Against the flow: Integrated taxonomy and environmental suitability of a cold-water species dispersing to warmer waters

The number of species dispersing to higher and/or lower latitudes, in association with temperature warming, is growing exponentially with only a few described opposite cases of dispersal to warmer regions. Here we describe the dispersal of the solitary ascidian Ascidia virginea, considered native to the seas of North Europe, to disperse to warmer regions: the Eastern Mediterranean and a Red Sea marina. This case highlights an emerging trend of taxa being introduced by marine vessels and successfully establishing populations in unfavourable environmental conditions by opportunistically utilizing niche areas. We provide molecular and morphological data that facilitate the identification of A. virginea in regions in which it may have previously been overlooked. We also employ ecological-niche modeling to project habitat suitability for this species, predicting the coasts of North America, South America, the northwest Pacific, and the Red Sea as moderately and highly suitable habitats. Phylogenetic analyses based on the mitochondrial COI gene reveal that the A. virginea sequences obtained in this work belong to a well-supported clade, including previously published sequences of A. virginea from California and Florida, localities where its presence was not predicted by our model. Furthermore, publicly available COI sequences of A. malaca and A. sydneiensis are assigned to the same A. virginea clade, implying a case of synonymy or misidentification. Our results provide useful data for the accurate identification of this species when expanding into other areas in which it may become a nuisance, and highlight the need to further explore the potential of cold-water species to establish in ports and niche areas in warmer regions.