PATTERNS OF INTRODUCTION AND SPREAD OF THE INVASIVE ASCIDIAN, STYELA CLAVA: EVIDENCE FROM MITOCHONDRIAL SEQUENCE DATA

A major element of ecological change in marine and coastal systems concerns the introduction of invasive species. Analyses of genetic variability of introduced populations can provide information concerning the identity of source populations, pathways of introduction, number of founding individuals and the occurrence of population bottlenecks. This study examined sequence polymorphism of the mitochondrial COI gene from 2 native and 13 introduced populations of the Asian ascidian Styela clava, which has successfully invaded several regions in both Europe and North America. Overall, introduced populations were genetically less diverse, compared with the native populations, but levels of variation were not suggestive of significant founder events. Only the monomorphic population of Mission Bay showed evidence of having passed through a severe population bottleneck. Evidence suggesting the occurrence of multiple introductions was also apparent in several populations. A median-joining network revealed the presence of some highly divergent haplotypes within the introduced populations indicating the possibility that colonisations have also resulted from primary introduction events. A M O V A revealed a lack of genetic structuring at the regional level, suggesting that human-assisted dispersal methods contribute significantly to gene flow between populations. The proposed hypotheses for the spread of S. clava through Europe are discussed in relation to the molecular evidence, together with the supposed vectors of dispersal. Further extensive sampling within the native range and the use of more variable molecular markers will provide clearer insight into the patterns and processes of invasion by S. clava. Keywords: Styela clava, invasive species, cytochrome oxidase I, genetic differentiation, multiple introductions, founder effectFaculty of Science, The Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth, PLl 2PB, U