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Adaptations involved in temperature regulation
Southern bluefin tunas often migrate vertically through the water column in search of their preferential temperature, as well as spend time in cooler waters searching for prey. Some have hypothesized that they take refuge in warmer areas of water fronts and eddies after these foraging periods, but others suggest that these migrations are only associated with the aggregation of prey. It is clear, however, that Southern bluefin tuna have developed complex physiological mechanisms to maintain their body temperature significantly above the ambient temperature in these changing conditions. As the metabolic heat is carried from tissues to the gills, it is lost to the environment, because the rate of heat diffusion is much higher than the rate of oxygen diffusion. Tuna can, however, maintain the temperature of their muscles at 5-20 °C above the temperature of surrounding water, by employing complex vascular structures-rete mirabile. In tunas, large lateral cutaneous vessels that branch off into the arteries and veins of rete mirabile supply blood to the red muscle, instead of a centrally located aorta. Rete mirabile function as countercurrent heat exchangers that prevent metabolic heat loss at the gills. Warm-bodied fish, such the Southern bluefin tuna, maintain their body temp. by varying the efficiency of heat exchangers. Some oxygen is typically lost to outgoing venous blood in the process of heat exchange, depending on heat exchanger efficiency, which can be influenced by the rate of blood flow and blood vessel diameter. As tunas migrate to greater depths, often looking for prey, they encounter cooler water temperatures at the gill surface. In order to able to maintain normal levels of oxygen transport in these conditions, they have developed unique blood respiratory properties. The oxygen carrying capacity in Southern bluefin tuna is high, due to the high hemoglobin (Hb) concentration. The blood affinity for oxygen is also elevated. Normally, blood affinity for oxygen would change with changes in temperature experienced at gills (in comparison to warmer adjacent tissues); however, Hb in Southern bluefin tuna shows insensitivity to temperature, and a reverse temperature effect between 10 °C and 23 °C (Hb-O2 binding is endothermic). Due to their anatomical positioning, the heart and the liver are the coldest organs and a lot of work needs to be expanded for them to serve a regionally warmer body. It is likely the reversed temperature effect on oxygen binding was developed to ensure adequate unloading of oxygen at the heart and liver, especially in colder waters when the difference in temperature between these organs and the swimming muscle is the greatest. Overall, Southern bluefin tunas do not have a set body temperature point, but they do maintain the body temperature within a narrow range, with variations of 4-5 °C over time and from individual to individual.