Fish account for more than half of all the vertebrate species known in the world, with more than 24,000 species identified so far. They are found in almost every watery habitat- salt, fresh, or brackish- from large puddles to polar seas. They range in size from gobies, less than an inch long, to whale sharks, 59 feet long. Fish come in all colors and in thousand of patterns, but some are colored drably in with surroundings.
Fish fall into three main group: jaw-less, cartilaginous, and bony. The category of jawless fish includes hag-fish and lampreys; the category of cartilaginous fish includes sharks and rays; and the category of bony fish, to which the largest number of fish belongs, includes both the lobe-finned and the ray-finned fish species.
Most fish have scales that protect their bodies. Nearly all fish have fins, used for swimming. Flying fish can launch themselves into the air for brief periods, and some fish, such as walking catfish, use their fins to drag themselves along on land.
Most fish reproduce by means of eggs laid by the female and fertilized by the male, outside the female’s body. Many sharks, however, reproduce by internal fertilization and live birth from the mother’s body.
Many fish eat algae or insect larvae and other small animals, but some, such as sharks, are tireless predators than hunt even large sea mammals.
Fish frequently travel in large group called schools or shoals. Using low-frequency sonar, researchers detected a shoal of some 20 million fish.
HOW DO FISH BREATH?
Most fish breathe with gills, arched structures on either side of the body that contains features known as filaments and lamellae. Water, which contains oxygen, flows in through the fish’s open mouth and then out across the gills. Rakers filter out particles as the water flows through.
Blood coursing through the filaments and lamellae also dump carbon dioxide from the body into the gills, and water carries it out of the body. Many species, including sharks and bony fishes, have a pumping mechanism that aids respiration.
A fish out of water suffocates because its gill structures collapse and the oxygen exchange cannot continue.