Write a brief note on Scale of Fishes

Scales are the exoskeleton of fishes according to the mode of origin of scales; there are two types of scales.

Those, which are formed due to the secretory activity of both epidermis and dermis, as the placoid scales of elasmobranches.

Non-placoid scales that are derived from the dermis only as the scales of teleosts.

Structurally the scales are classified a cosmoid, ganoid or rhomboid, placoid, cycloid and ctenoid. The last two are called as bony ridge scales.

Cosmoid Scale:

Cosmoid scales are found in extinct crossopterygii and Dipnoi. External layer of the scale is thin and enamel like.

Ganoid Scale:

There are characteristics of primitive actinopterygians called the ganoid fishes and are of various forms and structure. Scales are heavy.

Placoid Scale:

Plazcoid scales or dermal denticles are characteristics of Shark. Each has a disc like base plate embedded in the dermis and a spine projecting out through the dermis.

Cycloid and Ctenoid Scales:

Cycloid and ctenoid scales are known as bony ridge scales, present in teleostem fishes. These are thin, transparent. The scale has ridges alternating with grooves. The central part of scale is called focus.

The cycloid scales are thin and roughly rounded in shape. It is thicker in the centre and thinner out towards margin.

The ctenoid scales are also circular with serrated free ridges. More over several spines are present on the surface of the posterior area of the scale.