Résumé
Non-native ascidians can be a major feature of sessile communities, particularly in artificial habitats, but may be overlooked because of poor understanding of species' taxonomy and biogeographic status. The styelid unitary ascidian Asterocarpa humilis, up to now only reported in the Southern Hemisphere, has been found on the coast of NW France from St Malo to Quiberon, on the south coast of England from Falmouth to Brighton, and also in north Wales. The first documented occurrence was in 2005 in Brittany, but the species was found to be relatively widespread at a regional scale and common in many places during surveys in 2009, 2010 and 2011. It has possibly been present but overlooked for some time. The identification based on morphology was confirmed by comparison with specimens from New Zealand, within the species' presumed native range, by molecular barcoding based on mitochondrial (COI) and nuclear (18S) genes.