Geographical variation in osmoregulatory abilities among populations of ten species of fiddler crabs from the Atlantic coast of Brazil: A macrophysiological analysis

Geographical variation is often the earliest stage leading to divergence and speciation. Osmoregulatory ability was assessed in 64 populations of 10 species of semi-terrestrial fiddler crabs along the Atlantic coast of Brazil between Amapá and Santa Catarina. In the laboratory, crabs were exposed for 5 days to media ranging in osmolality from 15 to 3550 mOsm/kg H2O. Hemolymph osmolality was measured in 10-μL aliquots using a Wescor 5520 osmometer. Survivorship, lower- (LLC50) and upper (ULC50) lethal osmolalities, and isosmotic concentrations [ISO] were estimated in populations for which habitat osmolality was also measured. All fiddler crab species were excellent hypo-/hyper-osmoregulators. Mean [ISO] was < 600 mOsm/kg H2O in the lone oligosaline species, between 650 and 770 mOsm/kg H2O in the seven mesosaline species, and > 800 mOsm/kg H2O in the two eusaline species. Intraspecific variation in [ISO] was significant only in Minuca rapax, emphasizing the importance of this parameter as a physiological set-point. Although ULC50 varied intraspecifically in six species, habitat osmolality varied significantly for M. rapax and M. victoriana alone. Thus, intraspecific variation in ULC50 likely results from local osmotic acclimatization. Since genetic variation appears to be minor and unstructured across populations in most fiddler crab species, the consistency of [ISO] reflects its importance as a physiological property. In contrast, intraspecific differences in ULC50 among populations indicate that this character is ecophenotypic. Unquestionably, physiological studies on fiddler crab populations distributed over a wide geographical range can provide insight into the biological basis of adaptation and the evolution of species in this semi-terrestrial genus.