Fins to the Left, Fins to the Right . . .
The image of a shark's dorsal fin, knifing through the waves toward some hapless victim, has become a shared cultural icon (Complete with theme music!). But, as is so often the case with these much-maligned animals, the most interesting part of the story is taking place beneath the surface.
Like other sharks, the Great White has two basic categories of fin — paired and unpaired. The pectoral and pelvic fins are each paired, arranged as mirror images of one another on opposite sides of the body. Pectoral ("chest") fins are located immediately behind the gill slits, while the pelvic ("hip") fins are situated on the abdomen at the posterior of the belly. In all extant male sharks, the pectoral fins sport paired, sausage-like intromittant organs called "claspers", which are used to transfer sperm into the body of receptive female sharks. The pectoral and pelvic fins correspond, respectively, to the arms and legs of humans. This is no accident, as both sharks and humans – as well as all other tetrapods (four-footed vertebrates) – share a distant common ancestor that also had a structural plan featuring paired fore- and hind-limbs.
Unlike us, however, sharks – including the Great White – have unpaired fins that occur along the mid-line of the body. Most sharks posses four un-paired fins: first dorsal, second dorsal, anal, and caudal. As in most other sharks, the Great White's first dorsal fin – the one that is usually depicted in movies slicing through the surf, something that rarely occurs in nature – is by far the larger of the two and located almost exactly half-way along the back. The second dorsal fin is located much farther rearward, near the anterior part of the tail stalk. As its name suggests, the anal fin is located near the cloaca (the shark analogue of an anus), posterior to the pelvic fins. Lastly, the caudal fin is located at the posterior end of the White Shark's body.
Each of these fins serves one or more functions. In the White Shark, the pectoral fins are moderately long and slightly falcate, quite broad near the body and have bluntly pointed tips. The resemblance of these fins to aircraft wings results from their similar function. The pectorals originate forward of the shark's center of gravity and have a hydrofoil-like cross-section: flattened on the lower surface and convex on the upper. Used as planing surfaces, they provide a substantial portion (perhaps 25% or more) of the shark's total lift. These pectoral foils combine the ability to carry a heavy load with an exceptionally low stalling speed, enabling even a large White Shark to stalk its prey slowly and almost imperceptibly.
Unlike aircraft wings, a shark's pectoral fins are highly mobile and not nearly so stiff as they may appear. Film of free-swimming White Sharks has revealed that their pectorals are under precise control: they can be raised and lowered, their angle of attack can be increased or decreased, they can be subtly curled, twisted, and even buckled so that their upper curvature is increased. All this fin-foil talent is put to good hydrodynamic use. A slight dip of a pectoral fin combined with a subtle decrease in angle of attack increases water resistance on that side, helping the shark pivot around the lowered pec like a compass; momentarily increasing a pectoral's upper curvature induces a temporary increase in lift on that side, permitting very delicate adjustments in the shark's overall trim. In addition, these large foils also serve the Great White as visual signaling devices, enabling individuals to transmit social messages to other White Sharks.
The White Shark's pelvic fins can also be finely controlled, allowing subtle modifications of the fish's hydrodynamic balance at the posterior part of the body. In some sharks in which mating has been observed, the female's pelvics are cupped during intromission, possibly signaling her acceptance of the male's sexual advances. Perhaps female White Sharks, too, cup their pelvics during copulation. Unfortunately, such details are lacking as, to date, White Shark mating behavior has been witnessed only once — and this from above the surface, hardly an optimal vantage point from which to observe the belly fins of an aquatic creature.
The Great White's first dorsal fin – that quintessential symbol of its sharkiness – is broadly triangular with a short free rear tip and muscular base. The primary hydrodynamic function of this fin is stabilization, preventing the shark's body from rolling uncontrollably. Although the first dorsal looks as stiff and unyielding as a fixed rudder, high-speed film of living White Sharks has revealed that it is actually surprisingly dexterous. The shark can apparently warp and buckle this fin at will, possibly inducing subtle changes in its hydrodynamic properties.
The White Shark's caudal fin is deeply forked with a high aspect ratio. Unlike that of most sharks, in which the upper lobe is considerably longer than the lower, the caudal fin of the Great White is nearly symmetrical in outline. The upper lobe is only slightly longer than the lower, with a relatively small subterminal notch. The tail stalk is flattened from top-to-bottom, forming a strong lateral keel on each side of the caudal fin. In theory, this is the high-thrust tail of a sustained cruiser capable of high-speed bursts of acceleration. Unfortunately, since the White Shark does not adapt well to captivity, no one has been able to measure its swimming efficiency. In a 1990 paper, marine biologist Jeffrey Graham and his co-workers managed to test the swimming efficiency of a close relative of the White Shark in an experimental water tunnel. Graham's team found that a small Shortfin Mako – 32 inches (82 centimetres) long and 8.5 pounds (4.9 kilograms) in mass – could maintain a speed of 0.75 body lengths per second with 18 to 26% fewer tail beats than could other species of similar-sized sharks tested. These results suggest that the Shortfin Mako can achieve a given speed with greater efficiency than can non-lamnid sharks. Since the White Shark is very similar in body shape and swimming style to the Shortfin Mako, it, too, is probably a relatively efficient swimmer.
The White Shark's second dorsal and anal fins are so small relative to the other fins that they may seem unimportant. This is not true. All the lamnids have small second dorsal and anal fins. These fins are reminiscent of the serial finlets that occur along the tail stalk of mackerels and tunas — fishes renowned for sustained high-speed cruising. Like them, the lamnids' second dorsal and anal fins have a pivoting base and a relatively long free rear tip. These fins' pivoting bases may help restore laminar (layered) flow over the posterior part of the body, while their longish free rear tip may help break up fin-tip vortices by swinging back and forth. Both these effects would reduce drag near the caudal fin, thereby making high-speed cruising more energy efficient. Yet another touch of engineering genius from the drawing boards of evolution.
All these fins contribute to making the White Shark a remarkably efficient swimming machine. But, as we shall see, its swimming adaptations do not stop there. From the tip of its snout to the notch in its tail, the Great White is indeed a hydrodynamic miracle.
