Bony Fishes

Abstract

The bony fishes, or Osteichthyes, with more than 25000 species are the most successful of the fish-like vertebrates, and gave rise to the land vertebrates, or tetrapods. They arose in the Silurian, possibly as a freshwater group, and radiated widely in both fresh water and the sea. Life in each of these environments presents physiological problems. In fresh water, the blood and tissue fluids of bony fishes are hypertonic to the surrounding medium, and there is a need to combat the constant uptake of water by osmosis. This is achieved by the excretion of large amounts of water through the kidney. In the sea, the problem is reversed for the body fluids of the bony fish are then hypotonic to sea water, and there is a consequent danger of water loss. In this case, large amounts of sea water are taken in through the gut, and the production of urine is reduced. Excess of salt taken in with the sea water is excreted from the gills and kidneys.