Salinity as a barrier for ship hull-related dispersal and invasiveness of dreissenid and mytilid bivalves

The benthic stages of Dreissenidae and Mytilidae may be dispersed over long distances while attached to ship hulls. Alternatively, larvae may be transported by water currents and in the ballast and bilge water of ships and vessels. To gain insight into dispersal potential and habitat suitability, survival of the benthic stages of two invasive dreissenid species (Dreissena polymorpha and Mytilopsis leucophaeata) and one mytilid species (Mytilus edulis) chosen based on their occurrence in fresh, brackish and sea water, respectively, were tested in relation to salinity. They were exposed to various salinities in mesocosms during three long-term experiments at outdoor temperatures. Mussel survival was studied without prior acclimation, reflecting conditions experienced when attached to ship hulls while travelling along a salinity gradient from fresh or brackish water to sea water, or vice versa. Initially, mussels react to salinity shock by temporarily closing their valves, suspending ventilation and feeding. However, this cannot be maintained for long periods and adaptation to higher salinity must eventually occur. Bivalve survival was monitored till the last specimen of a test cohort died. The results of the experiments allowed us to distinguish favorable (f.: high tolerance) and unfavorable (u.: no or low tolerance) salinity ranges in practical salinity units (PSU) for each species, viz. for D. polymorpha 0.2-6.0 PSU (f.), 7.0-30.0 PSU (u.), for M. leucophaeata 0.2-17.5 PSU (f.), 20.0-30.0 PSU (u.) and for M. edulis 10.5-36.0 PSU (f.), 0.2-9.0 and 40 PSU (u.). At the unfavorable salinities, all mussels died within 14 days of initial exposure with the exception of M. edulis (23-30 days). The maximum duration of survival of single specimens of D. polymorpha was 318 days at a salinity of 3.2 PSU, of M. leucophaeata 781 days at 15.0 PSU and of M. edulis 1052 days at 15.0 PSU. The number of days survived was compared with the duration of actual ship voyages to estimate the real world survival potentials of species dependent of salinity changes, travel distances and durations. The conclusion is that salinity shocks during the trip were survived within the favorable salinity range but that the species tolerate only for a few weeks the unfavorable salinity range. This functions as a barrier for dispersal. However, at faster and more frequent shipping in the future salinity can become no longer very important as a dispersal barrier.