Locomotion in Fish

Fish swim, everybody knows that. They are in fact much better at swimming than we are, but then so are all the mammals that live their lives in the water. Fish make swimming look easy, and for them it is, millions of years of evolution have created many fascinating adaptations, many of which we do not yet understand. What we do know is that fish, and aquatic mammals are incredibly efficient at swimming. The energy required to propel a Whale Shark through the water at 10 km an hour is far less than the energy required to propel a submarine of similar size at the same speed.

 

While we do not understand all there is to know about how fish swim so effectively we do know that its flexible body plan helps it to greatly reduce the turbulence it creates, and that a swimming fish experiences far less drag, about 10th only, of the drag generated by a rigid model of a fish being being propelled at the same speed. Part of the efficiency also comes from the slime that a fish produces, while this is annoying when you are trying to hold a fish it reduces the friction a fish experiences by at least 65%.

Water is of course far more dense than air, (about 800 times more) and therefore it resists the movement of any body through it much more strongly than air. However water is also noncompressible which means it is far easy to generate thrust by pushing against it. Further more, the density of water is very close to the density of a living body, which means that fish have to expend little or no energy in resisting gravity. You may have noticed, when you are tired, how heavy your head becomes, this is because gravity is pulling your head down, and your muscles have to work all day just to hold it up. A fish, or mammal, living in water doesn't have this problem.

All this means that water is actually the easiest medium to move through and that swimming is the most efficient form of locomotion known. The energetic costs of travelling 1 km (per kilogram of body weight) are 5.43 kcal for a walking Ground Squirrel, 1.45 kcal for a flying Gull and only 0.39 kcal for a swimming Salmon, which makes swimming about 7 times as efficient as walking for creatures well adapted to their respective mediums and methods.

The absolute speed at which a fish swims is relevant both its size and its shape, and like you it can only keep going at top speed for a relatively short period of time before it gets tired. But at a more leisurely pace it can keep going all day. For your typical Trout, Herring or Sardine shaped fish the maximum speed is around 10 body lengths per second, naturally this means that larger fish swim faster. For instance a 30 cm (1 ft) Sea Trout (Salmo trutta) has a top speed of around 10.8 km (7.2 miles) per hour while a 20 cm specimen as a top speed of about 8.1 km (5.4 miles) per hour, while a 60 cm (2 ft) Salmon as a top speed of around 22.5 km (14 miles) per hour. Naturally fish that have evolved less dynamic shapes and attitudes in order to allow them to survive in specialist habitats, such as coral reefs, the sea floor, the deep oceans or environments with dense vegetation have lower relative top speeds. However fish can swim at many different speeds and as a general rule, excepting anguilliform swimmers, fish, when actively swimming, not just drifting with slow tail beats, develop a speed that relates to its length and the frequency of its tail beats in the following way. V = 1/4 [L(3f-4)] where 'V' is the velocity in centimetres per second, 'L' is the length of the fish and 'f' is the frequency of tail beats per second. So there you have it.

Maximum Swimming Speed of Fish in Relationship to Length
Common NameScientific NameLength in cmMax. Speed in Km/hr
Sand GobyPomatoschistus minutus6.50.9
Sea SticklebackSpinachia spinachia102.63
GoldfishCarasius auratus154.8
HerringClupea harengus205.4
DaceLeuciscus leuciscus155.85
PikeEsox lucius16.57.2

A fish uses its fins to swim with, mostly it is the caudal (tail) fin that is used for propulsion while the remaining fins are for balance control and fine maneuvering. However slower moving fish, fish which simply are not in a hurry, or those working in habitats where movement is restricted are quite capable of delicate and direct movements powered only by the dorsal, pectoral and pelvic fins. The pelvic and pectoral fins are both capable of sculling, basically rowing the fish forward. The extreme proponents of pectoral fin locomotion are ofcourse the skates and rays, many of whom have given up the traditional arrow-shaped fish form for a more bird like one in which the pectoral fins are greatly enlarged and very well muscled and the fish seem to fly through the water.

The dorsal and anal fins or many fish are capable of undulating movements in which a series of oscillating waves travel along the fin. These muscularly generated waves provide a steady, if not intense forward thrust. Good examples of fish that rely on dorsal and anal fin undulations to move around are the Tube-mouths (Pipe-fishes and Sea horses) however many other fish use them as well, even larger fish such as Esox lucius the Pike. Most of the fish that use their fins for propulsion are also capable of using body flexure and the caudal fin as well. However some fish such as those in the order Plectognathi ( Trunk-fish, Parrot-fish, Butterfly-fish, Porcupine-fish, and Trigger-fish etc) have all lost the ability to swim using body flexure and can only move using their other fins. The Sun-fishes (Molidae) are by far the largest fish to have given up body flexure and swim their lives through the vast open seas propelled entirely by the paddling of their dorsal and anal fins.

Normal swimming involves sinuous movements of the fish's body to varying degrees. The fish flexes its muscles to produce a series of waves of contraction along each side of the body , these waves of muscular contraction alternate from one side of the fishes body to the other and the result is that the tail of the fish is moved from side to side. In long thin fish such as eels the fish's whole body undulates in series of open s-shaped curves. For most species the thrust is developed as the caudal fin and to some extent the anterior part of the body push against the water, however for species like eels, where the fins are small, but the body somewhat is flattened the whole rear section of the fish acts as a caudal fin. Scientists now divide active swimming like this into three categories depending on the amount of flexure the fish's body undergoes.

Fish that form a deep sinuous wave while they are swimming, such as eels, lampreys, lungfish and some sharks, as in the image above, are termed 'Anguilliform swimmers'. Fish that swim actively using the caudal fin, but which flex the fin in a manner that leaves the body relatively steady, such as Boxfish and Trunkfish, see image below, are termed 'Ostraciform swimmers'.

In between these two extremes there are a large number of fish that flex their body to an intermediate degree, the body may make an s-shape, but it is a shallow wave, as in many sharks, or they may merely flex the anterior half of the body, as in Tuna. Such fish are referred to as 'Carangiform swimmers'. Carangiform swimmers and anguilliform swimmers only contract a portion of the muscles on either side of their body at any one moment, by controlling and varying the muscular activity on both sides of their body they create the wave like movements we see when they swim. In comparison, ostraciform swimmers contract all the muscles on one side, and then all the muscles on the other, this beats the tail, but does not through the body into a sine wave.

Flying fish do not really fly. Scientists generally define flying as 'powered flight', and within this designation what flying fish do is glide, not fly. This is because all the momentum they possess whilst travelling through the air is gained while they are in the water and not from the air. In other words they don't flap their enlarged fins in the air, but only hold them out stiff.

 

Air travel, or gliding, has evolved in four different families of fish, all of which are marine in their habits; the Belonidae or Gar-fish, the Dactylopteridae or Flying-gurnards, the Exocoetidae, or Flying-fish and the Hemirhamphidae or Half-beaks. Of these it is the Exocoetidae, with about 50 species, which are the traditional Flying-fish. These fish have greatly enlarged pectoral fins, which they hold folded up alongside the body while they are swimming, but which they open out once their body is out of the water. They also have assymetrical dorsal fins, with the lower lobe being larger.

The fish swim rapidly, and close to the surface of the oceans they inhabit, holding their bodies with the head up and the tail down. When they wish to leave the water the tail begins to beat very rapidly, up to 50 times a second in some species. This increase in speed pushes the fish's body out of the water whereupon the pectoral fins can be unfolded. However the lower lobe of the tail or dorsal fin remains in the water a while longer and is therefore able to continue to supply propulsion even as the body escapes the resistance of the water. This final flurry of exertion drives the fish fully into the air where it glides for up to 5 or 10 seconds. As the fish's body is angled relative to the water surface the tail re-enters the water first and by flexing it rapidly a fish may enter an almost immediate second, and even 3rd, 4th and 5th take off. During such a number of repeated takeoffs a fish may rise up to one metre above the surface of the water and travel for several hundreds of metres at speeds as great as 30 km/h ( 20 miles/h) spending more than 40 seconds above the ocean's surface.

Two other familes of fish, both of which inhabit fresh waters, include species that were once thought to have powered flight, the Pantodontidae and the Gasteropelecidae. The Pantodontidae currently contains only one species, Pantodon buchholizi, the African Butterfly Fish while the Gasteropelecidae contains nine species including the Marbled Hatchet Fish, Carnegiella strigata. Both these species have deep bodies with a lot of muscular support for their enlarged pectoral fins, or in the case of Pantodon, pectoral and pelvic fins. It is now considered that both these species simply jump, if somewhat spectacularly, out of the water using their enlarged fins and extensive musculature to build up speed whilst in the water.

Finally having started with the statement that fish swim, we need to end with a mention of those fish that prefer to walk. Yes it is true, many fish are quite happy walking, because of the support that water offers their bodies they do not need strong bones for support and as long as the pectoral fins are reasonable firm, and long enough to hold the body off the substrate, then a fish can walk. Other fish have learned to leave the water and swim across the land and some have even learned to hop, or jump.

Several species of fish are known to cross short land barriers between one area of water and another using basically the same actions as they use in swimming, these include eels (Anguilla sp.), cuchia (Amphipnous sp.) and Snake-heads (Ophicephalidae) and several catfish in the genera Clarias and Saccobranchus. A rather more unusual means of terrestrial locomotion is shown by the Indian Climbing Perch (Anabas scandens), which uses sharp spines on the lower parts of its gill covers to hold onto the ground whilst it pushes itself forward using its tail and its pectoral fins.

However none of these fish could truly be said to be walking. There are however several species of fish which have become quite adept at fin-walking, on land, and in the water. In the water the best known, and most proficient of these are the Blennies, of which there are many species, but note should also be taken of the Lizard-fishes (Synodontidae) and the Gurnards (Trigilidae) both of which are happy to crawl across the ocean floor on their fins.

On land only a few species of fish, mostly in the genus Periopthalmus (Mud Skippers) have really taken to fin-walking. Two these are the African Mud Skipper (Periopthalmus papilio) and the Asiatic Snake-head (Ophicephalus striatus). Mud Skippers are the most adept walkers. They possess a strengthened girdle, and extra muscles to facilitate their walking, which is done using the pectoral fins with the tail supperting the end of the body. They are able to climb tree roots and survive for long periods of time out of the water. They use their unusual ability to hunt down terrestrial insects and surface dwelling crabs. Mud Skippers as well as some other fish, particularly Gobies (Gobiidae), are very good at jumping from a substrate, or hopping. In the coastal, tropical environments where they live they happily leap from one pool of water to the next at low tide. A fine example of a fish that hops by first curving up its tail and then suddenly straightening it, whilst pushing it against the ground, is the Sheep's-head Molly Miller (Bathygobius soporator).

As an absolute end to this section I must mention one fish that has learned to use its fins to hold onto and to climb around on the seaweed where it lives. The pectoral and pelvic fins of Sargassum-fish (Histrio histrio) are long and flexible and they can use them to grasp the fronds of the sargassum that creates an underwater jungle where they live. They will also move their fins in an alternating pattern to crawl along fronds. However they also swim very well.

Well I hope you have enjoyed this brief look into the world of fish locomotion. Remember that with over 29,000 species the range of possible variations is immense and there is a lot more that you could learn about fish locomotion if the subject really interests you, but for that you will have to visit a university or college library.


The last two images of this page are of Blennies, fish which are closely related to Gobies, unfortunately I have no images of Gobies. These two images, and the one at the top of the page, are reproduced, from the book 'The Fish of Bulgaria' with the kind permission Lyubo Penev of Pensoft Publishing. They were both painted by the talented Georgi Pchelarov.

 

 

 

 

Bibliography
 
The Fish Anatomy Menu
Anatomy Fins Blood Nerves Magnetism Swim-bladder
Skeleton Sight Scales Hearing Electricity Osmoregulation
Digestion Gills Smell Muscles Lateral Line Thermoregulation


 

 

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