Effects of hypercapnia on acid–base balance and osmo-/iono-regulation in prawns (Decapoda: Palaemonidae)

Anthropogenic carbon dioxide-induced ‘ocean acidification’ is predicted to have major implications for marine organisms. As the oceans becomes increasingly hypercapnic (elevated CO2) and seawater pH decreases, the ability of organisms to maintain extracellular pH homeostasis (acid– base balance) may be compromised. Acid–base regulation occurs by ionic transport, where hydrogen and bicarbonate ions (HCO3) are exchanged for sodium and chloride, respectively (H+/Na+; HCO3/Cl), as exemplified by decapod crustaceans. Palaemonid prawns, in particular, are efficient hypo-ionic/osmotic regulators in seawater. We demonstrate that hypercapnic exposure (0.3 kPa) results in short-term (5 to 14 d) extracellular acidosis in 2 efficient ionic/osmo-regulators (thus, acid–base regulators), i.e. Palaemon elegans and P. serratus. Complete hypercapnic compensation was observed in both species after 30 d exposure with no effect on osmotic capacity, but at the expense of extracellular acid–base alteration (alkalosis). Furthermore, the predominantly subtidal species P. serratus was observed to be as tolerant as the intertidal species P. elegans, although 2 differing mechanisms of ionic regulation may be at work, with P. elegans and P. serratus displaying lower and elevated haemolymph ion concentrations (i.e. sodium, chloride and calcium), respectively.