SCALES

Scales are overlapping series of hard plates that cover a fish’s body

Scales serve to protect the body from the outside dangers such as predators and infection.

What are Scales?

Present on palezoic sharks, and are still present on today’s sharks, skates and rays

Often referred as denticles

They do not increase in size as the fish grows (new scales are added).

 

Placoid Scales

Flattened rectangular base plate which is embedded on the fish, spines which project posteriorly on the surface.

It is composed of a vascular inner core of pulp, a middle layer of dentin and a hard enamel-like outer layer of vitrodentine.

The function of these scales is for protection against predators, although in some sharks, they may also have a hydrodynamic function.

help the shark swim more quickly because their streamlined shapes helps decrease the friction of the water flowing along the shark's body

FUNCTION

found in the majority of bony fishes (the Teleostei) and modern lobe finned fishes.

Ctenoid scales have a variously developed spiny posterior margin (the word 'ctenoid' comes from the Greek cteno, meaning comb, and refers to the comb-like ctenii on the margin of the scale).

Cycloid scales have a smooth posterior margin lacking ctenii. The word 'cycloid' comes from the Greek cyclo, meaning circle.

Cycloid and Ctenoid Scales

anterior part of each scale is usually overlapped by the posterior portion of the scale in front. ◦ This arrangement of imbricate (overlapping)

scales gives the fish greater flexibility than in those species with cosmoid and ganoid scales.

crenate, with simple indentations in the margin, spinoid, with developed spines that are continuous with the main body of the scale, and ctenoid, in which the ctenii are formed as separate bony growths which are distinct from the main body of the scale.

Three types of ctenoid scales are recognised;

As a fish with cycloid or ctenoid scales grows, its scales also grow. This results in a pattern of concentric growth rings on the scale.◦ These are sometimes used to determine the age of the

fish.

◦ thin, large, round or oval,

Protection and Hydrodynamics

FUNCTIONS