The unique physiology of tuna
The circulatory and respiration systems in tuna are unique among fish. The circulatory system, which consists of the heart and the network of vessels through which blood is pumped, is designed to conserve or to dissipate heat as needed.
When a tuna is relatively inactive, heat conservation is required; as the fish’s activity increases, heat dissipation may be needed.
Temperature regulation in tuna is complex; it depends not only on the level of activity and the size of the fish, but also on the temperature of the surrounding water.
Unlike most other fish, a tuna usually maintains its body at temperatures higher than the temperature of the water in which it swims. Scientists believe that this high body temperature may allow an increased rate of glycolysis — the breakdown of sugar in the muscles – enabling the fish to make rapid use of this chemical energy in a sudden burst of activity.
Presumably, digestion proceeds at a faster rate when the temperature of the abdomen is elevated. The tuna may also benefit from better performance of its brain and eyes at a higher body temperature.
The heat needed to maintain the tuna’s body temperature above the temperature of the surrounding waters is generated by the fish’s high metabolic rate. A high metabolic rate requires a good supply of oxygen. A larger proportion of the dissolved oxygen is taken from the water by certain tuna than by any other fish.
The size of the oxygen-gathering surface in these tuna approaches that of the respiratory surface area found in the lungs of mammals of comparable weight. Another factor in oxygen absorption is the concentration of hemoglobin, the oxygen-transporting pigment in blood. Hemoglobin concentration is as high in the tuna as in humans.
To supply its gills with oxygenated water, the tuna must swim constantly. These fish have lost many of the structures required to pump water over their gills.
This continuous swimming also compensates for the loss or reduction of the swim bladder in tuna — the organ that makes most fish buoyant.
The tuna’s extended pectoral fins apparently act as lifting hydrofoils, counteracting the weight of the fish in the water. The constant forward motion keeps the fish from sinking.
The musculature of the tuna seems to be adapted to make such continuous activity possible. Like most fish, tuna have two types of muscles: white muscle that functions in short bursts of activity, and red muscle that functions in continuous swimming.
In tuna, the mass of red muscle is relatively large, allowing the fish to swim for long periods without fatigue.
Because of its stamina, its high oxygen uptake and its speed, the tuna can be thought of as both a sprinter and a marathon runner.
Tuna are noted for their ability to maintain speeds for long periods of time and for bursts of activity in which they can attain remarkable speeds.
The slowest swimming speed of many tuna exceeds one body length per second. At that rate, a fish three feet long could cross the Pacific Ocean in less than two months.