Contrasting strategies of osmotic and ionic regulation in freshwater crabs and shrimps: gene expression of gill ion transporters

Abstract

Owing to their extraordinary niche diversity, the Crustacea are ideal for comprehending the evolution of osmoregulation. The processes that effect systemic hydro-electrolytic homeostasis maintain hemolymph ionic composition via membrane transporters located in highly specialized gill ionocytes. We evaluated physiological and molecular hyper- and hypo-osmoregulatory mechanisms in two phylogenetically distant, freshwater crustaceans, the crab Dilocarcinus pagei and the shrimp Macrobrachium jelskii, when osmotically challenged for up to 10 days. When in distilled water, D. pagei survived without mortality, hemolymph osmolality and [Cl−] increased briefly, stabilizing at initial values, while [Na+] decreased continually. Gill V(H+)-ATPase, Na+/K+-ATPase and Na+/K+/2Cl− gene expressions were unchanged. In M. jelskii, hemolymph osmolality, [Cl−] and [Na+] decreased continually for 12 h, the shrimps surviving only around 15 to 24 h exposure. Gill transporter gene expressions increased 2- to 5-fold. After 10-days exposure to brackish water (25 ‰S), D. pagei was isosmotic, iso-chloremic and iso-natriuremic. Gill V(H+)-ATPase expression decreased while Na+/K+-ATPase and Na+/K+/2Cl− expressions were unchanged. In M. jelskii (20 ‰S), hemolymph was hypo-regulated, particularly [Cl−]. Transporter expressions initially increased 3- to 12-fold, declining to control values. Gill V(H+)-ATPase expression underlies the ability of D. pagei to survive in fresh water while V(H+)- and Na+/K+-ATPase and Na+/K+/2Cl− expressions enable M. jelskii to confront hyper/hypo-osmotic challenge. These findings reveal divergent responses in two unrelated crustaceans inhabiting a similar osmotic niche. While D. pagei does not secrete salt, tolerating elevated cellular isosmoticity, M. jelskii exhibits clear hypo-osmoregulatory ability. Each species has evolved distinct strategies at the transcriptional and systemic levels during its adaptation to fresh water.