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 β-adrenergic Na ⁺ /H ⁺ exchanger (RBC β-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 β-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 β-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 β-adrenergic stimulation in both species, providing evidence for the presence of a rapid, active RBC β-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 β-NHE short-circuiting, assuming a - 0.2 ∆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.
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