Many people assume that sharks, being fish, are covered in scales like most other aquatic creatures. However, this common assumption is not accurate. Sharks possess a skin covering that is fundamentally different from the flat, overlapping scales found on bony fish. Their unique skin is a specialized adaptation, providing distinct advantages in their marine environment.
The Truth About Shark Skin
Shark skin is covered by thousands of tiny, tooth-like structures called dermal denticles, also known as placoid scales. These structures are embedded within the shark’s skin, giving it a rough, sandpaper-like texture when stroked against the grain. The name “dermal denticle” literally translates to “skin tooth,” which accurately describes their composition and appearance.
Anatomy of Dermal Denticles
Each dermal denticle is a complex, miniature tooth-like structure. A flat, rectangular basal plate anchors the denticle within the shark’s dermis, the inner layer of the skin. Rising from this base is a spine or cusp, which points backward, towards the shark’s tail.
This spine encloses a central pulp cavity, which contains blood vessels and nerves, similar to the pulp found in a tooth. Surrounding this pulp cavity is a layer of dentine, a hard, calcified tissue that forms the bulk of the denticle. The outermost layer, covering the dentine and forming the visible crown of the denticle, is a hard, enamel-like substance called vitrodentine or enameloid. This composition provides the denticle with its durability and tooth-like properties.
How Dermal Denticles Benefit Sharks
The unique structure of dermal denticles provides sharks with several significant advantages, particularly in terms of hydrodynamics and protection. The backward-pointing cusps and often ridged surfaces of the denticles help to reduce drag and turbulence as the shark moves through water. They achieve this by channeling water flow and disrupting the formation of vortices, which allows for more efficient swimming.
Beyond streamlining, dermal denticles also offer protection. They form a tough, abrasive layer that shields the shark from physical damage, such as scrapes against rough surfaces or bites from other marine animals during feeding or territorial disputes. This protective layer also acts as an effective anti-fouling mechanism. The rough, irregular surface created by the denticles makes it difficult for parasites, algae, barnacles, and other microorganisms to attach and grow on the shark’s skin. This resistance to biofouling helps sharks maintain their agility and reduces the risk of infections.
Distinct Differences from Fish Scales
Shark dermal denticles differ significantly from the scales found on most bony fish in several key aspects. Unlike fish scales, which are typically thin, flat, and overlapping plates that grow larger as the fish grows, dermal denticles do not increase in size. As a shark grows, it produces more denticles to cover the expanding surface area of its skin.
The developmental origin of these structures also varies; fish scales are derived from the mesoderm layer of the dermis, while dermal denticles, like teeth, originate from neural crest cells. This shared embryonic origin with teeth underscores their tooth-like composition and structure. The texture they impart is different: bony fish scales generally feel smooth, whereas shark skin, due to its denticles, feels rough and abrasive when rubbed in one direction, much like sandpaper.