Osmoregulation in Six Sympatric Fiddler Crabs (genus Uca) from the Northwestern Gulf of Mexico

Abstract. 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  ‰) 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  ‰). Uca spinicarpa and U. minax survive only below 1400  mOsm (44  ‰). U. longisignalis is intermediate, with limited survival above 2500  mOsm (79  ‰). 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  ‰). Uca longisignalis lost its ability to control hemolymph osmolality above 2200  mOsm (69  ‰). On the other hand , U. spinicarpa and U. minax become less effective hyporegulators in media above 1200 (38  ‰) and 1500  mOsm (47  ‰), 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.