Canals and invasions: a review of the distribution of Marenzelleria (Annelida: Spionidae) in Eurasia, with a key to Marenzelleria species and insights on their relationships

Recent invasions of the North and Baltic Seas by three Marenzelleria species have extensively altered benthic communities in the region. Despite several studies on the morphology and biology of the worms, their morphological identifications are often challenging. Here we summarize and map the available records of Marenzelleria from Eurasia, distinguishing those based on morphology versus molecular data. Based upon the genetic similarity ( p = 0.08% for COI ) between individuals from the Baltic Sea and individuals from the Barents and White Seas we propose, for the first time, a possible route for the invasion of the Baltic Sea by M. arctia from the White Sea through the White Sea–Baltic Sea Canal. At the same time, our analysis of the sequences of COI fragments showed a significant genetic distance ( p = 4.28– 4.29%) between individuals identified as M. arctia from the Baltic, Barents and White Seas and those from the Kara Sea. This genetic distance, as well as the isolated estuarine habitats of these Arctic worms, and the large geographic distance between the type locality of M. arctia in the Beaufort Sea (Alaska) and northern Europe, raise doubts about the conspecificity of North American, North European and Northwest Pacific populations. We report M. neglecta for the first time for the British Isles (River Thames). We also review the evidence for the role of the Baltic Sea−Volga Canal and the Volga−Don Canal in facilitating the dispersal of M. neglecta to the Caspian Sea and the Sea of Azov, respectively. We further provide new insight on the phylogeny of Marenzelleria , an updated diagnosis of the genus and a key for morphological identification of Marenzelleria adults greater than 1.2 mm wide.