Fish locomotion
Fish locomotion is the various types of animal locomotion used by fish, principally by swimming. This is achieved in different groups of fish by a variety of mechanisms of propulsion, most often by wave-like lateral flexions of the fish's body and tail in the water, and in various specialised fish by motions of the fins. The major forms of locomotion in fish are:
- Anguilliform, in which a wave passes evenly along a long slender body;
- Sub-carangiform, in which the wave increases quickly in amplitude towards the tail;
- Carangiform, in which the wave is concentrated near the tail, which oscillates rapidly;
- Thunniform, rapid swimming with a large powerful crescent-shaped tail; and
- Ostraciiform, with almost no oscillation except of the tail fin.
In addition, some fish can variously "walk", burrow in mud, leap out of the water and even glide temporarily through the air.
Swimming
Mechanism
Fish swim by exerting force against the surrounding water. There are exceptions, but this is normally achieved by the fish contracting muscles on either side of its body in order to generate waves of flexion that travel the length of the body from nose to tail, generally getting larger as they go along. The vector forces exerted on the water by such motion cancel out laterally, but generate a net force backwards which in turn pushes the fish forward through the water. Most fishes generate thrust using lateral movements of their body and caudal fin, but many other species move mainly using their median and paired fins. The latter group swim slowly, but can turn rapidly, as is needed when living in coral reefs for example. But they can not swim as fast as fish using their bodies and caudal fins.Consider the tilapia shown in the diagram. Like most fish, the tilapia has a streamlined body shape reducing water resistance to movement and enabling the tilapia to cut easily through water. Its head is inflexible, which helps it maintain forward thrust. Its scales overlap and point backwards, allowing water to pass over the fish without unnecessary obstruction. Water friction is further reduced by mucus which tilapia secrete over their body.
The backbone is flexible, allowing muscles to contract and relax rhythmically and bring about undulating movement. A swim bladder provides buoyancy which helps the fish adjust its vertical position in the water column. A lateral line system allows it to detect vibrations and pressure changes in water, helping the fish to respond appropriately to external events.
Well developed fins are used for maintaining balance, braking and changing direction. The pectoral fins act as pivots around which the fish can turn rapidly and steer itself. The paired pectoral and pelvic fins control pitching, while the unpaired dorsal and anal fins reduce yawing and rolling. The caudal fin provides raw power for propelling the fish forward.
Body/caudal fin propulsion
There are five groups that differ in the fraction of their body that is displaced laterally:Anguilliform
In the anguilliform group, containing some long, slender fish such as eels, there is little increase in the amplitude of the flexion wave as it passes along the body.Subcarangiform
The subcarangiform group has a more marked increase in wave amplitude along the body with the vast majority of the work being done by the rear half of the fish. In general, the fish body is stiffer, making for higher speed but reduced maneuverability. Trout use sub-carangiform locomotion.Carangiform
The carangiform group, named for the Carangidae, are stiffer and faster-moving than the previous groups. The vast majority of movement is concentrated in the very rear of the body and tail. Carangiform swimmers generally have rapidly oscillating tails.Thunniform
The thunniform group contains high-speed long-distance swimmers, and is characteristic of tunas and is also found in several lamnid sharks. Here, virtually all the sideways movement is in the tail and the region connecting the main body to the tail. The tail itself tends to be large and crescent shaped.Ostraciiform
The ostraciiform group have no appreciable body wave when they employ caudal locomotion. Only the tail fin itself oscillates to create thrust. This group includes Ostraciidae.Median/paired fin propulsion
Not all fish fit comfortably in the above groups. Ocean sunfish, for example, have a completely different system, the tetraodontiform mode, and many small fish use their pectoral fins for swimming as well as for steering and [|dynamic lift]. Fish in the order Gymnotiformes possess electric organs along the length of their bodies and swim by undulating an elongated anal fin while keeping the body still, presumably so as not to disturb the electric field that they generate.Many fish swim using combined behavior of their two pectoral fins or both their anal and dorsal fins. Different types of Median paired fin propulsion can be achieved by preferentially using one fin pair over the other, and include rajiform, diodontiform, amiiform, gymnotiform and balistiform modes.
Rajiform
Rajiform locomotion is characteristic of rays and skates, when thrust is produced by vertical undulations along large, well developed pectoral fins.Diodontiform
Diodontiform locomotion propels the fish propagating undulations along large pectoral fins, as seen in the porcupinefish.Amiiform
Amiiform locomotion consists of undulations of a long dorsal fin while the body axis is held straight and stable, as seen in the bowfin.Gymnotiform
Gymnotiform locomotion consists of undulations of a long anal fin, essentially upside down amiiform, seen in the South American knifefish Gymnotiformes.Balistiform
In balistiform locomotion, both anal and dorsal fins undulate. It is characteristic of the family Balistidae. It may also be seen in the Zeidae.Oscillatory
Oscillation is viewed as pectoral-fin-based swimming and is best known as mobuliform locomotion. The motion can be described as the production of less than half a wave on the fin, similar to a bird's wing flapping. Pelagic stingrays, such as the manta, cownose, eagle and bat rays use oscillatory locomotion.Tetraodontiform
In tetraodontiform locomotion, the dorsal and anal fins are flapped as a unit, either in phase or exactly opposing one another, as seen in the Tetraodontiformes. The ocean sunfish displays an extreme example of this mode.Labriform
In labriform locomotion, seen in the wrasses, oscillatory movements of pectoral fins are either drag based or lift based. Propulsion is generated either as a reaction to drag produced by dragging the fins through the water in a rowing motion, or via lift mechanisms.Dynamic lift
Bone and muscle tissues of fish are denser than water. To maintain depth, bony fish increase buoyancy by means of a gas bladder. Alternatively, some fish store oils or lipids for this same purpose. Fish without these features use dynamic lift instead. It is done using their pectoral fins in a manner similar to the use of wings by airplanes and birds. As these fish swim, their pectoral fins are positioned to create lift which allows the fish to maintain a certain depth. The two major drawbacks of this method are that these fish must stay moving to stay afloat and that they are incapable of swimming backwards or hovering.Hydrodynamics
Similarly to the aerodynamics of flight, powered swimming requires animals to overcome drag by producing thrust. Unlike flying, however, swimming animals often do not need to supply much vertical force because the effect of buoyancy can counter the downward pull of gravity, allowing these animals to float without much effort. While there is great diversity in fish locomotion, swimming behavior can be classified into two distinct "modes" based on the body structures involved in thrust production, Median-Paired Fin and Body-Caudal Fin. Within each of these classifications, there are numerous specifications along a spectrum of behaviours from purely undulatory to entirely oscillatory. In undulatory swimming modes, thrust is produced by wave-like movements of the propulsive structure. Oscillatory modes, on the other hand, are characterized by thrust produced by swiveling of the propulsive structure on an attachment point without any wave-like motion.Body-caudal fin
Most fish swim by generating undulatory waves that propagate down the body through the caudal fin. This form of undulatory locomotion is termed body-caudal fin swimming on the basis of the body structures used; it includes anguilliform, sub-carangiform, carangiform, and thunniform locomotory modes, as well as the oscillatory ostraciiform mode.Adaptation
Similar to adaptation in avian flight, swimming behaviors in fish can be thought of as a balance of stability and maneuverability. Because body-caudal fin swimming relies on more caudal body structures that can direct powerful thrust only rearwards, this form of locomotion is particularly effective for accelerating quickly and cruising continuously. body-caudal fin swimming is, therefore, inherently stable and is often seen in fish with large migration patterns that must maximize efficiency over long periods. Propulsive forces in median-paired fin swimming, on the other hand, are generated by multiple fins located on either side of the body that can be coordinated to execute elaborate turns. As a result, median-paired fin swimming is well adapted for high maneuverability and is often seen in smaller fish that require elaborate escape patterns.The habitats occupied by fishes are often related to their swimming capabilities. On coral reefs, the faster-swimming fish species typically live in wave-swept habitats subject to fast water flow speeds, while the slower fishes live in sheltered habitats with low levels of water movement.
Fish do not rely exclusively on one locomotor mode, but are rather locomotor generalists, choosing among and combining behaviors from many available behavioral techniques. Predominantly body-caudal fin swimmers often incorporate movement of their pectoral, anal, and dorsal fins as an additional stabilizing mechanism at slower speeds, but hold them close to their body at high speeds to improve streamlining and reducing drag. Zebrafish have even been observed to alter their locomotor behavior in response to changing hydrodynamic influences throughout growth and maturation.