Redescription of Melinna palmata (Annelida, Melinnidae), a recently established species in Sweden, with a synoptic table for Melinnidae species occurring in Sweden and Norway

Melinna palmata was found in Swedish waters for the first time during recent environmental surveys. In this paper the species is redescribed based on type material and recently collected material from environmental monitoring expeditions in Sweden, the English Channel and the Bay of Biscay, Western France (2021–2022). The description is illustrated with photographs of both live and fixed specimens as well as scanning electron microscopy photographs. Haplotype networks, based on COI and ITS1, have been constructed for M. palmata showing a lack of population structure between the different areas. The establishment and spread of M. palmata in Sweden are documented and illustrated with maps. The identity of M. palmata and its phylogenetic relationships with other species of Melinnidae have been assessed using the standard barcoding COI fragment. COI sequences from close to type localities are provided for M. palmata, Melinna cristata, Melinna elisabethae, Melinna albicincta, Melinna monoceroides and Melinnopsis arctica. A synoptic table of key morphological features for the same species is given.