Abstract
The most abundant species of intertidal crab along rocky shorelines of southern New England is an invasive species, the Asian shore crab Hemigrapsus sanguineus. The invader rapidly colonized and increased its density along the northeastern US coastline since its first discovery in 1988, and recently established on the shores of northern Europe. The Asian shore crab reaches high densities in its invaded ranges, and exceeds abundances found in the native region. However, within the invaded ranges, population densities are variable within and between coastal habitats. This investigation examines factors that explain Asian shore crab population abundance across different rocky intertidal habitats in southern New England. Biological monitoring was conducted biweekly for 6-7 months over 3 consecutive years at relatively well-studied sites to describe dynamics of H. sanguineus populations within and across reproductive seasons, and to determine if populations in different coastal ecosystems differ in reproductive characteristics such as the onset and length of the reproductive season, and the proportion and sizes of ovigerous females. We found the highest recorded density of Asian shore crabs in the upper reaches of Narragansett Bay estuary, at times reaching greater than 600 crabs per m². Moreover, female crabs reproduced earlier in the season and at smaller sizes at that location than at the outer coast. Results confirm the importance of reproductive characteristics as a factor contributing to different H. sanguineus densities between coastal and estuarine ecosystems. We also conducted a series of field tethering experiments to test relative predation of adult H. sanguineus at high tide and low tide, and during daytime and nighttime. A series of laboratory feeding trials tested the influence of high and low levels of habitat refuge on predation of sexually mature H. sanguineus by the fish predator Tautoga onitis. Results indicate that predation of invasive Asian shore crabs by aquatic species is greater than that by terrestrial species, and that high habitat refuge provided better shelter from predators. Finally, we measured genetic differentiation among populations in southern New England, as well as broadly along the invaded range from Maine to Maryland, to answer questions about larval dispersal. Genome-wide markers reveal high connectivity among populations along the coast, with slightly restricted dispersal between sites located north and south of Long Island Sound. Genetic data do not suggest self-recruitment as a primary mechanism underpinning high densities in Narragansett Bay. The data collected here suggests reproductive characteristics as well as environmental factors are important determinants of invasive population sizes, while predation does not strongly impact adult crabs because of the spatial and temporal refugia in rocky intertidal habitats. This work also describes high larval connectivity in the invaded region and provides evidence of long-distance dispersal of H. sanguineus larvae. Taken together, these investigations highlight the complexity of invasive population dynamics and provide fine-scale demographic information on a wide-spread invasive species. Understanding regional and habitat-specific population characteristics is critical for making predictions about invasive populations and managing their impacts.