On the Distinctness of the Fiddler Crabs Uca minax (Leconte) and Uca longisignalis Salmon & Atsaides in Their Region of Sympatry (Decapoda Brachyura, Ocypodidae)

Uca longisignalis is morphologically and ecologically distinct from all fiddler crab species in the western Atlantic. In particular, it is obvious among the congeners U. pugnax and U. minax. It shows an affinity for the minax-superspecies grouping within the Minuca subgenus (Crane, 1975). Since Von Hagen (1980) had access to few U. longisignalis and little information concerning the ranges of species within the Gulf of Mexico, his evolutionary considerations may be premature. The tomentum of the carapace and ambulatories (Von Hagen, 1970b) may offer little precision in evaluating the evolutionary origins of this species. On the one hand, ambulatory pubescence may aid a relatively terrestrial species to absorb and retain water in semi-arid and sub-humid habitats like those in the western Gulf (Hedgpeth, 1953). On the other, carapace tomentum like that found in U. vocator, U. speciosa or U. thayeri may enhance transevaporative water loss and be deleterious in these environments. Due to the degree of terrestriality expressed by U. longisignalis, it has been relatively successful in adjusting to non-marine environments. Consequently, it may be possible for a species to evolve and adapt to new biomes by maintaining or dispensing with primitive characters depending upon their fortuitous value in a newly invaded habitat. The exact relationship of Minuca species to each other is not clear. Within this subgenus, Uca longisignalis appears to be more closely related to a burgersi-mordax-minax-pugnax lineage than a galapagensis-herradurensis-marguerita-rapax grouping as suggested by Crane's treatment (1975). We have no conclusive evidence as to whether Gulf species are recently evolved or represent relic species from the lower Tertiary. In a geological perspective, Uca minax has been successful in maintaining a post-glacial disjunct distribution and, perhaps, represents a form older than either U. longisignalis or U. pugnax. Since the Gulf of Mexico began forming through tectonic activity at least 165 mybp (White, 1980) and apparently reached its present equilibrium during the late Miocene and early Quaternary, 13 mybp (Carey, 1963), the congruent distributions of five endemic species of fiddler crab within the Gulf of Mexico (Barnwell & Thurman, in preparation) appear to support a recent origins hypothesis. Otherwise, one would expect to observe more than one species of North American fiddler crab to exhibit a Carolinian-like distribution. /// Uca longisignalis est m