Comparative Study of Scales in fishes : Placoid, Cycloid & Ctenoid

Comparative Study of Scales in fishes : Placoid, Cycloid & Ctenoid.

A] Scales in fishes

        The Body of vertebrates has exoskeletal covering on the body often made up of either epidermal or dermal scales. Epidermal scales are present in many terrestrial vertebrates such as reptiles, birds & mammals & are the part of malpighian layer of epidermis. Whereas dermal scales are originates from mesenchyme & well developed in fishes. The dermal scales found in fishes are small, thin cornified, calcareous or bony plates that fit compactly or overlap each other.

⚫Cycloid Scale :-

    (1) The cycloid scales are found in lower Teleosts (Labeo, Catla, Cod) dipnoi & Holostei (Amia). They form a protective covering over the skin.

    (2) The scales are thin, flexible translucent dermal plates & roughly circular or rounded in shape. 

    (3) They are located in the dermal pockets and possesses concentric lines of growth hence it is cycloid.

    (4) It is composed of thin upper layer of bone & lower layer of fibrous connective tissue but pulp cavity wanting.

    (5) Scales covering lateral line are frequently perforated permitting the passage of small connection of lateral line to outside.

⚫Ctenoid Scale :- 

    (1) Ctenoid scales are found in higher Teleosts like perch, Sunfish & Actinopterygian fishes.

    (2) Ctenoid scales are thin, soft & dermal translucent plates whose grove like part is firmly attached & its hind teeth bearing part remains free.

    (3) These scales are embedded in small dermal pockets & concentric lines of growth with teeth like projections posteriorly hence it is - ctenoid.

    (4) Scales are arranges obliquely so the posterior end of one overlaps the anterior end of the next scale.

    (5) These scale are derived from ganoid scales in which ganoine, cosmin & bone cells are lost.

⚫Placoid Scale :-

    (1) Placoid scales are also called dermal denticles which is the characteristic feature of scoliodon.

    (2) Placoid scales are arranged in regular oblique rows covering the entire surface of body.

    (3) Placoid scales are small pointed & tri-radiate denticles & composed of a basal bony plates embedded in dermis from which spines are projected upward & points posteriorly.

    (4) Spine is composed of dentine covered by a hard material vitrodentine.

    (5) It is partly dermal & partly epidermal in origin.

    (6) Placoid scales contains a pulp cavity contains odontoblast, dentin forming cells, blood capillaries.

⚫Ganoid/Rhomboid Scale :- 

    (1) Ganoid or rhomboid scales are found in primitive Actinoptergyii fishes e.g. Polypterus & lepidosteus.

    (2) Ganoid scales are Compactly fitted on body wall.

    (3) These scales are heavy & have outer layer of hard inorganic enamel like material called ganoine.

    (4) The middle layer is cosmin containing numerous branching tubules.

    (5) The innermost layer is thickest & is made up of lamellar bone isopedene.

    (6) These scales are dermal in origin.