Placoid scales are the tiny, tough scales that cover the skin of elasmobranches, or cartilaginous fish—this includes sharks, rays, and other skates. While placoid scales are similar in some ways to the scales of bony fish, they are more like teeth covered with hard enamel. Unlike the scales of other fish, these do not grow after an organism has fully matured. Placoid scales are often called dermal denticles because they grow out of the dermis layer.

The Function of Placoid Scales

Placoid scales are packed tightly together, are supported by spines, and grow with their tips facing backward and laying flat. Placoid scales are rough to the touch and the structure they form is nearly impossible to penetrate.

These scales function to protect a fish from predators and can even be used to injure or kill prey. The v-shape of a placoid scale reduces drag and increases turbulence as a fish moves through the water so that they may swim more quickly and quietly, all while expending less energy. Placoid scales form a matrix that is so dynamic and fluid that swimsuits have been designed to mimic their composition.

The Structure of Placoid Scales

The flat rectangular base plate of a placoid scale is embedded in a fish's skin. Like teeth, placoid scales have an inner core of pulp composed of connective tissues, blood vessels, and nerves. They are a part of the fish. The pulp cavity is nursed by a layer of odontoblast cells that secrete dentine. This hard, calcified material forms the next layer of scales, which fits tightly between the old layers. The dentine is coated in vitrodentine, which is an enamel-like substance that is produced by the ectoderm and is even harder than dentine. Once the scale erupts through the epidermis, it cannot be coated in any more enamel.

Different species of cartilaginous fish support their scales with unique spines based on the shape and role of the fish. A species can be identified by the shape of its scales. Because rays are flat and sharks are more angular, the spines of their placoid scales are slightly different to allow both fish to swim quickly. The placoid scales of some sharks are shaped like a duck foot with spikes at the base. These spines are what make the skin so rough in texture that some cultures have been using it to sand and file for centuries.

Shark Skin Leather

In addition to being used as sandpaper, shark skin is often made into leather called shagreen. Shark scales are ground down so that the surface of the skin is still rough but smoothed enough that the leather can be handled without causing injury. Shark skin leather can take on dye colors or be left white. Years ago, sturdy shark skin leather was used to encase sword hilts and add grip.

Other Types of Fish Scales

The four main types of fish scales include placoid, ctenoid, cycloid, and ganoid scales. This list gives a brief description of the characteristics of all scale types other than placoid.