Question

Difference between placoid and cycloid scale ?

Answer 1

The 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. 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). Many authors feel that lumping all scales with a spiny posterior margin under the term 'ctenoid' is an oversimplification. Three types of ctenoid scales are recognised; 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. Cycloid scales have a smooth posterior margin lacking ctenii. The word 'cycloid' comes from the Greek cyclo, meaning circle. Cycloid and ctenoid scales consist of two main regions, a surface 'bony' layer, composed of an organic framework impregnated largely with calcium based salts, and a deeper fibrous layer composed mainly of collagen. Cycloid and ctenoid scales are derived from ganoid scales that have lost the ganoine and thinned the bony embedded plate. 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, which look a little like the growth rings in the trunk of a tree. These are sometimes used to determine the age of the fish.