Settlement and Growth of Mytilus galloprovincialis Pediveliger Larvae in Response to Biofilm-Based Microalgae and Chemical Neuroactive Compounds

The sustainability of mollusc aquaculture relies, in part, on overcoming the challenges of spat production in captivity, particularly during the metamorphosis and settlement stages. The optimization of rearing technologies at these stages would ensure possible solutions for sustainably producing mollusc spat while simultaneously improving stock performance. The current work represents a large-scale trial examining the effect of biological and chemical inducers on larval settlement in <i>Mytilus galloprovincialis</i>. For this purpose, one batch of pediveliger larvae was directly transferred to settlement on microalgae-based biofilm (mature cylinders), while another batch was pretreated with gamma-aminobutyric acid GABA (10^-4 M, 10^-5 M and 10^-6 M) and potassium chloride KCl (20 mM and 30 mM) according to two different exposure times (6 h and 24 h), before being transferred for settlement (immature cylinders). The impact of different treatments on larval performance was evaluated in terms of larval settlement rate (<i>Sr</i>), post-larval growth rate (<i>Gr</i>), and spat production rate (<i>Pr</i>). The biofilm treatment had the highest settlement rate and spat production (<i>Sr</i> = 65% and <i>Pr</i> = 46.4 spat/cm²) compared to chemical treatments. The highest settlement rate among chemical treatments occurred under short exposure times (6 h) to low GABA concentrations, i.e., <i>Sr</i> 40% and 45% at GABA 10^-5 M and 10^-6 M, respectively). GABA and KCl treatments ensured a faster post-larval growth rate than the biofilm, i.e., 15.54 ± 7.67 µm/day, 18.26 ± 9.39 µm/day, and 11.35 ± 6.73 µm/day, respectively, while control trials showed the lowest growth rate (6.80 ± 4.39 µm/day). These findings reveal a key trade-off: biofilm is the most effective measure for promoting spat production, while a targeted use of GABA and KCl at short exposure times (6 h) appears to significantly enhance post-larvae growth.