Settlement success, spatial pattern and behavior of mussel larvae Mytilus spp. in experimental 'downwelling' systems of varying velocity and turbulence

This paper presents results of the effect of water flow in experimental 'downwelling' systems on settlement success, settler distribution and behavior of Mytilus spp. larvae. The hypothesis of active habitat selection in relation to hydrodynamics was tested by direct observation of larvae during settlement using an endoscope camera and the comparison of distribution of settled larvae with that of inert particles mimicking passive behavior. The downwelling system consisted of cylinders in which water inflow was controlled. Six water flows, from 0 to 50 1 h(-1), were used in quadruplicate. An acoustic Doppler velocimeter (ADV) was used to measure velocity fluctuations in 3 dimensions along 4 transects per experimental unit. Settlement success was positively correlated with centrifugal and advective flow velocity and turbulence measured immediately above the mesh bottom. Mussel larvae actively selected microhabitats characterized by low shear stress and high advection when water flow in the cylinders ranged from 0 to 40 1 h-1. At higher water flow, mussel larvae were distributed similarly to passive particles and are apparently unable to overcome the flow regime, thus limiting their ability to exhibit settlement preference. In this study, total conformity between settlement and inert particle patterns was never reached, suggesting that settlement was, to a certain extent, under behavioral control.