About Fishes
Placoid Scales
Broadnose Sevengill Shark placoid scales. Photo: S. Lindsay © Australian Museum.
Placoid scales are found in sharks and rays, and can vary greatly in external appearance. They do not increase in size as the fish grows, instead new scales are added. Placoid scales are often referred to as denticles.
Placoid scales consist of a flattened rectangular base plate which is embedded in the fish, and variously developed structures, such as spines, which project posteriorly on the surface. The spines give many species a rough texture. There are large differences in the development of these spines between different species (see the Shark Scale Brain Teaser and Manta Ray scales).
Placoid scales are composed of a vascular (supplied with blood) inner core of pulp, a middle layer of dentine and a hard enamel-like outer layer of vitrodentine. (Leave the Australian Museum fish site to learn more about dentine and vitrodentine.)
Further reading
- Helfman, G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
- View Whale Shark scales.
- View White Shark scales.
