Metabolism and growth rate: life in the slow lane
Many aspects of the biology of sharks point to a strong emphasis on efficient energy use when compared with bony fishes. In addition to the anatomical features such as fin and scale morphology mentioned above, physiological attributes of sharks indicate a premium placed on energy conservation. Resting metabolic rates of a 2 kg Spiny Dogfish average 32 mg O2/kg body weight/h, about one-third of the average resting rate for comparably sized teleosts. In dogfish, the active metabolic rate is only triple that of the resting rate, whereas teleost active rates often go up 10-fold (Brett & Blackburn 1978). Extrapolated prey intake rates indicate that a 2 kg dogfish would need only 8 g fish prey per day for maintenance, whereas a similar-sized salmon would require four times that amount.
Comparable data for larger sharks are unavailable, for obvious logistic reasons. Estimated oxygen consumption suggests that sharks consume about half the oxygen of equivalent-sized bony fishes (Moss 1984).
For example, calculations of energy consumption have been made for a 4.5 m White Shark that was harpooned with a temperaturerecording ultrasonic transmitter (Carey et al. 1982). Based on the rate at which the temperature of the shark’s muscle mass changed as it passed through a thermocline (region of rapid temperature change), its oxygen consumption rate was calculated to be about 60 mg O2 or 0.2 kcal/kg body weight/h (a 60 kg human consumes about 1.6 kcal/kg/h).
Low metabolic requirements may translate into reduced energetic needs compared to bony fishes. White Sharks feed commonly on dead whales. Based on the caloric value of 30 kg of whale blubber found in a 940 kg White Shark’s stomach and on the above calculations of metabolic rate, it was estimated that White Sharks can maintain themselves by feeding on whales only once every 6 weeks (Carey et al. 1982).
Food consumption rates vary greatly among shark species but are apparently most closely related to degree of activity (Duncan 2006). Relatively sedentary sharks, such as the Nurse Shark, consume 0.2–0.3% of their body weight per day and digest an average meal over at least 6 days. Moderately active sharks, such as Sandbar and Blue sharks, consume 0.2–0.6% of body weight per day, digesting a meal in only 3–4 days. Very active sharks such as the Mako, which are “warm-bodied” (see Temperature relationships), eat 3% of their body weight per day, digesting their meals in 1.5–2.0 days. Translated into annual consumption rates, the Mako eats about 2 kg/day, or about 10–15 times its body weight per year. Although such fi gures appear large, they are about half the annual consumption of an individual teleost, emphasizing the relative energy efficiency of feeding in sharks.
To the list of possible energy-saving mechanisms of sharks can be added an intriguing but as yet puzzling characteristic, namely heat conservation. Lamnid sharks and to a lesser extent thresher sharks are able to conserve some of the heat generated during muscle contraction and thereby maintain their muscle and stomach temperatures at about 7–10°C above ambient water conditions (Carey et al. 1982; McCosker 1987). Heat generated during muscle contraction in most sharks is dissipated because cold, oxygenated blood coming from the gills moves into the deeper parts of the body. In lamnid sharks, a countercurrent exchange arrangement helps warm arterial blood flowing from the gills. This is not true thermal regulation as happens in birds and mammals or even tunas; body temperature is not constant but varies with external temperature. The potential adaptive signifi cance of this elaborate structure and process is a matter of conjecture. Higher body temperatures may permit maintenance of a higher metabolic level and generation of more muscle power, thus facilitating the capture of fast swimming prey (including endothermic marine mammals), and may increase the rate at which food is digested (Carey et al. 1982; Bone 1988). All of these factors might extend the ability of these large predators to invade cool waters at high latitudes (Block & Finnerty 1994).
Low metabolic demands may be linked to relatively slow growth and long life spans. After an initial rapid growth phase of 15–60 cm increase per year (see above), growth in most sharks slows considerably. Growth rates of juveniles and adults of 12 species of medium- to large-sized sharks averaged only 5 cm/year (Thorson & Lacy 1982; Branstetter & McEachran 1986). Longevity estimates vary considerably among and within species, but chondrichthyans on average live longer than bony fishes (Cailliet & Goldberg 2004). Among batoids, sawfishes live 30–44 years, stingrays 3–28 years, and skates 9–24 years (except in Europe’s largest skate, the Critically Endangered Common Skate, Dipturus batis, that can live to 50 years). Sharks are similarly long-lived, again with much variation. Angel sharks live to 35 years, carpet sharks 19–35 years, dog sharks 12–70 years (including the longest lived species, the deepwater Centrophorus squamosus), mackerel sharks 10–25 years, and ground sharks 4–32 years (most chimaeras live 5–10 years, with a maximum longevity of 29 years in Chimaera monstrosa). Age estimation in sharks is frustrated by a lack of retained, calcified structures; growth rings in vertebrae are the most commonly used indicator of age when more direct measurements are unavailable (Cailliet 1990; Cailliet & Goldman 2004).