Chemical Induction of Metamorphosis in Ruditapes decussatus and Venerupis corrugata: Concentration and Time-Dependent Effects of Epinephrine and GABA

Metamorphosis is a critical bottleneck in the bivalve life cycle, usually associated with high mortality and asynchronous development under hatchery conditions. Synthetic chemical inducers are widely used to overcome these constraints. The neuroactive compounds epinephrine (EPI) and γ-aminobutyric acid (GABA) have been reported to be effective metamorphosis inducers in bivalve species. This study evaluated the inductive efficiency and toxicity of EPI and GABA in two economically important clam species, Venerupis corrugata and Ruditapes decussatus. Competent larvae were exposed to different concentrations (10−2, 10−4 and 10−6 M) for 24, 48 and 72 h. Metamorphic progression and mortality were assessed immediately after exposure and following a six-day post-exposure period. Both compounds induced metamorphosis following concentration–time and species-dependent responses, with narrow efficiency windows and pronounced delayed mortality at higher concentrations and/or longer exposure durations. In V. corrugata, the highest metamorphosis rates were obtained with both compounds at 10−6 M after 48 h. In contrast, R. decussatus required higher EPI concentrations (10−4 M for 48 h) for optimal induction, while GABA at 10−6 M was effective after 24 and 48 h. These findings highlight the need for post-exposure assessments and species-specific optimisation of chemical induction protocols to improve metamorphosis success and spat production in bivalve hatcheries.