Cycloid scale from a chub (a type of freshwater fish in the carp family, Cyprinidae - Squalius cephalus) 插畫檔

Fish scales are often used by biologists as a "biological diary" to determine a fish's age and growth history, much like counting rings on a tree trunk.Anatomy of the ScaleThe intricate patterns on the scale are composed of several key structures:Focus: The very center of the scale, which is the first part to develop when the fish is young.Circuli: The fine concentric ridges radiating from the focus. These are formed by calcium salts secreted as the fish grows.Radii: The deep grooves that cut across the circuli, radiating from the center toward the edges. These provide the scale with flexibility so it can bend with the fish's body.Fields: The scale is divided into regions based on how it sits on the fish:Anterior Field: The part embedded in the skin (often where the radii are most prominent).Posterior Field: The exposed, overlapping part of the scale.