Ecologia e Fisiologia - Comunicações orais

Osmoregulatory investigations on crabs from high Neotropical latitudes are entirely lacking. Seeking to identify the consequences of evolution in cold climes, we examine osmotic and ionic regulation and gill ion transporter gene expression in two subAntarctic Eubrachyura from the Beagle Channel, Tierra del Fuego. Despite sharing the same osmotic niche, Acanthocyclus albatrossis tolerates a wider salinity range (2-65 ‰S) than does Halicarcinus planatus (5-60 ‰S); their respective lower and upper critical salinities are 4 and 12 ‰S, and 63 and 50 ‰S. Acanthocyclus albatrossis is a weak hyperosmotic regulator while H. planatus is a hyper-osmoconformer; isosmotic points are 1,380 and ≈1,340 mOsm kg-1 H2O, respectively. Both crabs hyper/hyporegulate [Cl-] well with iso-chloride points at 452 and 316 mmol L-1Cl-, respectively. Incongrously, [Na+] is hyper-regulated at all salinities. mRNA expression of the gill Na+/K+-ATPase α-subunit is salinity-sensitive in A. albatrossis, increasing ≈1.9-fold at 5 compared to 30 ‰S (reference salinity), decreasing at 40 to 60 ‰S. In contrast, expression in H. planatus is very low salinity-sensitive, increasing ≈4.7-fold over 30 ‰S, but decreasing at 50 ‰S. V(H+)-ATPase B-subunit expression decreases in A. albatrossis at low and high salinities as seen in H. planatus. Na+/K+/2Cl- symporter expression in A. albatrossis increases 2.6-fold at 5 ‰S, but decreases at 60 ‰S compared to 30 ‰S. Chloride uptake at low salinity may be mediated by increased Na+/K+/2Cl- expression but Cl- secretion is independent of symporter expression. These two unrelated eubrachyurans exhibit similar overall osmoregulatory characteristics and regulate better in dilute media; however, the gill transporter gene expressions underlying ion uptake and secretion show marked interspecific divergences. Cold clime crabs may limit osmoregulatory energy expenditure by allowing hemolymph Na+ to increase, hypo-regulating hemolymph [Cl-] alone, possibly apportioning resources for other energy-demanding processes like aerobic metabolism, growth, reproduction and seasonal molting.