Evolutionary trade-offs in osmotic and ionic regulation and the expression of gill ion transporter genes in high latitude, Neotropical cold clime crabs from the 'end of the world'

Seeking to identify consequences of evolution at low temperature, we examine hyper/hypo-osmotic and ionic regulation and gill ion transporter gene expression in two sub-Antarctic 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 Halicarcinus planatus (5-60 ‰ S); 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 hyper-osmoconforms; isosmotic points are 1,380 and ≈1,340 mOsm kg-1 H2O. Both crabs hyper/hypo-regulate [Cl-] well with iso-chloride points at 452 and 316 mmol L-1 Cl-, respectively. [Na+] is hyper-regulated at all salinities. mRNA expression of gill Na+/K+-ATPase is salinity-sensitive in A. albatrossis , increasing ≈1.9-fold at 5 ‰S compared to 30 ‰S, decreasing at 40 to 60 ‰S. Expression in H. planatus is very low salinity-sensitive, increasing ≈4.7-fold over 30 ‰S, but decreasing at 50 ‰S. V(H+)-ATPase expression decreases in A. albatrossis at low and high salinities as 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 may be mediated by increased Na+-K+-2Cl- expression but Cl- secretion is independent of symporter expression. These unrelated eubrachyurans exhibit similar systemic osmoregulatory characteristics and are better adapted to dilute media; however, the gene expressions underlying ion uptake and secretion show marked interspecific divergences. Cold clime crabs may have limited energy expenditure by regulating hemolymph [Cl-] alone, apportioning resources for other metabolic processes. ### Competing Interest Statement The authors have declared no competing interest.