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In 2014, the Atlantic mud fiddler crab Uca pugnax was found 80 km north of its previously known northern range limit. Two years before this shift was noted, we collected a total of 781 male and female specimens from 6 populations along a latitudinal transect extending from Wareham, Massachusetts (41.7615°N), to Tybee Island, Georgia (32.0139°N), USA. By assessing latitudinal and seasonal patterns in the hepatosomatic index (HSI; a measure of stored energy) and the reproductive status of females, we sought to determine whether adult physiological and reproductive limits might slow the northern expansion of U. pugnax. We did not find a latitudinal cline for HSI, suggesting that U. pugnax is a thermal generalist; however, both males and females in the southern part of the range showed greater seasonal fluctuations in HSI compared to northern conspecifics. Across the range, ovigerous females had a significantly reduced HSI, revealing the cost of reproduction. Ovigerous females were found in the May 2013 collection in Massachusetts be fore ocean conditions were permissible for larval development and earlier than previously reported for this species. U. pugnax is expected to closely track warming conditions in the Northwest Atlantic because adults in northern populations are able to maintain energy stores comparable to that of their southern conspecifics, and they release planktonic larvae in early spring, maximizing their dispersal potential.

For crab larvae, swimming behaviors coupled with the movement of tides suggests that larvae can normally move upstream within estuaries by avoiding ebb tides and actively swimming during flood tides (i.e., flood-tide transport [FTT]). Recently, a 1-D transport model incorporating larval behavior predicted that opposing forces of river discharge and tidal amplitude in the Pee Dee River/Winyah Bay system of South Carolina, USA, could limit dispersal within a single estuary for downstream transport as well as become a dispersal barrier to recruitment of late stage larvae to the freshwater adult habitats of Uca minax (LeConte 1855). We sequenced 394-bp of the mitochondrial cytochrome apoenzyme b for 226 adult U. minax, from four locales along a 49-km stretch of the Pee Dee River/Winyah Bay estuary, above and below the boundary of salt intrusion. Results of an analysis of molecular variance (AMOVA) and an exact test of population differentiation showed a small, but statistically significant (α=0.05) population subdivision among adults of the 4 subpopulations, as well as all subpopulations being significantly differentiated (α=0.05). This pattern fitted with model predictions, which implies that larval transport within the tidally influenced river system is limited.