Blood oxygen-binding characteristics of bigeye tuna (Thunnus obesus), a high-energy-demand teleost that is tolerant of low ambient oxygen

We found blood from bigeye tuna (Thunnus obesus) to have a significantly higher O-2 affinity than blood from other tunas. Its P-50 (partial pressure of oxygen, Po-2 required to roach 50% saturation) was 1.6 to 2.0 kPa (13 to 15 mmHg) when equilibrated with 0.5% CO2. Previous studies employing similar methodologies found blood from yellowfin tuna (T. albacares), skipjack tuna (Katsuwonus pelamis), and kawakawa (Euthynnus affinis) to have a P-50 of 2.8 to 3.1 kPa (21 to 23 mmHg). These observations suggest that bigeye tuna are more tolerant of low ambient oxygen than other tuna species, and support similar conclusions derived from laboratory whole-animal studies, depth-of-capture data, and directly-recorded vertical movements of fish in the open ocean. We also found the O-2 affinity of bigeye tuna blood to be essentially unaffected by a 10 C degrees open-system temperature change (as is the blood of all tuna species studied to date). The O-2 affinity of bigeye tuna blood was, however, more affected by a 10 C degrees closed-system temperature change than the blood of any tuna species yet examined. In other words, bigeye tuna blood displayed a significantly enhanced Bohr effect (change in log P-50 per unit change in plasma pH at P-50) when subjected to the inevitable changes in partial pressure of carbon dioxide (P-CO2) and plasma pH that accompany closed-system temperature shifts, than when subjected to changes in plasma pH accomplished by changing P-CO2 alone. In vivo, the resultant large decrease in O-2 affinity (i.e. the increase in P-50) that occurs as the blood of bigeye tuna is warmed during its passage through the vascular counter-current heat exchangers ensures adequate rates of O-2 off-loading in the swimming muscles of this high-energy-demand teleost.