Enhanced oxygen unloading in two marine percomorph teleosts

Teleost fishes are diverse and successful, comprising almost half of all extant vertebrate species. It has been suggested that their success as a group is related, in part, to their unique O-2 transport system, which includes pH-sensitive hemoglobin, a red blood cell beta-adrenergic Na+/H+ exchanger (RBC beta-NHE) that protects red blood cell pH, and plasma accessible carbonic anhydrase which is absent at the gills but present in some tissues, that short-circuits the beta-NHE to enhance O-2 unloading during periods of stress. However, direct support for this has only been examined in a few species of salmonids. Here, we expand the knowledge of this system to two warm-water, highly active marine percomorph fish, cobia (Rachycentron canadum) and mahi-mahi (Coryphaena hippurus). We show evidence for RBC beta-NHE activity in both species, and characterize the Hb-O-2 transport system in one of those species, cobia. We found significant RBC swelling following p-adrenergic stimulation in both species, providing evidence for the presence of a rapid, active RBC beta-NHE in both cobia and mahi-mahi, with a time-course similar to that of salmonids. We generated oxygen equilibrium curves (OECs) for cobia blood and determined the P-50, Hill, and Bohr coefficients, and used these data to model the potential for enhanced O-2 unloading. We determined that there was potential for up to a 61% increase in O-2 unloading associated with RBC beta-NHE short-circuiting, assuming a - 0.2 Delta pH(a-v) in the blood. Thus, despite phylogenetic and life history differences between cobia and the salmonids, we found few differences between their Hb-O-2 transport systems, suggesting conservation of this physiological trait across diverse teleost taxa.

Automated summary

Teleost fishes have a unique O-2 transport system that may contribute to their success as a group. This study expanded the knowledge of this system to warm-water fish species, cobia and mahi-mahi, demonstrating the presence of a rapid, active red blood cell beta-NHE. Despite differences in phylogenetics and life history, there were few differences in the Hb-O-2 transport systems of these species, suggesting conservation of this physiological trait across diverse teleost taxa.