Larval recruitment of the blue mussel Mytilus edulis: The effect of flow and algae

The mussel Mytilus edulis settlement and distribution was studied on plastic panels with manipulated flow regime (faired, bluff, split and angled) with or without water soluble metabolites of the green alga Cladophora rupestris. The panels were exposed vertically on a device (hydrovane) that ensures their constant orientation in the current during the peak of larval settlement at 1 m depth. In order to investigate larval distribution on the panels, half of them were coated with a silicone vacuum grease that prevents larvae from de-attachment. This grease was not toxic and did not attract or repel larvae. Low densities of larvae on the un-greased plates compared to the greased ones suggested that some of larvae left the substratum. The blue mussel larvae initially settled in regions of reduced shear velocity and then redistribute to the regions of high shear velocity. The presence of the alga increased the density of blue mussel larvae and changed their distribution on the panels. Overall, our results demonstrated that larval recruitment of M. edulis is an active process affected both by boundary-layer hydrodynamics and algal waterborne compounds.