Marine Biology �2000)136: 1087±1098 Ó Springer-Verlag 2000

Blood oxygen-binding characteristics of bigeye tuna (Thunnus obesus), a high-energy-demand teleost that is tolerant of low ambient oxygen Abstract We found blood from bigeye tuna (Thunnus obesus) to have a signi®cantly higher O 2 anity than blood from other tunas. Its P 50 (partial pressure of oxygen, P O 2 required to reach 50% saturation) was 1.6 to 2.0 kPa (12 to 15 mmHg) when equilibrated with 0.5% CO 2. Previous studies employing similar method-ologies found blood from yellow®n tuna (T. albacares), skipjack tuna (Katsuwonus pelamis), and kawakawa (Euthynnus anis) 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 ®sh in the open ocean. We also found the O 2 anity of bigeye tuna blood to be essentially una€ected by a 10 C° open-system temperature change (as is the blood of all tuna species studied to date). The O 2 anity of bigeye tuna blood was, however, more a€ected by a 10 C° closed-system temperature change than the blood of any tuna species yet examined. In other words, bigeye tuna blood displayed a signi®cantly enhanced Bohr e€ect (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 CO 2) and plasma pH that accompany closed-system temperature shifts, than when subjected to changes in plasma pH accomplished by changing P CO 2 alone. In vivo, the resultant large decrease in O 2 anity (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 o€-loading in the swimming muscles of this high-energy-demand teleost.