E12 Regional Thermoregulation

Topic

Regional endothermy is the ability of certain fish—such as tunas, marlins, swordfish, mako sharks, and great white sharks—to maintain the temperature of their locomotor muscles, brain, or eyes significantly above that of the surrounding water, thanks to specialized counter-current heat exchangers known as a *rete mirabile* (thermal rete). Unlike mammals, which are full endotherms that regulate their entire body temperature, these fish are regional endotherms: only specific regions of their bodies are thermoregulated, while the rest remain in thermal equilibrium with the water.

The physical mechanism underlying this regional endothermy is the vascular counter-current heat exchanger: cold arterial blood arriving from the gills passes through capillaries situated directly alongside the venous capillaries returning warm blood from the muscle. Heat transfer between the adjacent artery and vein is rapid enough for the arterial blood to reach the muscle already warmed, thereby retaining metabolic heat within the muscle core rather than losing it to the gills and, subsequently, to the water.

The effectiveness of this heat exchanger—that is, the fraction of metabolic heat retained rather than lost—depends on the relationship between the exchanger's thermal conductance and the convective heat flow transported by the blood: the higher the thermal conductance relative to the heat flow to be retained, the more effective the exchanger is at heat retention.

The temperature elevation the muscle achieves relative to the surrounding water is proportional to the rate at which the red muscle—which operates continuously—produces metabolic heat, divided by the heat losses the muscle sustains to the environment. This regional endothermy allows tuna to swim in cold waters while maintaining high muscle power—since muscle contraction is faster and more powerful at higher temperatures, reflecting the same relationship between temperature and the speed of biological processes described in this collection's thermal collapse model; thus, regional endothermy provides tuna with a performance advantage in environments where water temperatures vary.

The expected behavior is that this excess muscle temperature scales positively with both swimming speed and the fish's body size; consequently, large tuna—weighing over fifty kilograms—can maintain their muscle temperature between ten and twenty degrees above that of the surrounding water, the same temperature differential described at the beginning of this model as the defining characteristic of regional endothermy.

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