Habitat expansion in response to sea-level rise by the fiddler crab <i>Minuca pugnax</i> (Smith, 1870) (Decapoda: Brachyura: Ocypodidae) in southern New England salt marshes

Abstract Global change phenomena are shifting species distributions across multiple spatial and temporal scales. We examined the expansion of the fiddler crab, Minuca pugnax (Smith 1870), into high-marsh habitats along the Connecticut, USA coast. Previously reported as primarily found in low-marsh habitats in northeastern USA, our results indicate that large populations now occupy high-marsh habitats related to increased tidal range and inundation. Where expansion was evident, high-marsh population abundances were ~2.6× that in the low marsh through the summer and fall, given the relative areas of, and densities in each. Crabs on the high marsh ranged in size from newly settled individuals to large adults, and exhibited typical feeding and reproductive behaviors. Crab numbers greatly exceed the number of burrows on the high marsh, were active during high tides, and did not retreat into burrows. Mark-recapture studies indicated crabs mostly remained in one or the other habitat, with some movement between the low and high marsh. During late fall and winter, individuals of M. pugnax are only found in the low marsh. Depending on the size of the high-marsh population that migrates into the low marsh to overwinter, this may increase intraspecific competition for available space and burrows, resulting in population bottlenecks that may affect subsequent habitat-specific population dynamics. Several severe winters during the study appear to have caused high mortality, decreasing crab abundances on the high marsh in subsequent seasons. As salt marsh environments change due to sea-level rise and other factors, resident and transient fauna may experience conditions that are, or are not, within their range of biological adaptations. Minuca pugnax is exhibiting a positive response to such changes in southern New England, and large populations on the high marsh during much of the year may have a significant impact on the future ecology of these ecosystems.