Among sharks, the lamnids (family Lamnidae: white sharks, makos, and porbeagles) have a unique physiology. Whereas all other sharks (as far as we know and excepting the possibility of the thresher sharks Alopias sp. joining the physiological ranks of the lamnids) are ectothermic--that is, their body temperature is controlled by the ambient sea water temperature--lamnids are able to warm their muscles, stomach, viscera, brain and eyes to a temperature above that of the ambient sea water. This is called regional endothermy, and is accomplished by the rete mirabile ("wonderful net"), capillary beds of parallel arteries and venules found near the "warm structures" (in white sharks, there is the orbital rete, for the brains and eyes, the suprahepatic rete for the stomach and viscera, and the subcutaneous rete for the muscles). The rete work as countercurrent heat exchangers: as warm, oxygen-poor blood passes through the venules in the rete (on its way to the gills), the heat it carries (gained from the shark's metabolism) is transfered to the parallel arteries, which contain cold blood with high levels of oxygen, having just come from the gills. This way, little heat is lost to the outside water, and most heat is kept within the animal's body. This allows the white shark to maintain an average stomach temperature of about 25 degrees celsius (with a narrow range of fluctuation), regardless of the outside sea water temperature. Of course, this means that in colder water, the white shark has to heat its stomach up even more than a white shark in warmer water, to maintain this optimal 25 degree C operating temperature.
Of what use is this regional endothermy in the white shark? Basically, it enhances the overall predatory and physiological efficiency of the great fish. The orbital rete increases neural activity and visual acuity, by warming the brains and eyes, respectively. The subcutaneous rete warm the muscles which increases efficiency and power (a warm muscle is a better muscle, as swimmers and other athletes well know). Finally, the suprahepatic rete warm the stomach and viscera, which speeds up and increases the efficiency of digestion (e.g. the warming increases the activity rates of protein-digesting enzymes such as trypsin and chymotrypsin). Efficient digestion is of great use to the white shark, as it often feeds on marine mammals, which are well-stocked with fat (blubber). Blubber is rich in energy, but is slow to digest, so an increased digestion rate in the white shark allows it to process large amounts of blubber, as well as plain-old meat, fast and efficiently. The more the shark can digest at one time, the more energy it can store in case of long periods with out food.
White sharks may even warm their stomachs to an even higher temperature than the normal visceral temperature, which would naturally increase digestion rates even more. However, this has not yet been verified. Recent research by Kenneth Goldman et al. (1996) suggests that the extra elevation, if it occurs at all, is probably small--about 2-3 degrees C--and that the heating process is a slow one, taking a few days to reach the peak, and another few days to decrease to normal operating levels. If this is true, it is quite different from the stomach elevation seen in the bluefin tuna (Thunnus thynnus), which can increase by as much as 15 degrees C in just 12-20 hours, before returning to the normal 5-6 degree C elevation (tunas, as well as some other fast-moving pelagic bony fish, also feature regional endothermy). Even if the white shark can't elevate its stomach temperature after feeding, its digestion is still fast and efficient because the stomach is already warm from the constant action of the suprahepatic rete.
Finally, there is evidence that the white shark has a high metabolism. Scott Emery (1985) found that the blood of the white shark contains high values of hemoglobin (a substance found in the red blood cells of vertebrates which carries oxygen) and hematocrit (the ratio of red blood cells to the volume of blood--typically, the higher the ratio, the higher the metabolism). These high values are comparable to "higher" vertebrates like birds and mammals. The reason this supports high metabolism is that it means more oxygen can be carried in the white shark's blood at any given time, and the more oxygen the more aerobic metabolism (as opposed to anaerobic metabolism, which doesn't require oxygen and is used only in short sprinting situations) that can take place. Emery also looked at the white shark's heart for clues to its rate of metabolism. He found that the white shark's heart weight to body weight ratio was at a level seen in the warm bodied bluefin tunas, and also comparable to mammals and birds. Also, the white shark's ventricle (which pumps oxygenated blood out of the heart to the body) is well muscled, allowing the heart to pump lots of blood at a fast rate, as in birds and mammals. The small volume of the ventricle is also indicative of a "high pressure pump", as in endotherms like birds and mammals, and the warm bodied tuna (La Place's Law says that a narrow chamber with an associated small volume needs less muscle tension than a rounded one to maintain a certain pressure--in this case, the pressure at which the blood leaves the heart). All of these traits point to the ability of the white shark to pump well oxygenated blood (thanks to the high hemoglobin and hematocrit levels) around the body at a fast rate (thanks to the heart's large size, and the small volume and well-muscled nature of the ventricle). This is indicative of a high metabolism and activity rate comparable to those in birds and mammals.
The current benefits from the warming effect of the rete mirabile (and the associated high metabolism) in the white shark are evidently very useful, but even more paramount is their evolutionary significance. The rete have probably allowed the white shark (and other lamnids) to inhabit cool temperate and cold waters around the world, all while remaining active and aggressive predators of fast-moving fishes and, in the case of the white shark at least, marine mammals. The development of regional endothermy in the white shark was essential, as most pinnipeds live in cool temperate or cold waters. Essentially, the white shark's rete allowed it to follow the evolutionary trend toward colder waters that appeared millions of years ago among marine mammals, and to establish itself as their main predator, at least in the case of the pinnipeds. Without their rete mirabile, the white shark and other lamnids would lead a notably less active lifestyle.
Carey, Francis et al. 1985. "Temperature, Heat Production and Heat Exchange in Lamnid Sharks." Memoirs of the Southern California Academy of Sciences vol. 9, pp. 92-108.
Ellis, Richard and John McCosker. 1991. Great White Shark. (New York: Harper Collins).
Emery, Scott H. 1985. "Hematology and Cardiac Morphology in the Great White Shark, Carcharodon carcharias." Memoirs of the Southern California Academy of Sciences vol. 9, pp. 73-80.
Goldman, Kenneth et al. 1996. "Temperature, Swimming Depth, and Movements of a White Shark at the South Farallon Islands, California." In Great White Sharks: The Biology of Carcharodon carcharias, edited by A.P. Klimley and D.G. Ainley (San Diego: Academic Press), pp. 111-120.
Copyright: Ben S. Roesch, 1997.
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