To become succesful in the aquatic environment there are few problems that need to be solved.
2.) Gaseous exchange
3.) Sensory perception
4.) Osmoregulation and excretion
Water, even though we consider it more friendly for living things, has its own problems if you plan to live in it.
1.) More resistance than air.
2.) Less amount of Oxygen than that of air.
3.) Higher density than air.
4.) Do not let light to penetrate as easily as that of air. therefore a light source is needed to see.
eg : Bio luminescence in animals in deep water.
5.) Higher pressure in deeper waters. Therefore need a mechanism to withstand the high pressures.
(1.) Locomotion of fishes
For an object to move in water a propulsive force is needed to move forward. Resistance of the
medium creates a drag. Gravity works downwards. Buoyancy upwards.
If the object is shaped as image (1) and dragged as shown in the picture it is easy to move at start but will stop due to the low pressure region created behind it.
If it is to happen as image (2) it is hard to make it move at the start but once moved it will keep going.
Therefore the best solution for the efficient movement is a combined shape as shown in image (3)
Because of this for better locomotion fishes posses
1.) A stream-lined body.- A Fusiform shape
Fish are somewhat torpedo shaped, which makes it easier to glide through water. The body is compressed at the sides and tapers more at the tail than at the head.
2.) Lacks protrusions. Such as ears and protruded noses found in terrestrial animals.
3.) Have a smooth skin. More smooth the skin is lesser the resistance.
4.) Well developed muscles,to generate the force of propulsion.Body is moved to the sides but the forces created to sides cancel out and only the forward force remains. Thus the fish moves forward without moving to sides.
There are three types of basic forms according to the
amount of body used for generation of propulsion.
* Carangiform- the most common. Only the posterior part of the body flexes for the movement
* Ostraciiform - Fish move by "wagging" the tail.
* Anguillform - Most of the body moves
The way of locomotion depends on the size and the habitat of the fish.
Fins are basically used for balance and lift and sometimes for propulsion.
5.) Bony fishes have a swim bladder that can be used to control density of the body.When they pump air to the bladder using the blood stream . Density goes down. So the fish goes up.
To go down air is extracted back into the body. By adjusting the body density they are able to go at any level of density.
6.) Sharks and rays do not have a swim bladder so they have to swim continuously to avoid sinking. Cartilage makes body lighter than that of bony fishes. They also have Squalene a
neutral oily organic compound in their liver which also makes them lighter.