Variation in Denticles
| In the past, variation in shark denticles was considered a valid trait with which to identify an individual's genus, and sometimes even species. However, recently there has been more interest in studying the variation in denticles on a single shark. The purpose of this exercise would not be to identify the individual, but rather to discern any pattern of denticles that might hold clues as to how the drag-reducing properties of shark skin function. |
Denticle Styles in Various Parts of a Shark's Skin
This figure was adapted from Bargar and Thorson 1995.
Probable function of different types of denticles:
| Along the majority of a shark's body run longitudinal riblets, composed of many denticles (Bechert et al. 1986). The part of the boundary layer in contact with the body is channeled through these riblets. This dampens streamwise vortices, reducing frictional drag along most of the body's length (Bargar and Thorson 1995). |
| At the rostrum and on the leading edges of the fins the skin is almost totally devoid of riblets. This arrangement probably has hydrodynamic function, promoting smooth water flow to each side. If the leading edges of the body and fins were covered by the same denticles as the majority of the body, a swimming shark would deflect the boundary layer away from its body with its rostrum and fins. This would increase all three types of drag. Instead, relatively smooth leading edges of a shark's body cut into the water with a minimal loss of energy (Bargar and Thorson 1995). |
| Water approaching the posterior edge of the fins encounters unfavorable pressure gradients and frictional forces, causing the boundary layer to separate from the skin, and producing a wake. However, the posterior edges of fins are flexible and denticle-free. This may help to reduce turbulence, saving energy lost to the vortices occurring immediately behind the fins (Bargar and Thorson 1995). |
| It should also be noted that in addition to having denticle types uniquely suited to their position on the body, sharks potentially have active control of their skin. This is possible because shark muscles are attached not only to the skeletal system but also to the skin (Naresh et al. 1997). |
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