Taxonomy of Finfish

3.2.1 Elasmobranch fishes

The shark, ray and skate are cartilaginous fishes and they all come under the class Chondrichthyes.  In global waters, about 600 - 700 species are represented in this group.  The cartilaginous group are considered as primitive compared to their counter part, the bony fishes.  Some of the members of cartilaginous fishes are specialized in their own way as are the teleosts among bony fishes.  This group could be distinguished by the following characters:

1. Notochord constricted by vertebrae
2. Cartilaginous skeleton - The cartilage are calcified giving the appearance of bone. 
3. Swim bladder absent. 
4. Skull lacks sutures in living forms. 
5. Teeth usually not fused to jaws and replaced serially. 
6. Nasal openings on each side usually single and more or less ventral in position. 
7. Intestinal spiral valve present. 
8. Fertilization external or internal. 
9. Males with pelvic claspers. 
10. Embryo encapsulated in a leather like case.  

The cartilaginous fishes come under the class Chondrichthyes.  This class includes two subclasses - viz. (i) Elasmobranchii and (ii) Holocephali.        

The subclass, Elasmobranchii (sharks, rays and skates) includes 128 genera and 608 living species.  The characteristics that are diagnostic to elasmobranchs are:

1. Five to seven gill openings with spiracle (secondarily lost in some species). 
2. Body covered with placoid scales. 
3. Upper jaw not fused to cranium but attached with either amphistylic or holostylic suspension. 
4. Numerous teeth. 
5. Cloaca present. 
6. Males usually have intromittant organs.

The fossil forms of this group are recorded from Devonian time onwards.

The members, of the subclass Holocephali are called as ratfishes because of their long slender tail.  The chimaeras come under this subclass and about twenty five species are represented in this group and most of the species come under the family, Chimaeridae.  The following are the diagnostic characters of this group:

1. Four gill openings and spiracle absent. 
2. Upper jaw fused to skull. 
3. Teeth few in number, large, flat plates. 
4. Scales absent. 
5. In males, claspers are seen on head region (in addition to the pelvic claspers).