Aquatic locomotion or swimming is biologically propelled motion through a liquid medium. Swimming by different mechanisms has evolved repeatedly in organisms including arthropods, fish, molluscs, amphibians, reptiles, birds, and mammals. Many single-celled organisms including bacteria and ciliates use motile organelles, cilia or flagella, while others use pseudopodia, temporary projections of the cell body powered by the cytoskeletal protein actin. Larger organisms may swim by undulating their bodies, as in many fish and worms; by jet propulsion, as in cephalopods such as squid; or by moving flippers like wings, as in sea turtles.
Evolution of swimming
Swimming has evolved repeatedly in unrelated lineages. Fossils claimed to be jellyfish occur in the Ediacaran, but the first free-swimming animals appear in the Early to Middle Cambrian. These are mostly related to the arthropods, and include the Dinocaridids, which swam by means of lateral lobes in a fashion reminiscent of today's cuttlefish and rays. Cephalopods joined the ranks of the active swimmers (nekton) in the late Cambrian, and chordates were probably swimming from the Early Cambrian. Many terrestrial animals retain some capacity to swim; some have returned to the water and developed the capacities for aquatic locomotion. Most apes (including humans) have lost the swimming instinct.
Micro-organisms
Microbial swimmers, sometimes called microswimmers, are microscopic entities that have the ability to move in fluid or aquatic environment. They are found everywhere among microorganisms, such as bacteria, archaea, protists, sperm and microanimals.
Flagella and cilia
Many small organisms such as bacteria have flagella which enable them to move in liquid environments. They use the protons of an electrochemical gradient to move their flagella. Torque in the flagella of bacteria is created by particles that conduct protons around the base of the flagellum. The direction of rotation of the flagella comes from the occupancy of the proton channels along the perimeter of the flagellar motor. Rod-shaped bacteria swim using rotating flagella. Sperm swim in much the same way.
Ciliates such as Paramecium use small flagella called cilia to move through the water. One ciliate may have hundreds to thousands of cilia, densely packed together in arrays. During movement, an individual cilium deforms using a high-friction power stroke followed by a low-friction recovery stroke. The deformation of each cilium is in phase with the deformation of its neighbor, propagating deformation waves along the surface of the organism. These waves of cilia, displaying collective behavior in a metachronal rhythm, allow the organism to move using the cilia in a coordinated manner. Paramecium can propel through water at up to 500 micrometers per second.
Twitching and gliding
Some bacteria can move by twitching and gliding, without using flagella. Twitching depends on the extension, attachment to a surface, and retraction of type IV pili which pull the cell forwards in a manner similar to the action of a grappling hook, providing energy to move the cell forward. Bacterial gliding uses different motor complexes, such as the focal adhesion complexes of Myxococcus.
Pseudopodia
Movement using a pseudopod is accomplished through increases in pressure at one point on the cell membrane. This pressure increase is the result of actin polymerization between the cortex and the membrane. As the pressure increases the cell membrane is pushed outward creating the pseudopod. When the pseudopod moves outward, the rest of the body is pulled forward by cortical tension. The result is cell movement through the fluid medium. Furthermore, the direction of movement is determined by chemotaxis. When chemoattraction occurs in a particular area of the cell membrane, actin polymerization can begin and move the cell in that direction. An excellent example of an organism that utilizes pseudopods is Naegleria fowleri.
Larger organisms
Jet propulsion
Jet propulsion is a method of aquatic locomotion where animals fill a muscular cavity and squirt out water to propel them in the opposite direction of the squirting water. Most organisms are equipped with one of two designs for jet propulsion; they can draw water from the rear and expel it from the rear, such as jellyfish, or draw water from front and expel it from the rear, such as salps. Filling up the cavity causes an increase in both the mass and drag of the animal. Consequently, the animal's velocity fluctuates as it moves through the water, accelerating while expelling water and decelerating while vacuuming water. Even though these fluctuations in drag and mass can be ignored if the frequency of the jet-propulsion cycles is high enough, jet propulsion is a relatively inefficient method of aquatic locomotion.
All cephalopods can move by jet propulsion, a very energy-consuming way to travel compared to the tail propulsion used by fish. The relative efficiency of jet propulsion decreases further as animal size increases. Since the Paleozoic, as competition with fish produced an environment where efficient motion was crucial to survival, jet propulsion has taken a back role, with fins and tentacles used to maintain a steady velocity. The stop-start motion provided by the jets, however, continues to be useful for providing bursts of high speed – not least when capturing prey or avoiding predators. Indeed, it makes cephalopods the fastest marine invertebrates. Most cephalopods float (i.e. are neutrally buoyant), so do not need to swim to remain afloat.
Jellyfish use a one-way water cavity design which generates a phase of continuous cycles of jet-propulsion followed by a rest phase. They move through the water by radially expanding and contracting their bell-shaped bodies to push water behind them. They pause between the contraction and expansion phases to create two vortex rings. Muscles are used for the contraction of the body, which creates the first vortex and pushes the animal forward, but the mesoglea is so elastic that the expansion is powered exclusively by relaxing the bell, which releases the energy stored from the contraction. Meanwhile, the second vortex ring starts to spin faster, sucking water into the bell and pushing against the centre of the body, giving a secondary and "free" boost forward. The mechanism, called passive energy recapture, only works in relatively small jellyfish moving at low speeds, allowing the animal to travel 30 percent farther on each swimming cycle. Jellyfish achieved a 48 percent lower cost of transport (food and oxygen intake versus energy spent in movement) than other animals in similar studies. One reason for this is that most of the gelatinous tissue of the bell is inactive, using no energy during swimming.
Scallops swim by slowly opening the gape of their two half-shells (called valves) and rapidly clapping them together. Closing the valves propels water with a strong force near the hinge via the velum, a curtain-like fold of the mantle that directs water expulsion around the hinge. They can swim both forwards and backwards.
Salps are colonial marine animals. Each colony member is a barrel-shaped zooid that can pump water from an inlet at one end to an outlet at the other end. A salp colony consists of a chain of zooids, and thus propels itself with multiple small jets of water, operating asynchronously (not in time with each other). This produces a relatively steady swimming speed, which is more efficient than a single pulsed jet.
Undulating the body
Many fish swim through water by creating undulations with their bodies or oscillating their fins. The undulations create components of forward thrust complemented by a rearward force, side forces which are wasted portions of energy, and a normal force that is between the forward thrust and side force. Different fish swim by undulating different parts of their bodies. Eel-shaped fish undulate their entire body in rhythmic sequences. Streamlined fish, such as salmon, undulate their tails. Some fish, such as sharks, use stiff, strong fins to create dynamic lift and propel themselves. It is common for fish to use more than one form of propulsion, although one mode of swimming is dominant in each species. Gait changes have been observed in juvenile reef fish of various sizes. Depending on their needs, fish can rapidly alternate between synchronized fin beats and alternating fin beats.
Crocodiles and amphibians such as newts, as well as amphibian tadpole larvae, use their deep, laterally compressed tails in an essentially carangiform (stiff body, movement mainly in tail) mode of propulsion. Terrestrial snakes, in spite of their 'bad' hydromechanical shape with roughly circular cross-section and gradual posterior taper, swim fairly readily when required, by anguilliform (eel-like) propulsion.
Oscillating fins or flippers
Cheloniidae (sea turtles) have forelimbs adapted into flippers of high-aspect-ratio wing shape, with which they propel themselves in a bird's propulsive mode, as if flying through the water. Plesiosaurs of the Jurassic and Cretaceous periods propelled themselves mainly with their flippers. Some slow-swimming fish propel themselves by oscillating their fins.
Pushing with webbed feet
Frogs and toads lose the larval tail completely. They have evolved webbed feet and a mode of tailless aquatic propulsion that resembles a wide-spreading squat-jump, with their muscular hindlimbs flexing and then kicking rearwards together to push water and provide most of the propulsion, while the forelimbs are recruited for additional paddling when more agile maneuvers are needed. It is a rather efficient form of aquatic locomotion in shallow, slow-flowing waters because the legs are better streamlined for short accelerations, and the movements are "amphibious", i.e. the same jumping motion can also be used on land without needing to evolve a separate locomotive mechanism.
Escape reactions
Some arthropods, such as lobsters and shrimps, can propel themselves backwards quickly by flicking their tail, known as lobstering or the caridoid escape reaction.
Some bony fish use fast-starts to escape from predators. Fast-starts are characterized by the muscle contraction on one side of the fish, twisting the fish into a C-shape. Afterwards, muscle contraction occurs on the opposite side to allow the fish to enter into a steady swimming state with waves of undulation traveling alongside the body. The power of the bending motion comes from fast-twitch muscle fibers in the central region of the fish. The signal to perform this contraction comes from a set of Mauthner cells which simultaneously send a signal to the muscles on one side of the fish. Mauthner cells are activated when something startles the fish.
Fast-starts are split up into three stages. Stage one, the preparatory stroke, is characterized by the initial bending to a C-shape with small delay caused by hydrodynamic resistance. Stage two, the propulsive stroke, involves the body bending rapidly to the other side, which may occur multiple times. Stage three, the rest phase, cause the fish to return to normal steady-state swimming and the body undulations begin to cease. Large muscles closer to the central portion of the fish are stronger and generate more force than the muscles in the tail. This asymmetry in muscle composition causes body undulations that occur in Stage 3. Once the fast-start is completed, the position of the fish has been shown to have a certain level of unpredictability, which helps fish survive against predators.