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Life Under Water

Print Version  
Last Updated On: 31/07/2014  
Do fish drink water?

An oft- repeated question about the fact regarding fishes living in water is Do fish drink water. Many of us are curious to know the reply. 

The answer is Yes & No 

While the freshwater fish doesn’t drink water- the marine fish drinks large quantities of water.  

Since the liquid in a freshwater fish’s body is saltier (dense) than the surrounding water, the freshwater fish is in constant danger of soaking up water and swelling. As a result it doesn’t drink water and what ever water enters in fish body through skin & gills is carried to the kidney and used to flush out waste products in large quantities of urine.  

The marine (Salt-water) fish has exactly the opposite problem

Salt-water fish’s liquids are less salty (dense) than surrounding water and fish is in constant danger of dehydration. Thus the salt-water fish have to drink large quantities of water to make up for what it loses through its gills & skin. Some of the salt that it receives goes through the digestive tract and is excreted. While some salt that it receives is passed through special gill cells back into the ocean. A salt-water fish seldom urinate.  

How Fishes Swim?  

Though shapes of fishes differ widely but in general they are boat shaped which facilitate their movement in water. For a fish to move through aquatic medium, which is incompressible, it actually push it aside. The fish can do this by wiggling back & forth in a snake like motion, pushing water aside by the forward motion of its head, first to the left than do the right, also with the curve of its body and finally with its flexible tail. The water, tending to return to its original position, now flows back along the fish’s narrowing side, closing in at the tail and helping the fish to move forward.  

Skate and rays move by undulation of their greatly enlarged sidefins, which gives them appearance of flying through water some flattened fish crawl on the bottom like sea robin while some crawl right out of the water on the beach like mud skipper. The climbing perch and snakehead travel overland from pond to pond on their forefins. Some fishes can fly in the air. They can skim above the water for nearly a minute and if there is good breeze to lift them up they may reach a height of 10-20 feet, planing from wave to wave with their greatly extended forefins held rigidly out like wings.

How A Fish Sees? 

Most of us are aware that seeing under water is a different matter from seeing in the air. Light is diffused and quickly fades to a dim twilight zone. Even in clear water only relatively close objects can be sharply focussed. But this is the environment, the eyes of fishes must cope with, and their adaptation to the medium is extremely ingenious. Their primary need is to see movements and nearby shapes and this they do to perfection with their eyes set on the side of their head, many of the fishes can practically register every thing that moves around them at any time. Needing no eyelids or tear ducts in their liquid medium, they have evolved ways of coping with varying amount of light. 
The eyes of most of the fishes are placed so widely apart that they are considered to have monocular vision viz. each eye collects a separate un- coordinated image, which overlaps the least. In addition, the retinas of a fish pass impulses only to the side of the brain, opposite the eye, enhancing the monocular effects. Compared to this man has a double advantage his eyes are placed in front of his head giving him a wide field of overlapping vision. Mans eyes sends simultaneously impulses along nerves to both sides of the brain, resulting in true binocular vision.  
Some fishes have another remarkable feature of having Divided Vision. Fish like Anableps is adopted in seeing above a below the surface. Each eye of this four eyed fish is situated on top of the head in such a way that it can swim with half of the eye out of water, often ducking in water or moistening the eyeball.  

We can find age of fish by reading its Scale 

From head to tail, fishes are generally covered by flexible armour of rounded overlapping plates called Scales. These are embedded in the inner layer of the skin and form an important covering. In addition to this, fishes are further protected by a layer of mucous slime produced by numerous invisible glands scattered all over the body, this mucous in antiseptic helping to ward off bacteria and fungi as well as lubricating agent. 
In size and thickness, scales may vary greatly, from those of mahseer, which is reported to have scales (Fish size over 12 feet) as large as man’s head to the microscopic one of common eel. Few fishes especially catfishes have no scales at all. In some fishes like trunk-fish, scale are fused to form an inflexible box like covering or in pipe-fish and sea horse, rows of connected bony plates. 
Scales also grow along with the fish and especially in fishes of temperate zone leave a distinctive record of age and season. Since in the temperate zones, each scale grows fastest during the summer, when the fish is getting the maximum feed and this make possible to tell a fish age by counting growth rings on scale. Regarding typology, fish scales are categorized in four types. The primitive placoid scales found in shark, rays and skates are tooth like structure size of grains embedded on sand paper. 
Ganoid scales are present in few bony fishes. These are diamond shaped and attached to each other by joints. These are coated with ganoin a substance, which gives the fish appearance of polished ivory.  
The most common scales are cycloid and ctenoid. The former has a comb like edge while the later has a rounded border. Most fishes have one or the other types of scale. They are arranged in over lapping rows and as they are thin light and flexible, most fishes that have them are fast swimmers.

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