The fishes, or Pisces, are aquatic vertebrates that breathe with the aid of gills. They are thought to have originated 500 million years ago, a time that predates the origin of tetrapods by some 200 million years. It is uncertain whether the transformation from an invertebrate ancester took place in fresh water or in the sea (see Page 35), but subsequently fish have adopted both of these environments, as well as inland lakes and springs where the solute concentrations of the waters may even exceed those in the sea. The Pisces contains about 23000 species, more than all of the tetrapods, which occur in several widely divergent evolutionary classes. The principal of these are: (see Table 7.1) the Agnatha (the jawless lampreys and the hagfishes), the Chondrichthyes (sharks and rays) and the Osteichthyes (bony fishes). The tele-ostean fishes, which are a group of the latter bony fish, are today the predominant group in both fresh water and the sea; there is more information about the physiology of these than for the other classes of fish. From the biological viewpoint, however, further information about osmoregulation in the sparser relict fishes would probably be even more interesting, especially as it could aid our understanding of the evolution of osmoregulation in vertebrates. Such archaic fish include the lungfishes, the coelacanth, the sturgeons, the bowfin and garpike, and the lampreys and hagfishes. At present physiological information about these fishes is somewhat limited.
KeywordsFresh Water Marine Teleost Rectal Gland Spiny Dogfish Sodium Loss
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