Elasmoid

Elasmoid is a term used to describe a type of dermal scale characteristic of most ray-finned fishes (teleosts). Elasmoid scales are thin, flexible, and typically overlap one another, providing protection with minimal hydrodynamic resistance. This contrasts with other scale types such as placoid scales of cartilaginous fishes or ganoine- or cosmine-covered scales found in other lineages.

The two principal forms of elasmoid scales are cycloid and ctenoid. Cycloid scales have smooth posterior edges and are common in many bony fishes such as carp and salmon. Ctenoid scales have comb-like projections, or ctenii, along the posterior margin and are typical of many perch-like fishes. Both types are derived from dermal bone and generally grow by adding material at the edges, while maintaining a thin, flexible structure. They commonly articulate through peg-and-socket joints between adjacent scales.

Structurally, elasmoid scales consist of a thin base plate of bone beneath an outer surface that lacks the enamel or ganoine found in some other scale types. Growth rings, called circuli, can be observed on many elasmoid scales, signaling periodic growth. The scales are usually embedded in the dermis and are shed or renewed as the fish grows.

In paleontology and systematics, elasmoid scales are important for identifying teleosts and for understanding the evolution of scale types in ray-finned fishes. They illustrate a key adaptation for a lightweight, flexible integument that supports rapid growth and diverse life histories.