MRAZ, J*; THURMAN, C; BENNETT, C; BRODIE, R; High Point University; University of Northern Iowa; University of Northern Iowa; University of South Carolina: Osmotic tolerance in meglopal and adult crabs of U. minax: can early life stages withstand the same regimes as adults?

The fiddler crab, Uca minax, has a complex lifecycle. Juvenile and adult stages occupy benthic habitats in estuaries and along the banks of tidally influenced rivers, and larval stages develop in the plankton of the coastal ocean. Bridging the gap between coastal planktonic and estuarine benthic habitats is the megalopal stage, also known as the postlarval settlement stage. Megalopae ride nocturnal flood currents into estuaries to locate appropriate settlement sites. In the process, they move from stable ocean salinities to the fluctuating environment of the estuary. In this study, we compare the osmotic tolerance of adult and megalopal U. minax to determine if it is equivalent in both stages of life. Adult crabs were collected from benthic habitats in the Winyah Bay (oligohaline), North Inlet Estuary (euhaline) and the Great Pee Dee River (oligohaline) in South Carolina. Megalopae were collected from the Pee Dee and Waccamaw rivers using light traps. Both adult and megalopal U. minax were subjected to 12 concentrations of seawater between 40 � 3320 mOsm/Kg (pH 8.0). Survivorship was assessed in adults after 5 days and in megalopae after 24 hours of osmotic challenge. Both adults and megalopae had excellent survival in the lowest concentrations. The upper lethal concentration (ULC₅₀) was 1384, 1350 and 2200 mOsm/kg for adults from The Great Pee Dee, Winyah Bay, and North Inlet Estuary, respectively. Megalopae from the Pee Dee and Waccamaw rivers showed an ULC₅₀ of 1246 mOsm/kg. Since megalopae and adults from oligohaline habitats have similar osmotic tolerance, this suggests that the settlement stage of U. minax has well developed osmoregulation capabilities.