Taxonomy of Finfish

2.1.5.9 Operculum and Gills

Operculum and gills form part of the branchial apparatus. On either side of the fish the gill slits are situated which may be wide (Fig. 20 A), narrow or even in the form of a small aperture as in the case of the eels. In the snake eels (Ophichthyidae) the gill openings are in the pharynx as wide slits (Fig. 20 B). On the other hand in Moray eels (Muraenidae) they are small, round openings only (Fig. 20 D). In hill stream fishes they are greatly restricted to the ventral side (Bhavania australis, Fig. 20 C). Where the openings are wide they are covered by a group of flat thin opercular bones joined together by the skin which covers the gills inside. On the ventral side of the head numerous tiny thin bones are arranged fanwise from the lower side of the opercle. These are branchiostegal rays (Fig. 20 F) covered by a thin membrane. Comb-like plates, red on colour are seen on either side, which are the gills. The concave pharyngeal margins of the branchial arches are fringed with a double series of either cartilaginous or bony tubercles or filaments called the gill rakers. The anterior row of gill rakers on each arch usually interdigitate with those of the posterio'r row on the preceding arch and in this way the two rows form a sieve like mechanism to prevent any solid particles entering the pharynx with the respiratory current of water and from passing into the gill clefts and clogging it. The gill arches carry the gill lamellae and gill rakers or branchiospines. The first branchial arch (the anterior- most one) (Fig. 20E) carry rakers on the upper limb and filaments on the lower limb. Five gill arches are placed on either side of the head region (Fig. 20 G). The rakers on the upper and lower limb of the first arch are counted separately Usually the first gill arch alone is taken for counts.

Fig. 20. Operculum and gills. a. Normal. B. Eel as a moderate slit in pharynx near base of pectoral fin. C. Greatly restricted above base of pectoral fins Bhavania australis. D. Round and lateral in pharynx Mureanidae. E. Structure of a gill. (u.l) Upper limb. (gf) Gill filament. (ga) Gill arch. (ll). Lower limb. (gr) Gill rakers.