The presence of Melinna palmata (Annelida : Polychaeta) and Ensis directus (Mollusca : Bivalvia) related to sedimentary changes in the Bay of Seine (English Channel, France)

Since late 1990s the annelid polychaete Melinna palmata and the mollusc bivalve Ensis directus have been collected in the eastern part of the Bay of Seine (English Channel), indicating changes in the benthic communities. Melinna palmata was never collected prior to 2002, whereas it was reported in the muddy fine sands of the western part of the Channel, along the French (e.g. Bay of Cherbourg) and southern UK (e.g. Southampton Waters) coasts. Ensis directus was first reported in 1998 and now appears to be well implanted, given the abundant population collected in 2006. The colonization of Melina palmata seems to be a consequence of recent increase of the fine sediment in the eastern part of the Bay, while that of the invasive Ensis directus seems more likely to be related to its southwest expansion, from the Scheldt estuary (Belgium and Netherlands) towards the Bay. Since both species have complex life cycles including planktonic larval phases, their colonisation may also be favoured either by an accidental introduction via ballast waters or by larval dissemination from neighbouring populations.