Distribution and dispersal of the non-native caprellid amphipod, <i>Caprella mutica</i> Schurin 1935

The introduction of non-native species is the second most important anthropogenic threat to global biodiversity, with the first being habitat destruction. In the UK, we have been fortunate to date, that the majority of marine non-native species have not caused significant adverse ecological impacts. Consequently, the processes of species introduction to the region have been little studied. Caprella mutica is a native of the Sea of Japan area, but has subsequently been identified as introduced in several globally distributed locations. This thesis first establishes the current global distribution of C. mutica and uses molecular evidence to suggest global introduction pathways. A study of post-establishment (secondary) vectors and the environmental tolerance of C. mutica provide information regarding the modes of dispersal and potential future range of this and other non-native species. Information regarding the biology and ecology of C. mutica was collected through a study of its seasonal population dynamics. Globally, C. mutica has established populations on all oceanic coastlines in the northern hemisphere and has been found at two sites in New Zealand in the southern hemisphere. A phylogenetic analysis, using the cytochrome c oxidase subunit I gene, indicated that the global distribution of C. mutica is the result of multiple introduction pathways with stepping-stone events. A high genetic diversity was found in the native region but the source for the non-native populations could not be identified. Individuals from the east Pacific grouped together and were genetically most divergent, indicating a single introduction of a few individuals to this region. Several haplotypes were shared between populations from east and west Atlantic locations, indicating similar source populations, introduction mechanisms and stepping-stone pathways. Human vectors, primarily shipping and aquaculture activities, are likely to be responsible for the species’ global distribution. The distribution of C. mutica on the west coast of Scotland indicates effective secondary dispersal of C. mutica from sources of introduction. Phylogenetic analysis indicated that the populations may be the result of multiple introductions to the region, and several vectors may be responsible for the distribution. The most likely vectors are aquaculture and shipping activities and recreational boating. Field experiments confirmed the ability of C. mutica to disperse by rafting on drifting algae, with localised dispersal (< 5 km) by free-swimming. Physiological tolerance determines the fundamental niches of marine species. In laboratory experiments, C. mutica were tolerant of a broad range of temperature and salinity conditions, ii with 100 % mortality at 30 °C (48 h LT50, 28.3 ± 0.41 °C), and salinities lower than 15 (48 h LC50, 18.7 ± 0.24). Although lethargic at low temperatures (2 °C), no mortality was observed and it is known to survive at temperatures as low as -1.8 °C. It is unlikely that salinity will limit the distribution of C. mutica in open coastal waters, however, the species will be excluded from brackish-water environments such as the heads of sea lochs or estuaries. The physiological limits of C. mutica are beyond the physical conditions experienced in its native or introduced range, thus are unlikely to be the primary factors limiting its present distribution and future spread. C. mutica has several traits attributed as being characteristic of successful non-native species, such as an abundant and widespread distribution in the native range, broad physiological tolerances, broad food preferences, short lifespan and generation times and high fecundity and growth rate. The seasonal population dynamics of C. mutica were studied at four humanimpacted sites on the west coast of Scotland. Populations at two fish farm sites were more successful than those at a recreational marina and disused pontoon. At all sites, C. mutica was most abundant during late summer (August-September) with reduced abundance or absence during winter (January-April). The maximum recorded abundance was 319,000 individuals m-2, in August 2004. Females were dominant for most of the year and the year-round presence of juveniles at the fish farms indicates either continuous reproduction or delayed growth of overwintering juveniles. Fecundity was positively correlated with female size; the maximum number of eggs per female was 363. The relative anthropogenic influences at the four sites played an important role in the population dynamics and characteristics of C. mutica. Enhanced food supply, availability of space, and intensity, timing and sequence of disturbance events contributed to the relative success of C. mutica at each site. Species native to cold temperate regions, with similar life-history characteristics to C. mutica and exposed to human dispersal mechanisms have the potential to be introduced to the UK. Once established, there are several effective dispersal mechanisms that can rapidly spread species along the coastlines. Given the widespread distribution of C. mutica in the UK, eradication is not an option.