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We investigated the effect on Mytilus galloprovincialis larval settlement, as well as the toxicity, of serial concentrations in filtered seawater of acetylcholine (AC), γ-aminobutiric acid (GABA); 3-isobutyl-1-methylxanthine (IBMX); and the potassium ion in the form of potassium chloride (KCl) and potassium sulfate (K2SO4). All the substances assayed induced larval settlement and peak responses were above 90% in exposures to 10− 2 mol L− 1 (M) AC, 10− 4 and 10− 5 M epinephrine, 10− 3 M GABA and 20, 30 and 40 mM KCl. The optimal concentration of K+ varied depending on the anionic component of the compound assayed, and peak settlement response to KCl was higher (100%) than that achieved with K2SO4 (69.7%). The estimated LC50 of the compounds assayed ranged from 9.4 × 10− 6 M (GABA) to 3.1 × 10− 2 M (KCl). GABA, IBMX and K2SO4treatments displayed toxic effects in all the active concentrations. In contrast, AC 10− 5 M, epinephrine 10− 4 and 10− 5 M and KCl 20 mM treatments enhanced larval settlement without an acute short-term effect on mortality. These results provide new insights on the molecular mechanisms controlling settlement in M. galloprovincialis larvae, and yield promising outcomes for the mussel industry to find a reliable method to enhance larval settlement in hatcheries. Highlights► We study the effect of neuroactive compounds on mussel larval settlement. ► The larval response to K+ vary depending on the compound trialed (KCl or K2SO4). ► GABA, IBMX and K2SO4 display toxic effects in all the active concentrations. ► AC, epinephrine and KCl can induce larval settlement without acute toxic effect.

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Interest of mussel hatchery development has increased during the last years as natural seed collection does not always satisfy the demand of producers. Success on larval bivalve culture depends on several factors such as diet, temperature or stocking density. In this study, the effect of larval density in mussel culture success has been assessed testing three larval densities: high (50 larvae mL À1 ), medium (25 larvae mL À1 ) and low (5 larvae mL À1 ). Results showed higher growth and survival at low density but the amount of postlarvae was low. The highest number of settled larvae was observed at high density but mortality was also high and a delay on the larval development was detected producing a prolongation on the culture length. However, medium density resulted in a good survival and growth and also in a considerable amount of postlarvae, as this density could be adequate for massive mussel seed production. Cortisol content on the larvae and postlarvae was also analysed and significant differences were obtained among groups probably due to the stress associated to settlement and metamorphosis.

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Settlement responses were investigated for mussel (Perna canaliculus) larvae after exposure to catecholamines and their precursor metabolites. Settlement and mortality assays were conducted in Petri plates with chemical treatments (L-phenylalanine, L-tyrosine, L-DOPA, dopamine hydrochloride and epinephrine at various concentrations) and controls. 15The proteinogenic amino acids L-phenylalanine and L-tyrosine both were effective inducers (~65%) of larval settlement at 10 −5 mol L −1 compared with controls (4%). Exposure of larvae to L-DOPA, dopamine and epinephrine resulted in maximum settlement induction (50, 60 and 51%, respectively) at 10 −5 mol L −1 . Larval mortalities were low (comparable to controls) across all concentrations of L-phenylalanine and L-tyrosine treatments whereas high mortalities (>60%) were observed for L-DOPA, dopamine and epinephrine at concentrations ≥ 10 −4 mol L −1. Our results indicate that P. canaliculus 20 larval settlement is under endogenous regulation by a catecholaminergic mechanism. Furthermore, the inductive effects of all tested metabolites in the epinephrine biosynthesis pathway point to a putative involvement of adrenergic-type receptors in regulation of larval settlement in this mussel species.

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