Competent larval behaviour and settlement responses to adult conspecific cues of the horse mussels Modiolus modiolus and Modiolula phaseolina (Mytiloidea)

Larval settlement in response to conspecific adult pheromonal cues guides population connectivity and recruitment in many benthic invertebrates. However, evidence for this mechanism in bivalves, except for oysters and dreissenids, is limited. The evolution of such functional behaviour would have strengthened formation and maintenance of marine mussel aggregations of which several species and habitats are now considered threatened and in need of restoration. The two modioline bivalves, Modiolus modiolus and Modiolula phaseolina , form biogenic habitats in the subtidal. Experiments were conducted in vitro to test competent larval phototaxis, sinking rates, swimming behaviour and settlement responses to conspecific adult conditioned seawater (ACS) with pheromonal cues. Larvae of both species were maintained in ambient, aged seawater (ASW). Prior to settlement, the larvae became photonegative. Modiolus modiolus larvae sinking rates varied between 4.75 and 6.75 mm sec −1 , depending on larval size and seawater temperature. Competent pediveliger larvae initially required higher concentrations of ACS to induce settlement completed by metamorphosis. The odds of larval settlement were 8 to 85 times higher with exposure to higher ACS concentrations of compared to lower concentrations or absence of ACS. Low concentrations of ACS altered larval swimming behaviour but did not induce settlement completed by metamorphosis, and this is thus a threshold response. This first documentation of mytiloidean larval settlement in response to conspecific adult pheromonal cues, may play an integral role in the metapopulation dynamics of these species. Population declines could weaken plumes of pheromonal cues excreated from adults and, thereby, larval settlement and recruitment. This may explain, in part, the low recoverability of previously long-standing M. modiolus habitats after cessation of anthropogenic physical disturbance, such as bottom trawling. Application of these results may enable high metamorphosis and juvenile survival rates in aquaculture production and guide restocking efforts to enhance in situ recruitment and, thereby, support horse mussel habitat restoration in coastal waters. • Competent pediveligers expressed photonegative behaviour. • Shell morphology aids non-swimming larvae in descending to the seabed. • Mytiloidean larvae settle preferentially in response to conspecific adult pheromonal cues. • Larval settlement response induced by conspecific adult pheromonal cues is a threshold response. • Odds of settling was 8 to 85 higher after exposure to higher concentration of conspecific adult pheromonal cues.