Can hyper/hypo-osmoregulating fiddler crabs from the Atlantic coast of South America mobilize intracellular free amino acids as osmotic effectors during salinity challenge?

Weakly osmoregulating crustaceans use intracellular free amino acids (FAA) to attenuate cell volume changes consequent to alterations in hemolymph osmolality. Whether semi-terrestrial, strong hyper/hypo-osmoregulators exhibit this ability is not known. We investigate FAA mobilization in muscle tissue of ten fiddler crabs from the genera Minuca, Leptuca and Uca distributed along the Atlantic coast of South America. Crabs were subjected to severe hypo- or hyper-osmotic challenge at their upper or lower critical salinity limits for five days; control crabs were held in isosmotic media. Hemolymph osmolality was measured, chela muscle FAA were identified and quantified, and percent contribution to intracellular osmolality (%FAA) was calculated. At isosmoticity, total FAA were nominally 2-fold higher in Minuca species (≈116 mmol/kg wet mass) than in Uca (≈60 mmol/kg wet mass). Glycine, alanine, arginine and taurine constituted >80% of total FAA. On hyper-osmotic challenge, hemolymph osmolalities ranged from 843 to 1,282 mOsm/kg H2O. FAA increased, although %FAA remained unaltered. Hypo-osmoregulating crabs thus can mobilize FAA, likely owing to a lesser ability to secrete salt near their upper critical limits. On hypo-osmotic challenge, osmolalities were more tightly regulated, between 475 and 736 mOsm/kg H2O. Total FAA and %FAA showed little change, probably due to the crabs’ strong hyper-osmotic extracellular regulation, FAA consequently playing a diminished role in isosmotic intracellular regulation. Total FAA responses to hyper/hypo-osmotic challenge are thus asymmetrical. There was no effect of crab genus on total FAA or on %FAA at isosmoticity or on either osmotic challenge, reinforced by the absence of phylogenetic signal. ### Competing Interest Statement The authors have declared no competing interest.