Global Phylogeography of the Widely Introduced North West Pacific Ascidian Styela clava

The solitary ascidian Styela clava Herdman, 1882 is considered to be native to Japan, Korea, northern China and the Russian
Federation in the NW Pacific, but it has spread globally over the last 80 years and is now established as an introduced
species on the east and west coasts of North America, Europe, Australia and New Zealand. In eastern Canada it reaches
sufficient density to be a serious pest to aquaculture concerns. We sequenced a fragment of the cytochrome oxidase
subunit I mitochondrial gene (COI) from a total of 554 individuals to examine the genetic relationships of 20 S. clava
populations sampled throughout the introduced and native ranges, in order to investigate invasive population
characteristics. The data presented here show a moderate level of genetic diversity throughout the northern hemisphere.
The southern hemisphere (particularly New Zealand) displays a greater amount of haplotype and nucleotide diversity in
comparison. This species, like many other invasive species, shows a range of genetic diversities among introduced
populations independent of the age of incursion. The successful establishment of this species appears to be associated with
multiple incursions in many locations, while other locations appear to have experienced rapid expansion from a potentially
small population with reduced genetic diversity. These contrasting patterns create difficulties when attempting to manage
and mitigate a species that continues to spread among ports and marinas around the world.