Placoid Scales

Dermal Denticles on Sharks and Rays

Gills of Whitetip Reef Shark
Jeff Rotman/Photolibrary/Getty Images

Placoid scales are the tiny tough scales that cover the skin of sharks, rays, and other elasmobranchs. Even though placoid scales are similar to the scales of bony fish, they are modified teeth and are covered with a hard enamel.  They grow out of the dermis layer and this is why they are called dermal denticles.

Placoid scales are packed tightly together, supported by spines, and grow with their tips facing backwards.

This gives the fish's skin a rough feel. The function of these scales is for protection against predators. In some sharks, they may also have a hydrodynamic function, helping them swim more efficiently and quietly. The placoid scales are shaped such that little vortices form, reducing friction as the shark swims. They also direct the water around the fish.

The Structure of Placoid Scales

The placoid scales grow out of the dermis, with the flat rectangular base plate embedded in the skin of the fish. Like our teeth, placoid scales have an inner core of pulp made up of connective tissues, blood vessels, and nerves. Like the pulp cavity of a tooth, it is nursed by a layer of odontoblast cells that secrete dentine. This hard, calcified material forms the next layer. The dentine is covered by enamel-like vitrodentine, which is produced by the ectoderm. Once the scale erupts through the epidermis, no more enamel can be deposited on that portion of the scale.

Different species have different kinds of spines develop to support the scales. The spines give the scales their rough texture. It is so rough that it has been used as sandpaper by various cultures form many centuries. The species of fish can be identified by the shape of the scales and spines. On some sharks, they are shaped like a duck foot.

Scales in bony fish grow as the fish gets larger, but placoid scales stop growing after they reach a certain size, and then more scales are added as the fish grows.

Shark Skin Leather - Shagreen

The tough nature of the placoid scales makes shark rawhide leather, called shagreen. The scales are ground down so the surface is rough with rounded protrusions. It can take on dye colors or be left white. It was used in Japan to cover sword hilts, where its rough nature was appreciated to help form a good grip.

Other Types of Fish Scales

Ctenoid scales are another kind of toothed scales, but the teeth are only along the outer edge of the scale. They are found on fish such as perch that have spiny fin rays.

Clycloid scales have a smooth texture and they are found on fish with soft fin rays, including salmon and carp. They are rounded. and show growth rings as they grow with the animal.

Ganoid scales are diamond-shaped and they do not overlap, but they fit together like pieces of a jigsaw puzzle. The are seen on gars, bichirs, and reed fishes. They act like armor plates.