Two cryptic species of the Hediste diversicolor group (Polychaeta, Nereididae) in the Baltic Sea, with mitochondrial signatures of different population histories

A presence of two cryptic biological species of Hediste diversicolor complex polychaetes was corroborated in a geographical survey of some 30 populations from the eastern and southern coasts of the Baltic Sea, with data from four completely diagnostic allozyme characters. Species A was dominant in the northernmost part of the Baltic Hediste range (Bothnian Sea), whereas Species B alone was found in the south (Poland, Germany, Denmark). In the intervening region, comprising the majority of the sites studied in southern Finland and Estonia, the two species were usually found together, with no evidence of recent hybridisation (i.e., no heterozygote genotypes). While mitochondrial DNA also distinguished the two taxa, it was not similarly completely diagnostic, but there were rare cases (ca 5%) of lineage mismatch indicating that some introgression has occurred in the past. Comparison with published data suggests that species A also inhabits the North Sea-NE Atlantic-Mediterranean coasts, and species B is also present in the North Sea and the NW Atlantic (Canada). Within the Baltic, the two species show distinctly different patterns of mtDNA diversity, plausibly related to different colonisation histories. Species A shows a generally high haplotype and nucleotide diversity, whereas in species B we found only four deeply diverged groups of closely related haplotypes. Hypothetically this could indicate a recent expansion of species B from a small number of colonising individuals. Moreover, species B showed marked intraspecific geographical structuring, with co-incident genetic changes along the N Estonian-S Finnish coasts both in mtDNA and an allozyme marker; this pattern suggests a contact between two genetically distinct invasion waves of different origins. In all, species A and B represent good, reproductively isolated and partly sympatric species which require to be recognised in ecological work. A formal taxonomical description is needed, but awaits better, range-wide distributional and ecological characterisation and working out of morphological differences that enable a practical identification.