Sharks: The Animal Answer Guide

2 Form and Function of Sharks

23 What is the metabolism of a shark? Sharks live in the slow lane. They have what we consider to be very slow metabolisms and are built to conserve energy in almost all aspects of their lives, especially when compared with us or even with bony fishes. Their fins and scales and body shape, as discussed in questions in this chapter, are optimal for moving through the water efficiently. In addition, their resting metabolic rates are comparatively low, about one-third that of similar-sized bony fish. Their active metabolic rates are only about 3 times that of resting rate, whereas in a bony fish, active rates are 10 times that of resting rates. Feeding rates are also low: a 2-kg (4.5-lb) Spiny Dogfish, Squalus acanthias, needs only 8 g (about a third of an ounce) of fish per day to maintain itself, whereas a similar-sized salmon needs four times that amount. The above generalizations hold when we talk about small sharks such as the Spiny Dogfish, which are about the size of some bony fishes. It is considerably harder to measure the metabolism of large sharks because of difficulties of keeping them in captivity. But indirect measures can be used, such as estimating oxygen consumption by looking at body temperature as a shark with a transmitter passes through water of different temperatures. This was done for a 4.5-m-long (14.75-ft) White Shark. The resulting calculation indicated that the White Shark used about one-eighth of the oxygen per pound of body weight that a human being uses. The same calculations give us some idea of how often a White Shark must feed to maintain itself. If the same shark fed on a dead whale and ate 30 kg (66 lb) of whale blubber, which is not an unusual amount for a White Shark, it could survive for six weeks before it would have to feed again. Chapter 2 Form and Function of Sharks 24 Sharks: The Animal Answer Guide Are sharks cold-blooded or warm-blooded? The great majority of sharks, rays, and chimaeras are cold-blooded, meaning their body temperature is the same as the surrounding water. However, despite the classic description of sharks as “cold-blooded killers ,” some sharks—including the highly predatory lamnid sharks (White, makos, Porbeagle, Salmon) and the closely-related thresher sharks (Alopiidae )—are actually warm-blooded: they maintain a body temperature that is warmer than the water around them. Muscle activity warms the blood deep in the body of any shark. In most sharks, this blood is carried in veins to the gills to pick up oxygen. Because the gills are in direct contact with the outside water, heat is lost, and blood is quickly cooled to outside water temperature. It is then pumped by the heart and into the arteries and back into the body. Blood returns to the gills via veins close to surface of the shark, again losing heat to the outside. A warming and cooling cycle continues, with the net result being that blood is maintained at about the same temperature as that of the surrounding water . This is also the situation in most bony fishes except tunas and billfishes. But lamnids have a circulatory system different from most other sharks. Both arteries and veins lie deep in the body and in close contact with one another. This allows heat to be transferred from the veins to the arteries, warming the blood coming from the heart before it reaches the muscles. This arrangement is called a counter-current exchange system, because blood vessels carrying blood flowing in opposite directions exchange heat owing to their closeness. Hence, cool arterial blood is continually warmed by warm venous blood. As a result, the body and stomach temperature of a lamnid shark can be 7°C to 10°C warmer than the outside water temperature . How does being warm-blooded help these sharks? The answer is a matter of speculation, but a popular explanation invokes the relationship of body temperature to activity level. Lamnid and thresher sharks are among the largest active sharks in cooler oceans, where they feed on active prey. (Other large, high-latitude sharks include the somniosid sleeper sharks, whose activity levels are described by their name, and the cetorhinid Basking Shark, which feeds on slow-moving zooplankton.) White Sharks are more common in cool-temperate than tropical regions, as are the marine mammals (seals, sea lions, whales) that they prefer. Salmon...