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maintain its position in the water. Some species, particularly those that spend most of their time on the bottom, lack the swim bladder and have some difficulty in swimming up to the surface. A few species can use their swim bladders to help produce grunting or clicking sounds. Being permanently immersed in water, fish always run a risk of absorbing too much fluid. The kidneys, therefore, which are proportionately larger in fish than in other vertebrates, are also important organs, responsible for maintaining the correct balance of water in the fish's body. A few specialized fish—particularly migrant species like eels and salmon—have kidneys that can cope with variable salinity, and so are able to live in both fresh and salt water, but these species are unsuitable for aquaria. Most fish are unable to make this switch; if they are put in the wrong type of water, they die quickly. Fish obtain oxygen through their gills, the red comblike structures at the back of the

mouth. Each fish has two sets of gills protected by bony flaps on either side of the head; when these are open it is sometimes possible to glimpse the gills inside them. Water, bearing dissolved oxygen, flows through the mouth and passes over the gills, where the oxygen is taken into the bloodstream and carbon dioxide is released. The gills look red because they have a rich supply of blood vessels covered by a thin membrane. As a fish swims along, apparently “drinking” water, it is really breathing by taking in a fresh supply of oxygen-rich water every time it opens its mouth. Some fish use this process to help them feed as well, filtering the water through much tougher, comb- like structures called gill rakers to remove tiny food particles or small organisms. A few fish can breathe by gulping air from the surface of the water into specialized organs that act like lungs; these species normally live in habitats where the water is low in oxygen.

The remarkable ability to change both pattern and color is character- istic of many terri- torial cichlids. The dark stripes that give this cobalt- blue zebra, Pseudotropheus zebra, its name will rapidly return if this fish is forced out of its territory by another, more aggressive, fish.

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