THURMAN, Carl ; Univ. Northern Iowa: Osmoregulation in Fiddler Crabs: A Review

Fiddler crabs are common in coastal habitats that differ greatly in salinity. To study water balance physiology in the genus Uca, thirteen species were collected from 86 locations ranging from 14 to 2080 mOsm along the shores of the Gulf of Mexico as well as the Atlantic and Pacific Oceans in the United States. Although most species are considered members of either the Celuca or the Minuca subgenus, one is a member of the Boboruca subgenus. Among the seven Celuca, five species exhibit distributions in habitats which are primarily euhaline or hypersaline, while two are primarily oligo- and mesohaline. Among five Minuca, two species are primarily oligo- to mesohaline and three are meso- to euhaline in their habitat preferences. The Boboruca species occupies primarily mesohaline habitats. Detailed osmoregulation studies were conducted in eleven species challenged with artificial sea water varying from 25 to 3550 mOsm. Through these studies, average survivorship, hemolymph-media osmotic relationship and isomotic hemolymph concentration ([ISO]) were determined for each. Within the Celuca subgenus, estimations of [ISO] vary from 845 to 888 mOsm in U. pugilator, U. panacea, U. subcylindrica, and U. crenulata. For U. speciosa and U. spinicarpa [ISO]s are 798 and 715 mOsm, respectively. For temperate species in the Minuca group, U. minax, U. longisignalis and U. pugnax, [ISO]s are 666, 693, and 879 mOsm, respectively. In the tropical species U. rapax the [ISO] is 775 mOsm. The [ISO] of the Boboruca representative, U. thayeri,, is 819 mOsm. Consequently, it appears that species with [ISO]s < 725 mOsm prefer oligohaline, between 725 and 820 mOsm inhabit mesohaline, and >820mOsm live in euhaline habitats. As a result of this investigation it appears that fundamental osmoregulatory capabilities and properties are fairly consistent within species. Variation in physiological properties among species is clearly related to phylogeny and ecology.