Fishes

What are they?

Fish come under the class vertebrates (animals possessing a backbone) that live in water. There number of species of fish outnumber all the other kinds of vertebrates put together. The various kinds of fish seem so diverse as regard to shape, colour, and size that it seems that they all don't belong to the same group.

The smallest fish known to date is the Trimmaton nanus, a fish found in the Indian Ocean, which grows to only about 1 centimetre long. The largest fish found is the whale shark, a shark that is capable of growing more than 12 metres long and weigh more than 15 metric tons. It is harmless to most other fish and to human beings as it eats plankton, due to small teeth. The most dangerous, on the other hand weighs only a few kilograms. This include the stonefish, whose poisonous spines are capable of killing a full grown human in just a matter of a few minutes.

Lifestyle:

Most fish eat shellfish, worms and other marine beings animals. They also eat other fish. Some survive on mainly water plants such as algae. Others are cleaners of the seas - scavengers. They do that by eating the dead bodies of fish and animals.

Where there is water, there is almost always fish. They are found almost all over the earth, ranging from the freezing arctic water to the steaming water bodies of the tropical islands. They live in places ranging from the roaring mountain rivers to underground streams. Some fish make travel across the oceans, while others still spend their life in a quite secluded spot. Most fish are incapable of leaving the waters. Yet some fish have the unique features of leaving the water for months and finding new habitats. Thus fish are one of the most diverse species of the earth.

Fish have great impact on the lifestyles of humans. They provide food for millions of people annually. Fishing enthusiasts catch them for sport, and people keep them in homes as pets. In addition, fish play an important part in the food web. They eat plants and animals and, in turn, become food for other plants and animals. Fish thus, like any other animal, help keep in balance the total number of plants and animals on the earth.

All fish have two main features in common.

(1) They possess a backbone, and so they come under the class of vertebrates.

(2) They breathe through gills... mainly. There are a few exceptions to this. Nearly all the fishes are also cold-blooded animals. This means that they do not have a constant body temperature and hence cannot maintain a constant body temperature - it varies according to the surrounding temperature.

In addition to this, most fish have fins that help them in swimming, also providing them a method of slowing down while swimming. All other creatures of the seas differ from the fishes in atleast one way or the other. Some water animals are called fish, but they do not have a backbone and so are not fish. These animals include jellyfish and starfish. These should not be mistaken for 'true' fish. Clams, crabs, lobsters, oysters, scallops, and shrimps are called shellfish. But they also lack a backbone.

The first fishes appeared on the face of this earth at about 500 million years ago, giving them a lot of time for changing their features. They were the first creatures to possess a backbone - the first vertebrates. Most scientists believe that these early fish became the ancestors of all other vertebrates.

Harmful fish:

A few fish will sometimes attack humans. They include a few sharks like the hammerheads and white sharks and moray eels. Certain types of piranhas are violent fish with dagger-edged-sharp teeth. A school ( group ) of them can bite the flesh off from a human being or other large animal in minutes or even seconds. Some other kinds of fish, including sting rays and stonefish, have poisonous spines or stings that can kill anything that comes near them, or at least injure the object. The meat of certain fish like filefish, puffers, fugi and some other fish is poisonous and can cause sickness or death if consumed.

A few species of fish have become can drive out the natives of that particular region in which they have been introduced. In North America, sea lampreys that have been introduced into the Great Lakes and Asian catfish introduced into the inland waters of Florida have become a problem to native fish and have threatened their existence.

Role of fishes in Natural Balance:

All fish of a particular environment, contribute to the balance of that water body be it lake or stream. The fish form a social community with other animals of the water, maintaining the population of all the creatures. This forms the natural cycle in water. In such a society, be it on land or energy is transferred from one living being to another in the form of food. This is known as a food chain. Such a system is called a food chain.. All food chains start due to sunlight. Plants use this energy to make their food for their survival. In the underwater food chain, small animals called plankton ( they are small plants and animals ) utilize this food formed by plants. These plankton are eaten by fish and those by even bigger fish and those by still others and some of these fish may form the food of human beings or those of birds or animals. Some even die naturally and they sink to the sea bed and form food to the water plants and animals after they decay.

Every fish community is a part of another community made of all plants and animals of that particular region. These larger communities form a food web. These food chains join together to form a larger food chain called the food web. This web prevents plants or animals from increasing too rapidly thus maintaining a natural balance on earth. This helps in preserving the balance of nature. Unfortunately, man can upset this balance by mass killing of any part of this food web or any food chain.

Evolution of fish

The development of fish has been found out by studying the fossils of fish that are extinct. Fishes first came to the face of this earth around 500 Million years ago. They were covered totally by thick armour. They had no jaws. They were the first vertebrates. At around 400 million years ago, the jawed fish appeared. This group was acan-thodians. Another group ( placoderms ) were the largest fish till that time. Some grew to 9 metres like the Dinichtys.

A period called the Devonian Period started at around 410 Million Years ago. Most of the development occurred at this time. It lasted till around 360 Million years. Bony fish developed during this time. The bony fish were divided into two groups - the sarcopterygians and the actinoptergians.

The living relations of the former include the lungfish. The coelacanth is the only other survivor, while whether the Bichir belongs here, is still an undecided issue.

The fishes of the class actinopterygians had fins that were rayed. The chondrosteans, which were the first rayed fish, are the ancestors of the modern ray finned fish.

The primitive sharks appeared around the Devonian Period. The sharks haven't changed much through the years. Rays appeared after the sharks ( 200 Million years after ). By the end of the Devonian Period, all most all jawless fish were extinct.

Modern fish made their appearance during the Triassic Period, which occurred around 240 Million years ago. The chondrosteans gave rise to the holosteans ( a group of primitive bony fish ), which gave rise to teleosts.

Telosts lost the heavy armour that clad the previous fish. They all had soft ray fins. The first spiny rayed fins came about at around 138 million years ago. These gave rise to modern fish like tuna. Since then teleosts ruled as the important fish group.