Homeothermy in adult salmon sharks, Lamna ditropis

Salmon sharks, Lamna ditropis, belong to a small group of sharks that possess vascular counter-current heat exchangers (retia mirabilia) allowing retention of metabolically generated heat, resulting in elevated body temperatures. The capacity of free-swimming lamnid sharks to regulate rates of heat gain and loss has not been demonstrated. Using acoustic telemetry, we recorded swimming depth and stomach temperature from four free-swimming salmon sharks in Prince William Sound, Alaska. Temperature data were obtained over time periods ranging from 3.8 to 20.7 h. Temperature profiles of the water column were obtained concurrently for use as estimates of ambient temperature. Mean stomach temperature among four individuals tracked ranged from 25.0 to 25.7°C. These sharks defended specific elevated temperatures regardless of changes in ambient temperature, which ranged from about 5–16°C. The maximum observed elevation of stomach temperature over ambient was 21.2°C. Because stomach temperatures were so strictly maintained relative to changes in ambient temperature, a thermal rate coefficient, k, (°C min^–1 °C thermal gradient^–1) for cooling of 0.053 min^–1 was obtained via a `control' experiment with a dead salmon shark. We show that free-swimming adult salmon sharks maintain a specific stomach temperature independent of changes in ambient temperature through a combination of physical and physiological means, and essentially function as homeotherms. This unique ability is probably the underlying factor in the evolutionary niche expansion of salmon sharks into boreal waters and in their ability to actively pursue and capture highly active prey such as salmon.