Living at the frontier - salinity and temperature during larval development determine the potential for invasion of the Baltic Sea

Abstract

We study the potential of a recent invader, the Asian brush-clawed crab (Hemigrapsus takanoi), to expand its distribution range further into the Baltic Sea. H. takanoi has been documented in the southwestern Baltic Sea since 2014. The ability to persist and further expand into the Baltic Sea will depend on their potential to sustain all stages of their complex life cycle, including pelagic larvae, under the Baltic Sea’s conditions. Range limits may be established by the tolerance to low salinity, which in addition may be affected by temperature. A key question is whether local populations at the distribution limit (within the Baltic) are adapting to low salinities and hence promote further expansion. We quantified the combined effects of salinity (10-32 PSU) and temperature (15-24°C) on four populations of H. takanoi (two from the Baltic and two from the North Sea). Our results show substantial differences in larval performance between the populations from the Baltic and North Sea. Larvae from the North Sea populations show higher survival and faster development compared with those from the Baltic Sea. No evidence of elevated tolerance towards low salinity were found in the larvae from the Baltic Sea. In addition, larvae from the population located at the range limit (Neustadt) show very low survival. We did not find evidence of adaptation to low salinity by any of the local populations of H. takanoi we studied. Thus, in order to the population to persist, larvae must either migrate to waters of increased salinity, and then return to the habitat of origin; or such populations are sustained by source-sink dynamics.