Genetic species identification and low genetic diversity in Pseudocalanus acuspes of the Baltic Sea

Calanoid copepods of the genus Pseudocalanus are key species in temperate-boreal marine pelagic ecosystems. Pseudocalanus species are difficult to distinguish morphologically and there is uncertainty regarding the species present in the Baltic Sea. In this study, we investigated the species composition of Pseudocalanus in the Baltic proper and the Gulf of Finland using a restriction fragment length polymorphism approach. Screening of 888 individuals from 13 different stations, sampled during various seasons on a total of 22 different occasions stretching from November 2006 until July 2008, confirmed that P. acuspes is the only Pseudocalanus species normally present in the Baltic Sea. Mitochondrial diversity is exceptionally low in Baltic Sea populations of P. acuspes, as only two cytochrome oxidase I haplotypes were observed in samples ranging from the Gulf of Finland in the north to the Arkona Basin in the south (83 individuals). This unusually low level of genetic diversity indicates that planktonic organisms may experience loss of genetic diversity in marginal Baltic Sea populations, despite large population sizes. Low genetic diversity may negatively impact the species' capacity for adaptation to environmental change.