Mechanics and Dynamics of how fishes swim

Fishes are the first group of animals evolved with vertebral column. Fishes are aquatic animals. They swim in water in order to live. They are cold blooded animals. They breathe through gills by drawing oxygen from the surrounding water. Their body is covered with scales to maintain their body temperature constant. Fins are their locomotory organs.

Fins are composed of two groups, unpaired and paired. The unpaired fins are the dorsal, caudal, and anal; the paired fins are called pectorals and pelvic or ventral fins. Each fin on fish is designed for certain specific functions.

1.    Dorsal fin, gives stability, staying upright while swimming.

2.    Caudal fin or the tail fin helps in propelling their forward movement and helps in changing the direction of motion.

3.    Pectoral fins and the Pelvic fins help the fish for steering through water.

4.    Anal fin balances the whole body while swimming.

Fishes swim by coordinated motion of their body and all the fins. Fishes swim by flexing their body and tail back and forth. Fishes stretch or expand their muscles on one side of their body, while relaxing the muscles on the other side. This motion moves them forward through the water.

Fishes exhibit variety of movements while swimming. They are: jumping, diving, speeding ahead by wriggling their tail, moving steadily, gliding, flying etc. However they exhibit two types of swimming.

A.   Steady and the Sustained Swimming.  B. Unsteady Movements.

Steady swimming is characterized by the cyclic repetition of the propulsive movements.  Fishes use this mode of swimming to cover large distances at a constant speed.

Unsteady Movements are very rare. Fishes use them when they have to escape from their predators or for catching their prey. This article focuses on Steady swimming of fishes.

The mechanism or the physics of swimming:

Fishes are aquatic animals. Their body is adapted to swimming. The density of water makes it very difficult for the fishes to swim, but fishes can move very smoothly and quickly because of their skeleton frame work, muscle power, and fins for thrust and direction. The vertebral column levels the movement of the fish.

The muscles provide the power for swimming and they are arranged in multiple directions (myomeres) that allow the fish to move in any direction. A sinusoidal wave passes down from the head to the tail. The fins provide a platform to exert the thrust from the muscles onto the water. Thus, swimming involves transfer of momentum from the fish to the surrounding water and vice versa.   The forces acting on a swimming fish are weight, buoyancy and hydrodynamic lift in the vertical direction, along with thrust and resistance in the horizontal direction.   

Thrust is the force acting in animal's direction. Lift is the force acting opposite in right angles to the thrust. Drag is the force acting in the opposite to the direction of movement. Drag is minimized by the streamlined shape of the fish. To swim efficiently in water, fishes have to overcome the drag, maintain their vertical position in the water column, and maintain an upright position. They should be able to change their direction to move efficiently.

The drag forces fishes experience during swimming are:    

Pressure drag is the force needed to push water out of the way to swim forward.

Frictional drag can cause turbulence, making it harder for water to flow smoothly across the fishes.

Streamlined body shape reduces the pressure drag. The slime coat provides a smooth surface that allows laminar flow and minimizes the frictional drag. 

Fishes overcome the drag forces by pushing their body against water. If they have to move fast, their push should be hard. The fins give fish control over its movements by directing thrust, supplying lift and even acting as brakes. Thus, when thrust is greater than drag fishes will start swimming.

In addition to fins, fishes have swim bladders. The swim bladders are air filled sacs functioning like our lungs. They are hydrostatic organs that help them maintain buoyancy in the water. The swim bladder is a sac inside the abdomen that contains gas. This sac may be open, located closed to the gut. Oxygen is the largest percentage of gas in the bladder; nitrogen and carbon dioxide also fill in passively.

A swim bladder is serving as a hydrostatic organ and it provides neutral buoyancy to fishes while swimming. When the swim bladder is filled with oxygen gas, the fish has a greater volume and the bladder is expanded. Fishes displace more water and so there is greater force of buoyancy. Fishes are pushed to the surface.  When the swim bladder is completely deflated, the fish has minimum volume and sinks to the ocean floor.

To stay at a particular level, a fish fills its bladder to the point at which it displaces a volume of water that is equal to the weight of fishes. In this case, the forces of buoyancy and gravity cancel each other out, and the fish stays at that level.

Bernoulli's principle states that if the velocity of the fluid increases, then the pressure exerted by that fluid decreases. Bernoulli's principle means that for an in viscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Therefore fishes are able to swim across water with ease.

Fish’s body is designed in such a way that eyes are located at Zero pressure point so that its vision is not distorted while it swims. The heart is located at the lowest pressure point to increase the heart’s stroke volume during rapid swimming. The gills are located near aerodynamic shoulder so that the suction pressure helps them to exhale.

The fishes have to overcome the following resisting forces in order to swim in aquatic medium.

Viscous drag is created due to friction between the fish body surface and water. Thin and small body faces high viscous drag owing to larger surface area as compared to bulky body. Mucous coated streamlined body of fishes reduces the viscous drag.

Pressure drag is caused due to the displacement of water by fishes. Water exerts pressure on the fish body and fishes also exert pressure on water to resist the drag. Bulky fishes experience high inertial drag as they displace more water as compared to smaller fishes. Pressure drag increases with speed as well as depth. Streamline body of the fishes and their motion in a straight line help them to overcome the pressure drag.

We also observe certain other types of movements in fishes. Yawing is side to side movement of head created by the lateral reaction force generated by the sideways lashing of the tail fin. Pitching is up and down movement of the head produced by uneven drag on the body by pectoral fins. Rolling is the spinning of the body on its anterior-posterior axis done by the dorsal fin.

Fishes have different modes of swimming. They are:

·         Anguilliform (eel-like): This type of swimming is quite efficient at low speeds but consumes a lot of energy since the whole of the body is involved in locomotion. 

·          Carangiform (jack-like): Tail is lashed from side to side. The caudal fin increases the area and the force of backward push of tail.

·          Ostraciform (boxfish-like): This type of movement is commonly seen in fishes whose body is not flexible. Tail fin propels the body forward.

When fish is propelling itself at a constant speed, forces and moments acting on it are well balanced. Therefore, the total thrust it exerts against water has to equal the total resistance it encounters moving forward. Pressure drag, lift and the acceleration reaction will contribute to both thrust and resistance. Viscous drag always contributes to resistance forces. Finally, body inertia, contributes to the water resistance, as it opposes acceleration from rest and tends to maintain motion once begun.

h  

The study of the swimming pattern of fishes should be researched well by the marine biologists. The remarkable pattern of swimming of fishes can inspire innovative designs to improve the ways of man-made under water systems. The man-made under water vehicles or robots help in interacting with the aquatic environment. Example: Autonomous Underwater Vehicles.

The highly efficient swimming mechanisms of certain kind of fishes living in oceans can help us modify the propelling system currently in use. We can also develop noiseless propulsion in under water vehicles for defence purposes.

A careful study and research of how fishes escape from their predators should be conducted to design new and indigenous under water fish like robots for military applications.