A shark’s skin surface is highly adapted to its aquatic environment. While appearing smooth when stroked from head to tail, a shark’s skin feels rough in the opposite direction, resembling sandpaper. This distinctive texture is due to specialized structures covering nearly their entire body. These structures play a significant role in the shark’s survival, offering both protection and enhancing their movement through water.
Placoid Scales: The Shark’s Unique Armor
Sharks are covered in tiny, tooth-like scales known as placoid scales, also called dermal denticles. Each placoid scale has a flat base plate embedded in the dermis, with a spine projecting outwards.
The structure of a placoid scale mirrors that of a tooth, featuring an inner pulp cavity containing connective tissues, blood vessels, and nerves. This pulp cavity is surrounded by dentine, a hard, calcified material. An even harder, enamel-like substance called vitrodentine covers the dentine, forming the outermost layer of the scale. The specific shape and arrangement of these denticles can vary considerably between different shark species, often reflecting their particular lifestyle and habitat.
Beyond Protection: The Many Roles of Dermal Denticles
Dermal denticles provide sharks with multiple advantages beyond simple physical protection. Their unique shape and arrangement contribute significantly to hydrodynamics, allowing sharks to move efficiently through water. The tiny ridges, or “riblets,” on the surface of the denticles reduce drag and improve swimming efficiency by disrupting turbulent water flow.
These scales also form a tough, abrasive outer layer that acts as a physical defense. This protects sharks from predators, injuries, and parasitic organisms. The texture created by the denticles makes it difficult for parasites, algae, and barnacles to attach to the shark’s body, offering an anti-fouling benefit. Some research suggests that certain denticles possess sensory capabilities, aiding in the detection of water movements or electrical fields, although this function is less understood.
How Shark Scales Differ from Other Fish Scales
Shark scales, or dermal denticles, fundamentally differ from the scales found on most bony fish. Placoid scales originate from the dermis, the deeper layer of the skin, similar to how teeth develop. In contrast, the scales of bony fish, such as cycloid, ctenoid, or ganoid scales, are bony plates that are also dermal in origin but are typically covered by the epidermis.
Another key difference lies in their growth pattern. Placoid scales do not grow larger throughout a shark’s life; instead, new scales are continually produced and added as the shark grows, filling in any newly exposed skin. Bony fish scales, however, typically grow in concentric layers throughout the fish’s life, often displaying growth rings that can indicate the fish’s age. Furthermore, shark scales are individually embedded and generally do not overlap in the same manner as the overlapping, tile-like arrangement seen in many bony fish.