Dispersal of Platorchestia platensis (Kröyer) (Amphipoda: Talitridae) along Swedish coasts: A Slow but Successful Process

Abstract

The distribution of Orchestia gammarellus (Pallas) and Platorchestia platensis (Kröyer) (Amphipoda: Talitridae) along the Swedish coast was studied in 1995–96. The latter species had expanded its distribution since the 1960s both on the Swedish west coast and in the Baltic Sea. The survival of the two species in salinities between 0·5 and 51 was investigated at 12°C. Platorchestia platensis was found to endure lower salinities better than Orchestia gammarellus with reversed conditions in higher salinities. This would imply that the inner distributional limit for Platorchestia platensis in the Baltic has not yet been reached, or that other environmental factors are operating. Southerly currents which dominate for most of the year also hamper dispersal northward along the Swedish Baltic coast. The life-cycle of the two species was studied for 2 years both in allopatric and sympatric populations in the Baltic Sea. In the locality with sympatric populations the dominance of Platorchestia platensis had increased from tenths of a percent in an earlier study (1991) to 90–95% in 1997. The dominance in 1997 was significantly higher than in 1996. The breeding cycle of both species began simultaneously in late May but ended in August/September in Orchestia gammarellus, whereas gravid female Platorchestia platensis were found as late as November/December. In the latter species the generation that hatched in early summer became mature and reproduced the same autumn. Fecundity values were positively correlated with female body size and the maximum egg numbers were among the highest recorded for both species. Thus salinity does not affect brood size.