Intestinal Anion Exchange in Marine Teleosts Is Involved in Osmoregulation and Contributes to the Oceanic Inorganic Carbon Cycle

Marine teleost fish osmoregulation involves seawater ingestion and intestinal fluid absorption. Solute coupled fluid absorption by the marine teleost fish intestine has long been believed to be the product of Na+ and Cl− absorption via the Na+:K+:2Cl− co‐transporter (NKCC2). However, the past decade has revealed that intestinal anion exchange contributes significantly to Cl− absorption, in exchange for HCO3− secretion, and that this process is important for intestinal water absorption. In addition to contributing to solute coupled water absorption intestinal anion exchange results in luminal precipitation of CaCO3 which acts to reduce luminal osmotic pressure and thus assist water absorption. Most recently, activity of apical H+‐pumps, especially in distal segments of the intestine have been suggested to not only promote anion exchange, but also to reduce luminal osmotic pressure by preventing excess HCO3− concentrations from accumulating in intestinal fluids, thereby aiding water absorption. The present review summarizes and synthesizes the most recent advances in our view of marine teleosts osmoregulation, including our emerging understanding of epithelial transport of acid–base equivalents in the intestine, the consequences for whole organism acid–base balance and finally the impact of piscine CaCO3 formation on the global oceanic carbon cycle.