Effects of open- and closed-system temperature changes on blood oxygen dissociation curves of skipjack tuna, Katsuwonus pelamis, and yellowfin tuna, Thunnus albacares

Tunas often experience rapid temperature changes of 10 °C or more during their daily vertical movements. Their blood is therefore subjected to open-system (i.e., constant O2 and CO2 partial pressure, variable O2 and CO2 content) temperature changes during passage through the gills. In addition, tunas possess vascular countercurrent heat exchangers and can have deep red muscle temperatures as much as 20 °C above ambient. Their blood also experiences closed-system (i.e., constant O2 and CO2 content, variable O2 and CO2 partial pressure) temperature changes during passage through the heat exchangers. Temperature-independent blood O2 binding could be expected. We found blood oxygen dissociation curves of skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) to be temperature independent during open-system temperature changes. Although blood from both species showed unsually large Bohr effects (−0.986 and −0.865 Δlog P50∙ΔpH−1 for skipjack and yellowfin tuna, respectively) when subjected to CO2 partial pressure alterations in the open system, the oxygen dissociation curves of skipjack tuna blood were nearly temperature independent during closed-system temperature changes. In other words, blood from skipjack tuna showed a reduced Bohr effect when subjected to the inevitable CO2 partial pressure changes that accompany closed-system temperature shifts. Since skipjack tuna blood shows temperture-independent O2 binding during closed-system temperature changes whereas yellowfin tuna blood does not, this unusual feature is not obligatory in thermoconserving fishes.