How do currents influence the settlement of Zebra Mussels (Dreissena polymorpha) in the Szczecin Lagoon?

Unlike most freshwater bivalves, zebra mussels (Dreissena polymorpha PALLAS) have free swimming larvae and are thus dependent on water currents to guarantee the spreading of their offspring. Although planktonic, the larvae are able to carry out some directed movements. This paper presents investigations carried out in the Szczecin Lagoon, testing the settlement of larvae on different materials at three sites. Zebra mussel larvae preferred the central part of the lagoon for settlement and favoured horizontal over vertical orientation. The rank order referring to number of settled mussels was net > wood > stones > mussel shells > PVc. Water current velocity in the crucial phase was usually very low (<10 cm/s), i.e. not above the limit for settlement. Therefore, other parameters must be involved as well. Two factors are discussed in particular: 1) salinity, and 2) sensory capacities of larvae, linked to the question of their capability of choosing where they settle.