THURMAN, C.L.*: Osmoregulation by Uca from the Temperate Coasts of Eastern North America

Since fiddler crabs are common in brackish estuaries, this study examines osmoregulation in seven Uca species. Specimens were collected from 11 locations between Barnstable Co., MA, and Camden Co., GA, on the Atlantic and five sites between Wakulla and Escambia Co. on Florida’s Gulf coast. Crabs were placed in finger bowls containing 50 ml of artificial sea water (ASW) ranging from 15 to 3500 mOsm (0.5 to 110 ppt). After five days, both survivorship and hemolymph osmolality (HmOsm) were determined. Uca spinicarpa, U. minax, and U. longisignalis are limited in tolerating ASW over 1800 mOsm (57 ppt). Uca speciosa are tolerant of 2400 mOsm ASW (76 ppt), while U. pugnax survive 2500 mOsm (79 ppt). Uca pugilator survive 2600 mOsm (82 ppt) while U. panacea tolerate 3000 mOsm ASW (95 ppt). Each species is an excellent osmoregulator. Uca minax, U. longisignalis and U. spinicarpa are able to control HmOsm between 30 and 1400 mOsm (1 to 44 ppt) ASW. Uca minax populations from Point Lookout (MD), St. Mary’s River (GA), and Thompson’s Bayou (FL) are better able to control HmOsm under stress than specimens from the Ochlochnee River (FL). Uca speciosa and U. pugnax are successful in controlling HmOsm in media up to 2200 mOsm (69 ppt) and 2600 mOsm (82 ppt), respectively. Uca pugilator and U. panacea are most successful in maintaining constant HmOsm under osmotic challange as high as 3000 mOsm (95 ppt). Both an Alligator Pt. (FL) population of U. panacea and a Waquoit Bay (MA) population of U. pugilator can successfully regulate HmOsm in media between 2800 (88 ppt) and 3500 mOsm (110 ppt). These results support the notion that variation in osmoregulation exists among species of Uca from different habitats.