The answer to whether sharks have scales is that they do not possess the same scales found on most bony fish. Sharks, rays, and skates are cartilaginous fish, or elasmobranchs, and their skin is covered instead by structures called dermal denticles. This term literally translates to “small skin teeth,” which accurately describes their composition and function. The unique texture of a shark’s skin, which feels like sandpaper when rubbed backward, is due to these numerous, microscopic denticles.
The Structure of Dermal Denticles
Dermal denticles are minute, hardened structures structurally homologous to vertebrate teeth. Each denticle is composed of three main parts, mirroring the composition of a tooth. The core contains a pulp cavity, a hollow area supplied with blood vessels and nerves during development. Surrounding this cavity is a layer of dentin, a calcified tissue that provides the bulk of the structure. The outermost layer, which forms the exposed crown, is a hard, enamel-like substance called vitrodentine or enameloid. This highly mineralized cap makes the sharkâs skin rough and durable.
These structures originate from the same embryonic tissue as teeth, explaining their similar composition. Unlike the scales of bony fish that grow by adding concentric layers, dermal denticles do not increase in size throughout the shark’s life. Instead, new denticles are continuously produced to replace damaged or shed ones, maintaining the skin’s integrity.
Hydrodynamic and Protective Functions
The structure of dermal denticles provides a dual advantage for sharks in their marine environment. One primary function is to enhance swimming efficiency through unique hydrodynamic properties. The exposed crowns often feature microscopic ridges, known as riblets, which run parallel to the direction of water flow.
These riblets interact with the turbulent boundary layer of water that forms over the shark’s body as it swims. By channeling the water flow and preventing the formation of large, drag-inducing vortices, the riblets effectively reduce skin-friction drag. This mechanism allows the shark to move through the water with greater energy efficiency.
Beyond hydrodynamics, the denticles act as a robust form of armor plating. The hard, overlapping, and abrasive nature of the denticles provides excellent protection against physical abrasion, such as rubbing against rough surfaces or during aggressive interactions. Due to their composition, dermal denticles have been noted to be as hard as granite and as strong as steel.
Furthermore, the denticles possess anti-fouling properties, preventing the attachment and growth of parasites, algae, and barnacles on the shark’s skin. The continuous shedding and replacement of denticles, combined with the microscopic texture, creates a surface difficult for organisms to colonize. This self-cleaning function is crucial for maintaining the shark’s health and streamlining.
Dermal Denticles Versus True Scales
The fundamental difference between dermal denticles and true scales lies in their biological origin and composition. Dermal denticles are derived from the same tissue layer as teeth, the mesoderm, and are composed of dentin and vitrodentine. They emerge from the skin, piercing the epidermis, and are not covered by a layer of skin or mucus.
In contrast, the true scales of bony fish, known as cycloid, ctenoid, or ganoid scales, are thin, bony plates embedded entirely within the skin’s dermis. Cycloid and ctenoid scales, found on most modern bony fish, are formed largely from bone and fibrous connective tissue. They are arranged in an overlapping pattern, allowing for flexibility as the fish swims.
Unlike denticles, which maintain a constant size and are replaced individually, the scales of bony fish grow continuously throughout the fish’s life, adding concentric rings that can be used to estimate age. The hard, tooth-like structure of the denticle, with its pulp cavity and dentin, is absent in the flexible, overlapping scale types of bony fish.