ENVIRONMENTAL CONSTRAINTS ON THE DISTRIBUTION OF THE BLUE LAND CRAB IN THE BAHAMAS: IMPLICATIONS FOR PALEO-SHORELINE RECONSTRUCTION

Many coastal burrowing organisms rely on periodic flushing of their burrows as an important mechanism for transporting water, nutrients, and waste and for maintaining optimal ecological conditions. Additionally, quantifying the relationship between these biogenic structures and ambient hydrographic regime aids in assessing their potential as (paleo) water/sea-level, salinity, and shoreline indicators. The blue land crab (Cardisoma guanhumi Latreille, 1825), a prolific burrower in a variety of low-latitude supratidal habitats, exhibits preference for immersion in saline water within its basal chamber. To assess the range of physicochemical perturbations over several tidal cycles, 30 burrows were investigated along the margin of North Pigeon Creek (NPC) and two blue holes on San Salvador Island, the Bahamas. Burrow casts and in situ water samples, were used to determine the magnitude of tidal dampening and salinity fluctuations. The open-ocean mean tidal range of 0.90 m is reduced by >30% (~0.60 m) within blue holes and the NPC mangrove shoreline. Further dampening to a mean amplitude of 0.12 m was measured at NPC burrow sites. The average salinity of groundwater at the NPC was 30.09 ppt, compared to 21.25 ppt along the bioturbated phytodetrital margins of the blue holes. C. guanhumi burrows were not observed in otherwise suitable biotopes if groundwater salinity was below 15 ppt or exceeded 41 ppt. Our findings aid in establishing a transfer function for these biogenic structures (along with similar burrows produced by other Cardisoma and Discoplax spp.), as well as demonstrate their utility for reconstructing the overall hydrographic regime and salinity along microtidal paleo-shorelines and tidally influenced inland water bodies.