Physiology: Fire of Life

Its senses quivering with alertness, a Shortfin Mako scans its vast liquid universe for potential prey. As it swims, oxygen-bearing water is forced into the shark's mouth, through the pharynx, and out the gill slits. Within the gash-like slits, crimson filigrees of gill filament collectively dump waste carbon dioxide and greedily consume life-sustaining oxygen. Suddenly, a thrumming commotion in the distance catches the shark's attention. The Mako turns toward its source, accelerating rapidly as its large, dark eyes strain to pierce the water between it and whatever was making the pulsing noise. There. In the distance: a school of about three dozen Yellowfin Tunas attacking a fleeing armada of pelagic squid. Each fish resembles a silvery-flanked bullet with a black back and amber, scythe-like fins, propelled by a crescent-moon caudal fin whipping side-to-side some 20 times per second. Preoccupied with the jetting squid, the tunas are oblivious to the shark's approach. Within a distance of about 50 feet (15 metres), the Mako selects an individual tuna and launches its attack. As its heart rate and tailbeat frequency increase, oxygen-carrying blood and metabolic heat floods through the shark's swimming muscles, priming them for impending violence. Within inches of its prey, the Mako protrudes its palatoquadrate, revealing upper teeth that are broader and more knife-like than the gracefully recurved lowers. The hapless Yellowfin flinches to the right, its tail smashing into the shark's mouth and knocking loose a few teeth which explode from its mouth in tinkling cascade. At that instant, the Mako reflexively bites down, its remaining teeth severing the tuna's meaty tail stalk with the final authority of a guillotine. Mortally wounded, the Yellowfin lolls on its side, its truncated tail stalk wagging impotently as its large, staring eyes try desperately to keep its assassin in view through a brownish cloud of its own blood . . .

Just as each of us must balance a household budget, other animals must balance an energy budget. All of the multitudinous tasks that an organism needs to carry out in order to maintain itself - feeding, growth, self-repair, locomotion, reproduction, and so on - must be accomplished within its energy budget. Everything an animal does is limited by the number of calories it takes in. Like slicing a pie into however many pieces are needed to serve a given group, an animal's caloric budget can be apportioned in any number of ways, but - no matter how an individual organism slices up its energy pie - in the final analysis, there is only one pie. Thus, energy that is expended in escaping from predators or healing wounds is no longer available for growth or reproduction. Like ourselves, the White Shark (Carcharodon carcharias) is an animal and, like us, it must live within its means. In practical terms, this means finding and using caloric energy efficiently, conserving its reserves in the face of a changeable and uncertain environment. The elegant physiological mechanisms that the White Shark has evolved for dealing with this harsh reality are a wonder to behold. Although the separation of Anatomy and Physiology is somewhat artificial - one cannot have one without the other - it is expedient. The apparently disparate topics dealt with in this chapter are unified by a common theme: energy-handling mechanisms of the White Shark. These mechanisms add up to a spare, efficient animal that wastes very little. There is a lesson here for our own, highly wasteful species: energy conservation works.

 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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