Locomotion of Fish – Types, Mechanism, Examples

Fish locomotion is referred to as the movement of fish in water with the help of body muscles and fins. It is the process by which fish generate force against water to move from one place to another. This movement is carried out mainly for feeding, escaping from predators and migration. It is controlled by the coordinated action of muscles, body flexibility and fins. The locomotion in fish is mainly adapted to aquatic conditions where water resistance plays an important role.

Fish locomotion is broadly divided into two main types based on the body parts involved in movement. These types are Body–Caudal Fin (BCF) locomotion and Median–Paired Fin (MPF) locomotion. In these types the movement pattern and fin usage is different and is suited for different habitats and speeds.

Body–Caudal Fin (BCF) locomotion is the common type of locomotion seen in most fishes. In this type the body and caudal fin are involved in producing thrust. The body is bent side to side and wave-like movements pass from head to tail. This type is mainly used for fast swimming and long distance movement. Anguilliform movement is seen in eel where whole body participates. Carangiform and subcarangiform movement occurs in fishes like trout where posterior part of body is involved. Thunniform movement is seen in tuna where body remains stiff and tail produces thrust. Ostraciiform movement occurs when body remains rigid and only tail moves.

Median–Paired Fin (MPF) locomotion involves the movement of median fins like dorsal and anal fins or paired fins like pectoral and pelvic fins. In this type the body remains mostly straight and movement is slow but controlled. It helps in precise movement and stability. Labriform movement occurs by pectoral fins as seen in wrasses. Balistiform movement uses dorsal and anal fins. Rajiform movement is seen in rays where pectoral fins show wave-like motion. Gymnotiform movement involves undulation of long anal fin while body remains straight.

Thus fish locomotion shows different types of movements depending on body structure and habitat. Each type helps the fish to survive and adapt efficiently in aquatic environment.

Types of Fish locomotion

Fish locomotion is mainly classified based on the body parts that are involved in the generation of thrust. The major types of fish locomotion are as follows–

1. Body–Caudal Fin (BCF) Locomotion

In this type of locomotion the thrust is produced by the undulation of body along with the caudal fin. It is mainly seen in fast swimming fishes and is suitable for long distance movement.

• Anguilliform – In this type the whole body takes part in movement. A wave of bending passes from head to tail along entire length of body. It is commonly seen in eel and lamprey.
• Subcarangiform – In this type the anterior part of body shows less movement while the posterior half of body produces most of the thrust. This type occurs in fishes like trout.
• Carangiform – The body is comparatively rigid and movement is restricted to the posterior third of body and tail region. It is seen in mackerel and jack fish.
• Thunniform – In this type the body remains almost stiff and propulsion is mainly produced by caudal peduncle and crescent shaped tail fin. It is found in tuna and some sharks.
• Ostraciiform – The body does not undulate and remains rigid. The movement is carried out only by oscillation of caudal fin. It is seen in box fish and cow fish.

2. Median–Paired Fin (MPF) Locomotion

In this type the body remains more or less straight and thrust is generated by median fins or paired fins. This locomotion is useful for slow swimming and better control.

• Rajiform – The propulsion is achieved by wave like undulation of large pectoral fins. It is commonly seen in rays and skates.
• Diodontiform – In this type undulations pass through large pectoral fins to produce movement. It occurs in porcupine fish.
• Amiiform – The movement is carried out by undulation of a long dorsal fin while body remains straight. It is seen in bowfin.
• Gymnotiform – In this type a long anal fin shows undulatory movement and body axis remains rigid. It is found in knife fish.
• Balistiform – The dorsal and anal fins undulate simultaneously to generate thrust. It is seen in trigger fish.
• Labriform – The pectoral fins are used for rowing or flapping type of movement. It is commonly seen in wrasses.
• Tetraodontiform – The dorsal and anal fins flap together to move the body. It occurs in ocean sunfish.

3. Specialized Locomotion

Some fishes show special types of locomotion adapted to particular environmental conditions.

• Gliding or flight – Flying fish leap out of water using strong tail movement and glide in air with the help of enlarged pectoral fins.
• Walking – Some fishes like mudskippers and frogfish move on land or sea bottom using modified fins.
• Burrowing – Eel shaped fishes move through sand or mud by body movements.
• Kármán gaiting – In flowing water some fishes adjust their body with water vortices to save energy instead of active swimming.

Detail step by step mechanism of Fish Locomotion

Fish locomotion is a coordinated process in which nervous control, muscle activity and water interaction work together to produce movement. The mechanism of fish locomotion occurs in a stepwise manner as given below–

Step 1. Initiation by Nervous System

The process of locomotion begins with the generation of nerve impulses in the central nervous system. These impulses are transmitted from head region towards tail region in a sequential manner. This neural signal controls the timing and direction of muscle contraction.

Step 2. Muscle Activation

After nerve stimulation the body muscles are activated. Fish possess two main types of muscles. Red muscles are activated during slow and continuous swimming. White muscles are activated during rapid movement and sudden acceleration. The muscles on opposite sides of body contract alternately.

Step 3. Regulation of Body Stiffness

During fast movement the muscles on both sides of body may contract together. This increases stiffness of body and prevents excess bending. This step helps in proper transmission of muscular force to water without loss of energy.

Step 4. Formation of Propulsive Body Wave

Due to alternate contraction and relaxation of muscles a wave like bending is produced in body. This wave starts from head region and moves backward towards tail. The amplitude of wave gradually increases from anterior to posterior region.

Step 5. Movement of Tail and Fins

As the body wave reaches tail region the caudal fin moves from side to side. In some fishes fins also oscillate or undulate. The tail and fins push water backward and sideways.

Step 6. Interaction with Water

The backward push of water creates a reaction force. This reaction force acts on the body in forward direction. At the same time water vortices are formed behind the fish which help in thrust generation.

Step 7. Thrust Production

The combined effect of body undulation and tail movement produces thrust. This thrust helps the fish to overcome drag force and move forward in water.

Step 8. Control and Stabilization

While thrust is produced by body and tail, other fins help in control. Dorsal and anal fins prevent rolling of body. Pectoral and pelvic fins help in steering, braking and maintaining balance.

Step 9. Buoyancy Adjustment

In bony fishes swim bladder helps in maintaining buoyancy during movement. In fishes without swim bladder pectoral fins generate lift during swimming to prevent sinking.

Step 10. Special Adaptations

In flowing water some fishes synchronize their body movement with water currents. This reduces muscular effort and saves energy during locomotion.

Thus fish locomotion is a stepwise and well coordinated mechanism involving nerves, muscles, body movements and water forces which together result in efficient movement in aquatic environment.

Fish Locomotion by body movements

Fish locomotion by body movements is referred to as Body–Caudal Fin (BCF) locomotion. In this type the movement is produced by the undulation of body muscles along with caudal fin. The muscles on both sides of body contract alternately and form wave like movements from head to tail. These body waves push the surrounding water backward and help in forward movement of fish. This type of locomotion is commonly associated with fast swimming and rapid acceleration.

The different types of fish locomotion by body movements are as follows–

• Anguilliform movement – In this type the whole body participates in movement. Large amplitude waves pass along entire length of body. It is commonly seen in eel and lamprey. This movement helps fish to move in narrow spaces but creates more resistance in water.
• Subcarangiform movement – In this type the anterior part of body shows little movement while posterior half of body produces thrust. The wave amplitude increases towards tail region. It is seen in fishes like trout.
• Carangiform movement – In this type the body is more rigid and undulations are restricted to posterior one third of body and caudal fin. This movement allows higher swimming speed and is seen in mackerel and jack fish.
• Thunniform movement – In this type the body remains stiff and streamlined. The movement is confined to caudal peduncle and crescent shaped tail fin. It is the most efficient type for long distance fast swimming. It is seen in tuna and some sharks.
• Ostraciiform movement – In this type the body remains rigid and does not show wave like movement. The propulsion is generated only by oscillation of caudal fin. It is found in box fish.

Thus fish locomotion by body movements shows different patterns depending on body flexibility and extent of body wave involved. These movements help fish to adapt for speed, efficiency and survival in aquatic environment.

Fish Locomotion by fins and tail

Fish locomotion by fins and tail is carried out by the use of caudal fin, median fins and paired fins. In this type of locomotion the thrust and control of movement is produced either by tail or by fins depending upon the requirement of speed and direction. This locomotion is mainly divided into tail based movement and fin based movement.

1. Locomotion by Tail (Body–Caudal Fin Locomotion)

In this type the caudal fin along with posterior part of body produces thrust. The tail pushes water backward and helps the fish to move forward. It is mainly used for fast swimming and long distance movement.

• Anguilliform type – In this type the entire body along with tail shows wave like movement. The caudal fin helps in propulsion. It is seen in eel.
• Subcarangiform type – In this type the anterior part of body remains less active and the posterior half with tail produces thrust. It occurs in fishes like trout.
• Carangiform type – The body is comparatively stiff and the tail region produces powerful strokes. It is seen in mackerel and jack fish.
• Thunniform type – In this type the body remains rigid and movement is confined to caudal peduncle and tail fin. It is highly efficient for fast swimming and is seen in tuna and sharks.
• Ostraciiform type – The body remains rigid and propulsion is produced only by oscillation of tail fin. It is found in box fish.

2. Locomotion by Fins (Median–Paired Fin Locomotion)

In this type the body remains almost straight and the movement is produced by median or paired fins. It helps in slow movement, balance and accurate direction control.

• Pectoral fins (Labriform movement) – The pectoral fins move in rowing or flapping manner to produce thrust. It is commonly seen in wrasses.
• Dorsal and anal fins (Balistiform movement) – In this type the dorsal and anal fins undulate together to move the fish forward. It is seen in trigger fish.
• Pelvic fins – These fins help in maintaining balance, stopping movement and controlling pitch of body during swimming.

3. Fins as Control and Stabilizing Organs

Apart from propulsion fins also act as controlling structures.

• Dorsal and anal fins prevent rolling of body during swimming.
• Pectoral fins help in turning and braking.
• In some fishes pectoral fins produce lift to prevent sinking in water.

Thus fish locomotion by fins and tail provides both power and control. The tail mainly produces thrust while fins help in balance, direction and precise movement in aquatic environment.

Forces acting on the body for locomotion

During fish locomotion different forces act on the body which together help in movement, balance and control in water. These forces are mainly hydrodynamic, gravitational and internal forces. The major forces acting on the body during locomotion are as follows–

1. Thrust Force

Thrust is the forward directed force produced by fish during swimming. It is generated when the body muscles contract and produce wave like movements or when fins push against water. This force helps the fish to move forward by pushing water backward. Thrust is mainly produced by body undulations and caudal fin movement.

2. Drag Force

Drag is the resisting force that acts opposite to the direction of movement. It is produced due to friction and pressure of water against the body surface. As the swimming speed increases drag force also increases. This force reduces swimming efficiency and requires more energy to overcome it.

3. Side or Recoil Forces

During undulatory movement lateral forces are produced on both sides of body. These forces do not contribute to forward movement and represent loss of energy. Many fishes reduce these forces by having streamlined body shape.

4. Gravitational Force

Gravitational force acts downward on the body due to body weight. It tends to pull the fish towards bottom. This force must be balanced to maintain position in water.

5. Buoyant Force

Buoyant force acts upward and is exerted by water on the body. In most bony fishes buoyancy is maintained by swim bladder. This force counteracts gravitational force and helps the fish to remain suspended in water.

6. Lift Force

Lift is the upward force generated during forward movement. It is produced mainly by pectoral fins acting like wings. This force is important in fishes which lack swim bladder such as sharks.

7. Internal Muscular Forces

These forces are produced by contraction and relaxation of muscles. They help in producing body stiffness and effective transmission of force to water. During rapid movement muscles on both sides of body may contract to increase rigidity.

8. Environmental Flow Forces

External water currents and vortices also act on fish body. In some conditions fish use these forces to reduce energy cost by adjusting body movement with water flow.

Thus fish locomotion is controlled by the combined action of different forces acting on the body. Balance between these forces helps the fish to move efficiently in aquatic environment.