Sharks & Rays

Abstract

The cartilaginous fishes, or Chondrichthyes, are the sharks, the skates and rays, and the chimaeras. The group arose in the Silurian at about the same time as the bony fishes, or Osteichthyes. The Chondrichthyes are almost all marine fishes, with very few entering fresh water. The bony fishes, on the other hand, are widely distributed both in fresh waters and the sea. The fundamental physiological difference between them lies in the way they control their internal osmotic pressure. The chondrichthyans, with the exception of the freshwater stingrays, are adapted to tolerate large quantities of dissolved urea in their body fluids, and this raises their osmotic pressure approximately to that of sea water. In the bony fishes, on the other hand, the internal osmotic pressure is lower than that of sea water, so that in the sea they expend much metabolic energy in retaining enough water in the body.