The life of Fish





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Published on Jan 30, 2010


Kingdom: Animalia
Phylum: Chordata
All fish are aquatic and are found in fresh and salt water throughout the world. There are three main classes: the jawless fish (Agnatha), bony fishes (Osteicthyes) and cartilaginous fishes (Chondrichthyes).

The jawless fishes, such as lampreys and hagfish, are the only fish that have sucking mouthparts, which makes them dependent on a parasitic way of life. They are primitive fish and fins are either absent or poorly developed.

Bony fish possess true ossified, internal skeletons to support the body tissues and a rigid skull of fused dermal bones. Most also have external scales covering the body and a bony flap, the operculum, covering a single gill exit. The spiracle is reduced or lost in bony fishes and the majority have swim bladders evolved from the primitive fish lung. Their eyes are large and of primary sensory importance. The fins are flexible and often able to be folded against the body. This is the biggest class of fish and includes about 20,000 species.

There are around 900 species of cartilaginous fish. Sharks and rays are two examples that belong to one subclass (Elasmobranchii). These differ from bony fishes in having flexible, cartilaginous skeletons, sometimes strengthened by salt deposits.

Elasmobranchs are covered with tooth-like scales called denticles, which helps strengthen the body's structure. They have multiple, uncovered gill exits and they have a good sense of smell. They have long snouts and their jaws are set well back from the snout tip. Their teeth are replaced continually, and arranged in multiple rows. Sharks have rigid and fleshy fins that cannot be folded flat. Some species retain the spiracle behind the eyes which is used in respiration on the bottom of the sea. Swim bladders and lungs are absent and buoyancy is controlled by storing a high level of oil in the liver. Fertilisation is usually internal and females either give birth to young or produce yolky eggs.

Coral Seas is about coral reefs, which are so crowded that they play host to a perpetual battle for space, even among the coral itself. It starts life as a larva that becomes a polyp. Having multiplied, it hardens into a limestone skeleton and grows to form a reef. As the community flourishes, animals develop relationships with one another and such a place can feature a huge variety of ocean life. Although corals feed nocturnally on plankton, sunlight is vital because even though they are animals, each contains millions of single-celled algae. This in turn is the favoured sustenance of the humphead parrotfish, whose jaws are so powerful that it erodes much of the reef into fine sand. Algae also grows on the top of the reef and a battle for grazing rights between shoals of powder blue and convict tangs is shown, the former being initially overwhelmed by the latter's weight of numbers before regaining the upper hand. The night-time hunting of a marbled ray alerts other predators and a group of whitetip reef sharks moves in, from which few are safe. Several breeding strategies are examined, including the acrobatic habits of brown surgeonfish and the colourful courtship of the flamboyant cuttlefish. Humpback whales are visitors to the reef and males establish their seniority by the loudness and strength of their song. Being fixed to the seabed, corals must synchronise their reproduction with lunar phases and the rising spring temperatures.

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