Sharks, rays, and chimaeras belong to the class Chondrichthyes, or cartilaginous fishes. It is a common misconception that they are covered in the same flat, overlapping scales found on bony fish like salmon or tuna. When touched, a shark’s skin has a sandpaper-like texture, suggesting a covering far rougher than traditional scales. This surface is a sophisticated biological armor, a complex microscopic structure that reflects the shark’s unique evolutionary history and serves multiple purposes.
Dermal Denticles: The True Nature of Shark Skin
The skin of a shark is covered by thousands of tiny, tooth-like structures called dermal denticles, which literally translates to “skin teeth.” These denticles are considered the structural and developmental equivalent of teeth in other vertebrates. Unlike the flat scales of bony fish, which grow continually, dermal denticles do not increase in size after formation. Instead, new denticles grow between existing ones as the shark grows, ensuring the entire body remains covered. This covering gives shark skin its characteristic abrasive feel, which is smoother when stroked from head to tail and rougher in the opposite direction.
The morphology of these structures varies significantly across the shark’s body and between different species, reflecting specialized functions. For example, denticles on the fins of a fast-swimming mako shark differ in shape from those on a slow-moving bottom dweller. This variation shows that denticles are finely tuned to the specific hydrodynamic requirements and protection needs of their location.
Anatomy of a Dermal Denticle
Each dermal denticle is a complex, miniature organ anchored in the sharkâs dermis, mirroring the structure of a complete tooth. The base is a broad, flat basal plate embedded firmly in the connective tissue of the skin. Extending upward is the crown, which projects through the epidermis and onto the surface. The center houses a pulp cavity, a space containing blood vessels and nerves that supply the living cells.
Surrounding the central cavity is a layer of hard, calcified tissue known as dentin, which provides the primary bulk and rigidity. The outermost layer of the crown is composed of a hard, enamel-like substance called vitrodentine. This combination of materials makes the denticle exceptionally durable. This robust, tooth-like construction provides a strong and flexible layer of biological chainmail.
Hydrodynamics and Movement Benefits
A primary function of dermal denticles is optimizing the shark’s movement through water. The microscopic grooves, or riblets, running along the denticle crowns are precisely aligned with the direction of water flow during swimming. These structures actively interfere with the development of turbulent flow, which is the chaotic water movement that creates drag.
The denticles reduce skin friction drag by manipulating the boundary layer of water immediately next to the skin. They create tiny, stable micro-vortices that lift the water flow away from the surface, maintaining a more streamlined, or laminar, flow over the body. This mechanism allows the shark to glide through the water with greater efficiency, reducing the energy required for movement. In fast-swimming species, the denticle morphology is adapted to reduce drag across a wide range of speeds, enabling efficient cruising and rapid bursts during a hunt.
The hydrodynamic efficiency of this skin structure has inspired biomimicry in engineering. Researchers have designed coatings and materials that mimic the riblet patterns to improve the performance of vehicles in fluid environments. Materials based on denticle patterns have been shown in studies to reduce drag by a small percentage, leading to applications on aircraft, boats, and competitive swimwear.
Protective Functions and Human Uses
Beyond swimming efficiency, the dense covering of dermal denticles provides the shark with significant protection. The overlapping, abrasive structures offer defense against external physical damage, such as scrapes in rough environments. This layer also acts as a deterrent against parasitic organisms and fouling, which is the accumulation of algae, barnacles, and other marine organisms on the body surface.
The denticle’s surface topography makes it difficult for microorganisms and larvae to securely attach and proliferate. This gives the shark’s skin natural anti-fouling and antibacterial properties, a biological advantage that has inspired materials used in the medical field to prevent bacterial growth on hospital equipment.
Historically, shark skin was harvested and known as “shagreen,” valued for its durability and abrasive texture. Its roughness made it an excellent natural substitute for sandpaper, used by woodworkers to smooth and polish materials. Shagreen was also used as a decorative, water-resistant leather for items like sword hilts, book bindings, and small cases.