Fishery and biology of deep-sea chondrichthyans off the southwest coast of india

Class Chondrichthyes is one of the oldest groups of vertebrates, which \nsurvived for more than 400 million years since Devonian period, and the second \nlargest class of fishes after Osteichthyes. Chondrichthyes can be differentiated from \nOsteichthyes (bony fishes) based on the presence of placoid scales, which has been \nreplaced by scales in fishes and possessing simple internal skeletons formed of \ncalcified cartilage and no true bones (Last and Stevens, 2009). The body forms \nof Chondrichthyes shares many similar characteristics, but is highly varied between \norders, reflecting adaptations to lifestyle, habitat and environment (Compagno, 1999). \nClass Chondrichthyes is a monophyletic group and is divided into two subclasses, the \nHolocephali (holo=whole, cephalic =head) containing Chimaeras and the \nElasmobranchii (elasmo =plate, branchii =gills) containing sharks, skates and rays. \nElasmobranchii can be differentiated from chimaeras in having multiple paired gill \nopenings (5-7), where as the latter have only single gill opening (Fowler et al., 2005).