Fish scales comparing ctenoid and cycloid

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.

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