Styela clava, a solitary ascidian native to the NW Pacific, has become a conspicuous member of fouling communities in NW European waters. As its natural dispersal appears to be limited, the wide distribution of S. clava along coasts within its introduced range may be attributed to secondary spread assisted by human activities. Here, we used six microsatellite loci to examine the genetic diversity and extent of gene flow among S. clava populations in its European introduced range. Samples were collected from 21 populations within Europe (N = 808), 4 populations within the USA and two populations within the native range (Japan).

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The solitary ascidian Styela clava is recorded for the first time in Portuguese waters. It was found at three sites on the west coast of Portugal. This represents the southern limit of its recorded distribution in European waters. The Portuguese specimens are described and compared with UK specimens.

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