Osmoregulation in six sympatric fiddler crabs (genus Uca) from the northwestern Gulf of Mexico

As problems in taxonomy and systematic relationships among the Uca species are resolved, a definition of how each fiddler crab species partitions and utilizes habitat resources is needed. To this end, the osmoregulatory capabilities were studied in six species of fiddler crabs from the western Gulf of Mexico. Specimens were placed in 50 ml of artificial seawater ranging in osmolality from 50 to 3450 mOsm (2 to 109 parts per thousand) for five days. Survivorship was recorded for each species. Uca rapax, U. panacea and U. subcylindrica are hardy from 200 to 3200 mOsm (6 to 101 parts per thousand). Uca spinicarpa and U. minax survive only below 1400 mOsm (44 parts per thousand). U. longisignalis is intermediate, with limited survival above 2500 mOsm (79 parts per thousand). Hemolymph osmolality was determined for crabs surviving the osmotic regimen. Uca panacea, U. subcylindrica and U. rapax are equivalent in their regulation between 50 and 3200 mOsm (2-101 parts per thousand). Uca longisignalis lost its ability to control hemolymph osmolality above 2200 mOsm (69 parts per thousand). On the other hand, U. spinicarpa and U. minax become less effective hyporegulators in media above 1200 (38 parts per thousand) and 1500 mOsm (47 parts per thousand), respectively. Only U. longisignalis appears to exhibit clear differences in osmoregulatory capacity between populations. Population data for U. panacea and laboratory experiments with U. subcylindrica also support the notion of capacitative acclimation in Uca. These physiological capabilities correlate well with the known habitat characteristics for each fiddler crab species in the northwestern Gulf of Mexico.