Fish (Pisces) are an aquatic class of vertebrates. There are more kinds of fish than all other kinds of aquatic and terrestrial vertebrates put together. The smallest fish is the Trimmaton nanus, a goby of the Indian Ocean, which grows to about one centimetre long. The largest fish is the whaleshark, which may grow more than 12 metres long and weigh over 14 metric tons. It feeds on plankton and is completely harmless to most other fish and to human beings. The most dangerous fish weigh only a few kilograms. They include the deadly stonefish, whose poisonous spines can kill a human being in minutes.
Fish live almost anywhere there is water. They are found in the near- freezing waters of the Arctic and in the steaming waters of tropical jungles. They live in roaring mountain streams and in quiet underground rivers. Some fish make long journeys across the ocean. Others spend most of their life buried in sand on the bottom of the ocean. Most fish never leave water. Yet some fish are able to survive for months in dried-up riverbeds.
Fish have enormous importance to human beings. They provide food for millions of people. Fishing enthusiasts catch them for sport, and people keep them as pets. In addition, fish are important in the balance of nature. They eat plants and animals and, in turn, become food for plants and animals.
Fish thus help keep in balance the total number of plants and animals on the earth. All fish have two main features in common. (1) They have a backbone, and so they are vertebrates. (2) They breathe mainly by means of gills. Nearly all fish are also cold-blooded animals - that is, they cannot regulate their body temperature, which changes with the temperature of their surroundings. In addition, almost all fish have fins, which they use for swimming. All other water animals differ from fish in at least one of these ways. Dolphins, porpoises, and whales look like fish and have a backbone and fins, but they are mammals. Mammals breathe with lungs rather than gills. They are also warm-blooded - their body temperature remains about the same when the air or water temperature changes. Some water animals are called fish, but they do not have a backbone and so are not fish. These animals include jellyfish and starfish. Clams, crabs, lobsters, oysters, scallops, and shrimps are called shellfish. But they also lack a backbone.
The first fish appeared on the earth about 500 million years ago. They were the first animals to have a backbone. Most scientists believe that these early fish became the ancestors of all other vertebrates. Research Fish
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Nature
Research Fish