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

Osmoregulatory findings on crabs from high Neotropical latitudes are entirely lacking. Seeking to identify the consequences of evolution at low temperature, we examined hyperosmotic/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%0 S) than Halicarcinus planatus (5-60%0 S); their respective lower and upper critical salinities are 4%0 and 12%0 S, and 63%0 and 50%0 S. Acanthocyclus albatrossis is a weak hyperosmotic regulator, while H. planatus hyperosmoconforms; isosmotic points are 1380 and similar to 1340 mOsm kg-1 H2O, respectively. 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 similar to 1.9-fold at 5%0 compared with 30%0 S, decreasing at 40-60%0 S. Expression in H. planatus is very low salinity sensitive, increasing similar to 4.7-fold over 30%0 S, but decreasing at 50%0 S. V-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%0 S, but decreases at 60%0 S versus 30%0 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 expression of genes underlying ion uptake and secretion shows marked interspecific divergence. Cold clime crabs may limit osmoregulatory energy expenditure by hyper/hypo-regulating hemolymph [Cl-] alone, apportioning resources for other energy-demanding processes.

Automated summary

This study examines the osmoregulatory characteristics and gene expression of two sub-Antarctic Eubrachyura crabs in low temperature environments. The results show that while both crabs exhibit similar systemic osmoregulatory characteristics, their gene expression for ion uptake and secretion differs significantly.