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Introduction

Fish have certain specialized functions that enable them to survive in water: gills instead of lungs, swim bladders to maintain buoyancy, and the "lateral line system" to detect changes in the fish's surroundings by a form of echo location.

Breathing

Fish "breathe" by drawing water in through the mouth and passing it over the gills. Oxygen in the water is absorbed by the gill filaments and then passed into the blood. Meanwhile, carbon dioxide and other wastes are expelled. Some species have developed extra breathing organs for collecting oxygen in stagnating waters or in water with decaying plant, where oxygen levels are low. Members of the anabantid family, for example, have an auxiliary organ near the gills that holds atmospheric air gulped from the surface and extracts oxygen from it. This mazelike organ has prompted the popular name of "labyrinth" fishes. Some catfishes also gulp air and extract oxygen in a capillary-rich offshoot of the gut.

Swim Bladder

Most fish have a gas-filled bladder that acts as a buoyancy compensation device enabling them to maintain position anywhere in the water. The bladder automatically inflates or deflates to give the fish neutral buoyancy, equalizing the fish's weight with that of the surrounding water. Some species use their swim bladder to make or to amplify sounds.
Sight and Smell

Sight is not as important to fish as it is to humans, as many fish can navigate and locate food in the darkest and murkiest water by using their lateral line system (see below) to detect objects. A fish's eyes do not need eyelids because they are permanently lubricated by the surrounding water. A fish's sense of smell is much more sensitive than a human's, and a fish has extra taste buds, usually on barbells and fins.

Lateral Line System

A fish's nervous system is linked to the outside world through tiny perforations in a single row of scales known as the lateral line The row runs horizontally along the length of the fish. Vibrations caused by the fish's own movements are reflected back from obstacles or by other fish and are then detected by nerve endings deep inside the "portholes" in the lateral line scales.

Osmosis

A fish's skin acts as a semipermeable membrane, or a one-way transfer system for water. Osmosis causes fluid to diffuse through this membrane until there is an equal concentration on both sides of the membrane. The fluid of a freshwater fish's body is more concentrated than the liquid in which it lives. Thus, water constantly passes into the fish. To avoid bursting, freshwater fishes excrete as much water as possible and drink little. Conversely, marine fishes lose water to the more concentrated sea water outside, and must drink constantly but excrete little. Few fish can pass from one type of water to the other without distress.

Food-finding Organs

A fish's ability to sense food is aided by taste buds on the ends of barbells, such as those of the catfishes, and on the hair-like cirri carried by blennies. Other fish, notably the gourami's, carry sensory cells on the tips of their pelvic fins. Long-nosed Elephant fish have a long point on the lower jaw that can be used for digging and the Pirahna has distinct nostrils to help it locate meaty foods.

Credits

Source is Dorling Kindersley (DK) Eyewitness Handbooks ( Aquarium Fish). The poster lays no claim to coming up with any of the info posted herein.
 
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