The question of whether sharks possess hair often arises from the sandpaper-like texture of their skin, which is unlike the smooth surface of most bony fish. The definitive answer is no, sharks do not have hair. As cartilaginous fish, their skin is covered by a unique and complex array of structures that serve a dual purpose for survival. This specialized covering is neither hair nor a typical scale, but a remarkable evolutionary adaptation.
The Direct Answer and What Sharks Have Instead
Sharks, along with rays and skates, possess a covering made up of thousands of microscopic, tooth-like structures known as dermal denticles. The name literally translates to “skin teeth.” Dermal denticles give shark skin its rough, abrasive quality, a texture that is noticeably coarse if stroked from tail to head. Unlike the smooth, overlapping scales of bony fish, these denticles are individually embedded in the skin.
The denticles’ tooth-like appearance is rooted in their developmental biology. They share a vast gene expression signature with developing teeth, meaning they arise from a similar biological blueprint. This connection confirms that the shark’s skin covering is fundamentally different from the hair or scales found on other vertebrates.
Anatomy and Function of Dermal Denticles
The microscopic anatomy of a dermal denticle mirrors that of a vertebrate tooth, consisting of a basal plate, a stalk, and a crown that protrudes from the skin. Each denticle contains an inner pulp cavity supplied with nerves and blood vessels, surrounded by a layer of dentine. The crown’s outermost layer is covered by vitrodentine, a hard, enamel-like substance that provides a durable casing.
These mineralized structures provide two primary benefits, the first being physical protection. The denticles create a flexible, yet tough, suit of armor that shields the shark from abrasion, damage, and parasites. The crowns of the denticles are often highly sculptured with longitudinal ridges, and these features are related to the second main function: hydrodynamics.
The microscopic grooves formed by the denticles function as “riblets,” which manipulate the flow of water over the shark’s body. This surface texture reduces drag and turbulence, allowing the shark to glide through the water more efficiently. The shape and density of the denticles vary across different species and on different parts of the shark’s body, reflecting specialized needs for speed or defense.
Comparing Denticles to Other Structures
Dermal denticles stand apart from the coverings of other animals, particularly true hair and the scales of bony fish. Hair, a characteristic of mammals, is composed primarily of keratin, a fibrous structural protein. Hair is a flexible, epidermal structure that originates from the outer layer of the skin, and its function is primarily insulation or sensory.
In contrast, the scales found on most bony fish, such as cycloid or ctenoid scales, are thin, bony plates that lie in an overlapping pattern within the dermis. These bony scales grow continuously throughout the fish’s life by adding concentric layers. Shark denticles do not grow once they emerge; instead, new denticles are added to cover the increasing surface area as the shark matures. This difference in growth pattern and composition highlights the denticle’s unique classification as a structure more closely related to a tooth than a scale.