Ca 2+ (0–18 mM), Mg 2+ (0–40 mM) and Ca 2＋ / Mg 2+ (0–7.5/0–37.5 mM), respectively. Results showed excess Ca 2+ and Mg 2+ alone or in combination could induce settlement and metamorphosis of M. mercenaria . The o ptimal concentrations of Ca 2+ , Mg 2+ , and Ca 2+ /Mg 2+ were 10 mM, 20 mM and 3.125/15.625 mM, respectively. The supplementation of the above optimal concentration of ions from D-shaped larvae stage also significantly induced larval metamorphosis ( P < 0.05), and at the same concentration, the induction effect was better than that initiating from pediveliger stage. In addition, compared with the control group, ion supplementation from D-shaped larvae stage significantly increased the survival rate of pediveligers ( P < 0.05) and prolonged the planktonic duration. The longer the planktonic duration, the larger the larval size at the end of the planktonic stage and the higher metamorphosis rate of larvae. In the hatchery seed production of M. mercenaria , it is recommended to supplement 3.125/15.625 mM Ca 2＋ / Mg 2+ from the D-shaped larvae stage to induce the settlement and metamorphosis. • Ca 2＋ and Mg 2+ alone or in combination can induce settlement and metamorphosis. • The combined induction effect of two ions was better than that of a single ion. • Inductive effect of ion added from D-shaped larvae was better than from pediveliger. • Ion addition from D-shaped larvae prolonged larval planktonic duration. • The longer the planktonic duration, the higher the metamorphosis rate.