Placoids

Placoids are a type of dermal denticle, which are tooth-like scales found on the skin of cartilaginous fishes, such as sharks, skates, and rays. Unlike the scales of bony fishes, placoids are not true scales in the sense of being calcified dermal bone. Instead, they are composed of a pulp cavity, a vascularized connective tissue core, a dentine layer, and an enameloid cap. This structure is remarkably similar to the teeth of vertebrates.

The primary function of placoid scales is to reduce drag and increase hydrodynamic efficiency, allowing these aquatic animals to move through water with greater speed and less resistance. This streamlined surface also provides a protective covering for the skin, though their impact resistance is limited. Placoid scales are not shed and replaced in the same way as the scales of bony fishes. Instead, they grow with the fish, and if damaged, they can be regenerated.

The arrangement and size of placoid scales can vary significantly between different species of cartilaginous fishes, contributing to the unique texture and appearance of their skin. For example, the skin of a shark feels rough like sandpaper when rubbed against the grain due to the orientation of these placoid scales. This characteristic has led to the historical use of sharkskin as a natural abrasive.