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 semiterrestrial, strong hyper/hypo-osmoregulators exhibit this ability is unknown. We investigate FAA mobilization in muscle tissue of 10 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 lower or upper critical salinity limits for 5 days; reference 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 twofold higher in Minuca species (approximate to 116 mmol/kg wet mass) compared to Uca (approximate to 60 mmol/kg wet mass). Glycine, alanine, arginine, and taurine constituted >80% of the total FAA pool. On hyperosmotic challenge, hemolymph osmolalities ranged from 843 to 1282 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 regulated more tightly, between 475 and 736 mOsm/kg H2O. Total FAA and %FAA showed little change, probably due to the crabs' strong hyper-osmotic extracellular regulatory ability, FAA consequently playing a diminished role in isosmotic intracellular regulation (IIR). Total FAA responses to hyper/hypo-osmotic challenge are thus asymmetrical. The lack of phylogenetic signal in FAA mobilization suggests that closely related fiddler crabs do not share similar strategies of IIR.

Automated summary

Weakly osmoregulating crustaceans use intracellular free amino acids (FAA) to attenuate cell volume changes, but it is unknown if semiterrestrial, strong hyper/hypo-osmoregulators have this ability. This study investigates FAA mobilization in muscle tissue of 10 fiddler crabs from different genera along the Atlantic coast of South America. The results show that hypo-osmoregulating crabs can mobilize FAA, likely due to a lesser ability to secrete salt near their upper critical limits. On the other hand, hyper-osmoregulating crabs have a strong extracellular regulatory ability, so FAA plays a diminished role in isosmotic intracellular regulation.