The caprellid amphipod Caprella mutica, a native of northeast Asia, was first detected in the Southern Hemisphere in the Port of Timaru, New Zealand, in 2002. It has since become established in the Port of Lyttelton and at 2 aquaculture sites in the Marlborough Sounds in New Zealand. Direct sequencing of C. mutica mitochondrial DNA (cytochrome c oxidase subunit I gene) identified 3 haplotypes: 1 unique to New Zealand and 2 previously found in non-native Atlantic populations. Higher haplotype

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... ersity and lower F ST distances between the Port of Lyttelton and global populations suggest Lyttelton may be the introduction site in New Zealand. C. mutica populations were sampled on 7 occasions, primarily in winter, and densities generally exceeded 10 000 ind. m -2 , contrasting with the winter declines seen in native and European populations. Sex ratios were generally close to 0.5 and the proportion of brooding females ranged from 0 to 98%. In the Marlborough Sounds, juveniles comprised 32 to 38% of the population regardless of season, brooding females were present throughout the year and males were dominant in winter. Population structure and adult size in the Port of Lyttelton differed with habitat type in August 2008; densities were higher, adults significantly larger on floating than on fixed structures and juveniles and brooding females dominated on vessel hulls. Given the high level of anthropogenic activity and connectivity between coastal locations, it is likely that C. mutica will continue to spread in New Zealand.