Fish scales are composed of connective tissue
covered with calcium. Typically, soft-rayed fish have smooth
cycloid scales and spiny-rayed fish have ctenoid scales.
Scales can be used for aging fish. The
annulii, or growth rings like on trees, are counted from the focus
(where growth starts) outward. By measuring the distance
between annulii, along a radius, the growth rate of the fish can
even be estimated.