Swimming is the most economical form of animal locomotion because the body of aquatic animals is supported by water and hence the animals do not have to spend energy to counter gravity. While a squirrel spends 5.43 kcal for walking and a gull spends 1.45 kcal for flying, a salmon spends only 0.39 kcal for swimming one kilometer distance. Almost neutral buoyancy is achieved by the air bladder of bony fishes and by the fatty liver in sharks. Streamlined body of fishes provides least resistance while moving through viscous water. Locomotion in fishes was studies by Breder (1926) and Gray (1933).
Based on energy costs, swimming can be classified into 3 groups (Hoar & Randall, 1978), namely, sustained, prolonged and burst swimming.
Swimming speed is slow, almost 6-7 body lengths per second and is maintained for long periods. Energy required by muscles is provided by aerobic respiration and since the speed is slow, oxygen debt is not built up as fatigue comes very slowly. This type of locomotion is used for foraging over large areas or for long distance migrations.
2. Burst swimming
This type of locomotion is used for escaping predators, chasing a prey or for swimming against currents of water. High speeds of up to 20 body lengths/second are achieved but can be sustained only for short periods. Power is generated by anaerobic respiration in which creatinine phosphate and muscle glycogen are used up. Fatigue comes very rapidly and therefore burst swimming can be sustained for short periods.
3. Prolonged swimming
This type of swimming is intermediate between the above two types in speed and energy. Energy is supplied by both aerobic and anaerobic respiration. Prolonged swimming can last up to 3 hours and longer bouts can end up in fatigue. This type of locomotion is used occasionally as the situation demands.
Fishes live in viscous medium where they face two types of drags that must be overcome in order to move forward in water. Fishes are able to move through water causing almost no turbulence.
1. Viscous drag. As the fish moves forward 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. Therefore, larval fishes having larger surface area, experience viscous drag as a major force against them. Hence these tiny creatures must always actively swim since they are unable to glide forward without actively swimming. Mucous coated streamlined body of fishes reduces viscous drag.
2. Pressure drag or inertial drag. Displacement of water by fish body creates pressure drag. It is the pressure of water from all sides on fish body. Bulky fishes face high inertial drag as they replace more water as compared to smaller fishes. Pressure drag increases with speed as well as depth. Streamlining reduces pressure drag and fishes keep their body in straight line in order to minimize drag.
Aspect ratio of caudal fin. Aspect ratio is the dorsal to ventral width of caudal fin divided by anterior to posterior length. High aspect ratio gives efficient forward motion as in sharks. Caudal fins of trouts, minnows and perches are flexible and can change aspect ratio according to the needs.
Fishes swim by metachronal contraction of myomeres alternately on either side of axis. Lateral push of the caudal fin on water produces a reactive force on the opposite side at right angle to the axis of body. Reactive force has two components—forward thrust and lateral force. Thrust propels the body forward and overcomes drag while lateral force makes the head yaw from side to side. Push force of the caudal fin is always stronger than any other force.
Yawing is side to side movement of head created by the lateral reaction force generated by the sideways lashing of the tail fin. Yawing is countered by the use of pectoral fin so that fish can move in straight line.
Pitching is up and down movement of the head produced by uneven drag on the body or by heterocercal or hypocercal tail fin. Pitching is countered by pectoral fins.
Rolling is spinning of the body on its anterior-posterior axis. Rolling must be controlled while turning right or left and it is done by the dorsal fin. Bony fishes have foldable dorsal fin that is supported by fin rays, so that rolling can be controlled at will.
There are three types of locomotions in fishes depending on the shape of body.
Eels (Anguilla) and cyclostomes having serpentine body swim by lateral undulation of the entire body that is caused by metachronal rhythm in the contraction of myotomes. 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.
In majority of fishes lateral undulation of body is restricted to the posterior one-third of body. Tail is lashed from side to side in such a way that it always has a backwardly facing component of push and caudal fin increases the area and the force of backward push of tail.
This type of locomotion is found in box fishes and trunk fishes (family Ostraciidae) in which body is not flexible and hence cannot undergo lateral undulation. Therefore, only tail fin propels the body forward.
LOCOMOTION IN SHARKS AND DOGFISHES (PLEUROTREMATA)
Sharks and dogfishes have long, streamlined body with a heterocercal tail fin. Pectoral fins are located in front of the centre of gravity that lies just below the dorsal fin. Larger upper lobe of the caudal fin produces lift force on tail due to which head pitches downward. Pitching force is countered by the pectoral fins which also function as elevators. Heterocercal tail fin helps elasmobranchs to swim near the bottom of sea as most of the elasmobranchs are natural bottom dwellers.
Dorsal fins are antirolling devices and they stop rolling of the body while the fishes turn right and left. Pelvic fins in cartilaginous fishes do not contribute to swimming or balancing.
LOCOMOTION IN SKATES AND RAYS (HYPOTREMATA)
Like other elasmobranchs, rays also have heterocercal tail fin and two dorsal fins on the tail. But they have a doroventrally flattened body and enlarged pectoral fins are fused on the lateral margins of body. Pectoral fins can produce metachronal contractions and propel the body forward. Rays being dorsoventrally flattened have no problem of rolling and hence dorsal fins are reduced.
LOCOMOTION IN BONY FISHES
Majority of bony fishes possess homocercal or diphycercal tail fin that produces a straight forward push on the body to counter viscous as well as pressure drag. Dorsal fin is foldable and can be stretched whenever required. Pectoral fins are placed high and are used as brakes and for turning right and left. Anteriorly placed pelvic fins stop the upward lift of head while braking. Bony fishes also use operculum to eject water to help in quick turning. Swim bladder maintains the fish steady at a given depth.
LOCOMOTION IN FLYING FISH
Flying fishes, owing to their enlarged pectoral fins can glide in air for considerable distances. Caudal fin is hypocercal with enlarged lower lobe that helps to pull the tail down and keep head upwards while swimming so that they can swim upward rapidly and jump out of water to glide. Even pelvic fin is enlarged to give upward lift to the body.
LOCOMOTION IN SEA HORSE AND PIPE FISH
Sea horse and pipe fish have no fins except the single dorsal fin and hence this fin is used to push the body forward in a vertical position. Tail is prehensile to hold on to the sea weeds and corals where these creatures remain camouflaged and prey upon planktons.
Some bony fishes such as Amia have very long dorsal fin extending to almost the entire length of the back. This fin is capable of undulation to propel the body forward while swimming at slow speed. Similarly, Notopterus and Wallago have very long anal fin which almost continues up to the tail fin and is used to push the body forward.
Trigger fish that can produce fast bursts of speed at short distances, have very high caudal fin to increase the surface area. Dorsal fin and anal fins are also broad and placed posteriorly near the caudal fin to increase the aspect ratio of the posterior region so that a powerful push can be created to propel the body forward.