Aquatic locomotion
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
has evolved repeatedly in unrelated lineages. Supposed jellyfish fossils 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 Anomalocaridids, which swam by means of lateral lobes in a fashion reminiscent of today's cuttlefish. Cephalopods joined the ranks of the active swimmers 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 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
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, and they can out accelerate most fish. Oxygenated water is taken into the mantle cavity to the gills, and through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions. Most cephalopods float, so do not need to swim to remain afloat. Squid swim more slowly and use more power than fish do. The loss in efficiency is due to the amount of water the squid can accelerate out of its mantle cavity.
Jellyfish use a one-way water cavity design which generates a phase of continuous cycles of jet-propulsion followed by a rest phase. The Froude efficiency is about 0.09, which indicates a very costly method of locomotion. The metabolic cost of transport for jellyfish is high when compared to a fish of equal mass.
Much of the work done by scallop muscles to close its shell is stored as elastic energy in abductin tissue, which acts as a spring to open the shell. The elasticity causes the work done against the water to be low because of the large openings the water has to enter and the small openings the water has to leave. The inertial work of scallop jet-propulsion is also low. Because of the low inertial work, the energy savings created by the elastic tissue is negligible. Medusae can also use their elastic mesoglea to enlarge their bell. Their mantle contains a layer of muscle sandwiched between elastic fibers. The muscle fibers run around the bell circumferentially while the elastic fibers run through the muscle and along the sides of the bell to prevent lengthening. After making a single contraction, the bell vibrates passively at the resonant frequency to refill the bell. In contrast with scallops, the inertial work is similar to the hydrodynamic work due to how medusas expel water – through a large opening at low velocity. Thus, the negative pressure created by the vibrating cavity is lower than the positive pressure of the jet, meaning that inertial work of the mantle is small. Thus, jet propulsion is an inefficient swimming technique.
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 the caudal portions of their bodies. 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 they will display one dominant mode of swimming Gait changes have even 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 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 propulsion.
Oscillating fins or flippers
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.Some slow-swimming fish propel themselves by oscillating their fins.