Return to sender: The influence of larval behaviour on the distribution and settlement of the European oyster <scp><i>Ostrea edulis</i></scp>

Abstract Benthic marine invertebrates, such as oysters, rely on larval recruitment for their populations to persist. This can be by self‐recruitment to the natal population or recruitment from geographically distant populations. Marine invertebrate larvae are increasingly understood to influence their dispersal through vertical migrations, based on a combination of responses to external cues and the larvae's ontogenetic stage. This study examined the larval behaviour of the European oyster Ostrea edulis in laboratory experiments. The aim was to establish if larvae show systematic behaviour that could affect dispersal. Vertical distribution, swimming speeds, and behaviour of O. edulis larvae were quantified throughout their ontogenetic development, and under scenarios of light/dark, food/no food, and two temperatures. Most O. edulis larvae concentrated at the bottom of the aquarium, independent of developmental stage or treatment, and consistently over time. Larvae behaved actively in ~50% of all bottom observations, indicating a behavioural function other than resting. At the surface, larvae frequently formed aggregations. In the water column, larvae swam with high vertical directionality and their distribution was homogenous. Swimming speeds ranged from 0.001 to 9.07 mm s −1 . Advection close to the seabed is slower than in any other part of the water column. The demersal preference of O. edulis may be targeted towards increasing the likelihood of self‐recruitment, which is consistent with the larvae's preference to settle in the presence of conspecifics. Stronger hydrodynamic environments are likely to override the larvae's demersal behaviour. It is recommended to restore European oyster beds at sufficient scale, density, and rugosity to promote retention of larvae within the natal population and minimize larval loss and mortality, as well as to account for the observed behaviours in networks of restoration sites.