Skin of the Teeth

Shark scales are tiny compared with those of teleosts (bony fishes) and have a characteristic tooth-like structure. Although they are often termed placoid ("plate-like") scales in older texts, most biologists today prefer the more descriptive phrase, dermal denticles (literally, "tiny skin teeth"). These denticles typically have a broad basal plate, a narrow stalk, and a broad, ridged or otherwise highly sculptured crown. In general, the crowns of dermal denticles have cusps pointing tailward, which is why a shark feels relatively smooth if stroked from head-to-tail but sandpapery coarse if stroked the other way. (An interesting exception is the Basking Shark [Cetorhinus maximus], in which the crowns seem to point every which way; Norwegians, who have commercially harvested this species for decades, have come up with a clever use for this peculiarity: they paste a strip of Basking Shark skin on the soles of their boots, preventing slippage on wet, rolling decks.) The White Shark is furnished with dermal denticles, too, and it is worth taking a moment to consider briefly their many functions.

Dermal Denticles of a White Shark (head to the left).  
A. Dorsal(top) view of the crowns of three denticles. 
B. Lateral (side) view of a single denticle.

Redrawn after Radcliffe (1916)

Dermal denticles are built on the same engineering principles as the most durable of man-made compounds, such as fibreglass and reinforced concrete. Embedding a hard material inside a softer one combines the best properties of both, providing the rigidity of the former without brittleness and the plasticity of the latter without distortion. The dentine layer of dermal denticles is composed of a hard, crystalline mineral called apatite, embedded in a soft protein, our old friend collagen. Due to their microstructure, dermal denticles are about as hard as granite and as strong as steel. Not surprisingly, dermal denticles afford sharks no small measure of physical protection. Yet they do so without sacrificing mobility, like a built-in suit of chainmail armor. The dermal denticles of the White Shark have crowns shaped like miniature horseshoe crabs, so tiny as to be barely visible to the naked eye. These crowns overlap tightly, providing protection from both large potential predators including other Great Whites and tiny skin parasites.

The denticle crowns of the White Shark are highly sculptured, each with three longitudinal ridges that terminate in a rearward-pointing cusp. Although it may seem counter-intuitive for an aquatic animal to be anything but smooth as possible, there are actually sound hydrodynamic benefits to be gained from such sandpaper roughness. How strategic roughness can yield aero- and hydro-dynamic benefits has elicited a great deal of research in recent years. Consider the humble golf ball. Those characteristic dimples are not created equal: the indentations around the equator of the ball are actually slightly deeper than those at the poles. This deceptively simple design feature grants a golf ball in flight and with the proper backspin an additional two seconds of 'hang-time' increasing driving range by as much as 80 feet (24 metres) and reduces the incidence of hooks and slices by as much as 75%. Similarly, in fighter jets or fast ships, the secret to their phenomenal speed lies in fine, V-shaped grooves. These grooves must be very closely spaced about as close together as the grooves on an old-fashioned phonograph record (Anyone remember those?). Such closely-spaced grooves appear to reduce drag by preventing eddies from coming in contact with the surface of a moving body. Nowadays, there is hardly an American military aircraft or vessel that does not somehow benefit from the fluid dynamic efficiency of incorporating strategically-placed, V-shaped grooves along the fuselage, hull, and foils. But, whenever there is a physical principle that provides an elegant solution to a practical environmental challenge, it seems that Nature always beats us to the punch. Collectively, the tiny, three-ridged dermal denticles of the White Shark create closely-spaced grooves similar to those on high-speed air or water craft. These denticles very probably impart similar drag-reducing properties to the shark. Thus, without understanding the first thing about golf balls or military craft, the White Shark has been employing many of the same fluid dynamics principles for millions of years.

In a short but fascinating 1982 paper, Wolf-Ernst Reif and his co-worker A. Dinkelacker reviewed the hydrodynamics of dermal denticles in fast-swimming sharks. Reif and Dinkelacker found that the crowns of dermal denticles in the Shortfin Mako and other fast-swimming sharks are smooth and almost ridgeless on the tip of the snout and leading (anterior) edges of the fins, but elsewhere on the body the crown ridges are quite steep, with depths one-half to two-thirds their width. They also found that the alignment of these crown ridges varies over the body, closely approximating path-of-least-resistance flow of water over the surface of the shark. The smoothness of denticles on the leading edges of the snout and fins offer the least resistance to these areas of minimal boundary layer thickness. In contrast, the alignment of crown ridges with the 'natural' flow-direction of water over the shark's body can be expected to maximize drag reduction by reducing turbulence, thereby preventing eddy formation. The arrangement of dermal denticles in the White Shark is probably very similar to that exhibited by the Shortfin Mako. Thus, like a dimpled golf ball, a grooved Great White may glide farther on a given amount of energy than would a smooth one.

In a 1986 paper, biologists William Raschi and Jennifer Elsom reviewed the drag-reduction properties of shark dermal denticles. Raschi and Elsom examined the denticles of 15 species of shark and found that those of fast-swimming pelagic species such as the Shortfin Mako are consistently smaller and lighter than those of sluggish or bottom-dwelling species. Therefore, the relatively small, light-weight dermal denticles of the White Shark are probably adapted for fast swimming more than armor-like protection yet another compromise between form and function. In addition, they found that the Shortfin Mako and other fast-swimming species consistently had ridge characteristics nearer those values predicted as optimal for burst speeds. Raschi and Elsom also found that, despite growth-associated increases in the crown size of denticles, the height and spacing of the scales' longitudinal ridges remained nearly constant in all species examined. This suggests that some important functional feature may be maintained throughout a shark's life. That feature is very probably drag reduction in a 1984 report, Raschi and ichthyologist Jack Musick discovered that the longitudinal ridge system created by shark dermal denticles is responsible for drag reductions as great as 8%. That percentage represents a substantial energy savings, and it seems unlikely that the White Shark would not take advantage of the benefits afforded by this mechanism.

There is at least one further benefit of sharks' hydrodynamically sculpted dermal denticles: stealth. Despite pioneer undersea explorer Jacques-Yves Cousteau's poetic description of the marine environment as a "silent world", the ocean is full of noise. Mournful songs of lonely whales, exuberant squeals and clicks of cavorting dolphins, multitudinous croaks and yaps of reproductively-ripe fishes, and the incessant, static-like chorus of snapping shrimps vie with the mechanical rumble of ocean-going ships and the frenetic buzz of speedboats. If you were to lower a hydrophone (underwater microphone) near a school of teleost fishes, you would quite easily hear the sloshing sounds of water turbulence, created by their swimming movements. The large, overlapping scales of teleosts are not nearly as hydrodynamically 'clean' as the dermal denticles of sharks. If you were to place the same hydrophone near a cruising shark, no such swimming sounds would be heard. Sharks are, literally, "silent hunters". For the Great White, this hydrodynamic side-effect probably confers tremendous advantages when stalking prey: the hapless fish or sea lion almost never hears the shark that caught it.

 

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