Barriers in the pelagic: population structuring of Calanus helgolandicus and C. euxinus in European waters

Molecular studies of marine plankton have shown that ecological and/or environmental barriers play an important role in separating populations. Calanoid copepods are central in marine ecosystems, and dramatic biogeographical shifts in copepod assemblages associated with recent climate warming have been reported. We examined spatial population structuring in European waters of the Atlantic Ocean and Mediterranean Sea of Calanus helgolandicus and its sister species, C. euxinus, from the Black Sea based on genetic and morphometric characters. The aims were to identify barriers to dispersal, relate these to hydrographic characteristics and infer historical patterns of distribution and demography. We analysed a 408 bp fragment of the mitochondrial 16S gene (316 individuals), prosome to urosome length relationships (212 individuals) and sea surface temperatures obtained from 19 European sites. Estimates of genetic differentiation between samples and hierarchical analyses of molecular variance indicated strong spatial population structuring between, as well as within, basins. We identified 7 phylogeographic groups: Fjords, Oceanic inflow, NE Atlantic/Tyrrhenian, Adriatic, Mljet Island, Aegean, and Black Sea, which explained 39.7% of the total genetic variation. Based on genetic data, C. euxinus is considered to be a differentiated population within the C. helgolandicus distribution range because the most important genetic barrier separates western and eastern Mediterranean populations. Morphometric barriers largely reflect sea surface temperature barriers and are not congruent with the main genetic barriers. Contrary to recent findings for C. finmarchicus, we conclude that C. helgolandicus/C. euxinus populations are not connected by high levels of dispersal and have been vulnerable to past climatic changes.