Inhabitants of The Oceans: Fish
The house in which we live, the school we attend, the pavement on which we walk, the parks in which we play, the air we breathe are all things that belong to our world. There are birds, people, trees, plants and animals in this world. However, there also exists another world with which we are not very familiar. We know the existence of this world and sometimes see it on TV. There are also animals and plants that inhabit this world. These animals and plants have no idea about our world. Neither could we live in their world nor is there any chance that they could survive in our world. We could not even breathe one time in that world.
The "other" world we are talking about is the underwater world in which fish live. But, there is something we need to remember. Fish are not the only living beings inhabiting the underwater world. The underwater world harbours reptiles, insects and plants and shelters millions of kinds of creatures. The creatures living in this world have particular methods of eating, breathing and sleeping peculiar to them.
The respiratory system of fish is different from that of all other creatures. Instead of noses fish have gills. By means of gills, they can use the oxygen in water. The water that is continuously taken into the mouth passes through the gill arches. Meanwhile, capillaries in the gills take in the oxygen dissolved in the water and release the carbon dioxide of the body to the water. Most fish have nostrils but these are not used for respiration. The nostrils open to tiny sacs by which fish detect the odour of the water that fills these sacs. Sharks, for example, find their prey by means of their odours.
A fish does not have eyelids as do human beings. It looks at the world through a transparent curtain covering its eyes. This curtain resembles a diver's eyeglass. Since they usually need to see objects very close to them, fish's eyes have been created to meet this need. The spherical and hard structure of their eyes are adjusted to see objects close to them. When they want to look at an object at a distance, the lens system is drawn backward by means of a specific muscle mechanism in the eye.
Beside its five basic senses of sight, smell, hearing, touching and taste, the fish also perceives the outside world through its "lateral" lines. Sensitive nerves that lie along these lines perceive the size and direction of objects the fish passes by. In this way blind cave fish can move very easily in the dark. This system is a kind of underwater radar or "sonar" system.
Furthermore, many fish species have a long and slender balloon-like air-filled sac in the abdominal cavity. They maintain their balance in the water with the help of this sac.
Fish of various kinds created by the will of Allah amaze people with their beautiful colours and movements. You can hardly find such vivid colours as those seen in many fish in any other animal.
You may be familiar with what has been described so far. But, there is another piece of information about fish which will astonish you.
Larger fish usually need tiny cleaner fish to cleanse them of parasites. Sometimes, these cleaner fish readily enter the mouth of a much bigger fish without fear. They clean the teeth and gills of the host fish, a service in return for which the host causes them no harm.
Well, how can the cleaner fish be sure that the big fish will not suddenly swallow it? How does it know that the big fish will not cause it any harm? How can it trust it? It is as if they had an agreement. Let's assume that there exists such an agreement; how could the little fish be sure that the host fish would not violate this agreement and eat it, once it renders its services?
The truth is that the cleaner fish is always vulnerable to its host, but, because Allah has inspired both of them to co-exist, neither does the big fish cause harm to the cleaner fish nor does the little fish feel fear of the larger one. The large fish is cleaned while the little fish feeds on the parasites it cleans. By Allah's inspiration to them, both of them lead their lives in harmony and co-operation.