Fish have scales embedded in their skin and are a part of the fishes front line defence against attack from fungus, disease, and parasites. The outer layer of a fish is covered in mucous, then directly below that are the scales and then two layers of skin. But we are only interested in the scales here.

One of the main functions of the scales is to protect the fish from abrasions that would other wise lead to infection like a type of armour plating. Whilst at the same time allowing the fish to remain flexible. The scales are not normally shed or moulted but instead the constantly grow from the base. This growth varies through the seasons (in wild fish) and you can tell the age of a fish by counting the growth rings in one of its scales, just like the growth rings of a tree. If a scale is lost because of some trauma it will re-grow and the fish will be unaffected if it is other wise healthy.

The scales are very tough and are made from a substance similar to enamel or dentine. In most fish they are arranged like roof tiles and over lap each other. This means that only a small proportion of the scale is visible.

There are several types of scales found on fish species that are alive today,

Ctenoid Scales: This type of scale has a comb like structure to the outer surface and small serrations on the edge. Because of this the fish is likely to get tangled up if netted.

Cycloid Scales: This is the type of scale that is found on most ornamental fish, the scales are round and smooth and greatly overlapped which means that only a small proportion of each scale is visible.

Ganoid Scales: These scales are quite primitive, They are quite small and there is very little or no over lapping, so most of the scale is visible apart from a small attachment under the skin. They are found on Garfish and Reedfish.

Bony Plates: Found on armoured catfish. Unlike other scales these are formed in an entirely different way to ordinary scales. When the fish hatch the fry develop folds in their skin, these folds then harden and develop into the bony plates. This is not true bone though; it is called ossified bone.

Naked: No Catfish has true scales and those without the bony plates are naked. But this doesn’t mean they are defenceless because their outer skin is thick and tough and sometimes covered in bumps. Quite a lot of bottoms living fish are naked along their underside. Hagfish are Naked and so thick and tough is there skin that it is used as leather.


There are some scales that have become greatly modified like the scales of the Porcupine fish, When threatened the fish draws in water and swells its body making its scales stand up. These scales are long and pointed so that when they are erect the fish effectively becomes a spiky ball which deters most predators.

Scale photos by: Andy

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