A taxis (plural taxes //, from Ancient Greek τάξις (taxis), meaning 'arrangement') is the movement of an organism in response to a stimulus such as light or the presence of food. Taxes are innate behavioural responses. A taxis differs from a tropism (turning response, often growth towards or away from a stimulus) in that in the case of taxis, the organism has motility and demonstrates guided movement towards or away from the stimulus source. It is sometimes distinguished from a kinesis, a non-directional change in activity in response to a stimulus.
Taxes are classified based on the type of stimulus, and on whether the organism's response is to move towards or away from the stimulus. If the organism moves towards the stimulus the taxis is positive, while if it moves away the taxis is negative. For example, flagellate protozoans of the genus Euglena move towards a light source. This reaction or behaviour is called positive phototaxis, since phototaxis refers to a response to light and the organism is moving towards the stimulus.
Many types of taxis have been identified, including:
- aerotaxis (stimulation by oxygen)
- anemotaxis (by wind)
- barotaxis (by pressure)
- chemotaxis (by chemicals)
- durotaxis (by stiffness)
- electrotaxis or galvanotaxis (by electric current)
- gravitaxis (by gravity)
- hydrotaxis (by moisture)
- magnetotaxis (by magnetic field)
- phototaxis (by light)
- rheotaxis (by fluid flow)
- thermotaxis (by changes in temperature)
- thigmotaxis (by physical contact)
Depending on the type of sensory organs present, a taxis can be classified as a klinotaxis, where an organism continuously samples the environment to determine the direction of a stimulus; a tropotaxis, where bilateral sense organs are used to determine the stimulus direction; and a telotaxis, where a single organ suffices to establish the orientation of stimulus.
- Aerotaxis is the response of an organism to variation in oxygen concentration, and is mainly found in aerobic bacteria.
- Anemotaxis is the response of an organism to wind. Many insects show a positive anemotactic response (turning/flying into the wind) upon exposure to an airborne stimulus cue from a food source.
- Chemotaxis is a response elicited by chemicals: that is, a response to a chemical concentration gradient. For example, chemotaxis in response to a sugar gradient has been observed in motile bacteria such as E. coli. Chemotaxis also occurs in the antherozoids of liverworts, ferns, and mosses in response to chemicals secreted by the archegonia. Unicellular (e.g. protozoa) or multicellular (e.g. worms) organisms are targets of chemotactic substances. A concentration gradient of chemicals developed in a fluid phase guides the vectorial movement of responder cells or organisms. Inducers of locomotion towards increasing steps of concentrations are considered as chemoattractants, while chemorepellents result moving off the chemical. Chemotaxis is described in prokaryotic and eukaryotic cells, but signalling mechanisms (receptors, intracellular signaling) and effectors are significantly different.
- Durotaxis is the directional movement of a cell along a stiffness gradient.
- Electrotaxis (or galvanotaxis) is the directional movement of motile cells in response to an electric field. It has been suggested that by detecting and orienting themselves toward the electric fields, cells are able to direct their movement towards the damages or wounds to repair the defect. It also is suggested that such a movement may contribute to directional growth of cells and tissues during development and regeneration. This notion is based on the existence of measurable electric fields that naturally occur during wound healing, development and regeneration; and cells in cultures respond to applied electric fields by directional cell migration – electrotaxis / galvanotaxis.
- Energy taxis is the orientation of bacteria towards conditions of optimal metabolic activity by sensing the internal energetic conditions of cell. Therefore, in contrast to chemotaxis (taxis towards or away from a specific extracellular compound), energy taxis responds on an intracellular stimulus (e.g. proton motive force, activity of NDH- 1) and requires metabolic activity.
- Gravitaxis (known historically as geotaxis) is a response to the attraction due to gravity. The planktonic larvae of the king crab Lithodes aequispinus use a combination of positive phototaxis (movement towards the light) and negative gravitaxis (upward movement). Both positive and negative gravitaxes are found in a variety of protozoans (e.g., Loxodes, Remanella and Paramecium).
- Magnetotaxis is, strictly speaking, the ability to sense a magnetic field and coordinate movement in response. However, the term is commonly applied to bacteria that contain magnets and are physically rotated by the force of the earth's magnetic field. In this case, the "behaviour" has nothing to do with sensation and the bacteria are more accurately described as "magnetic bacteria".
- Pharotaxis is the movement to a specific location in response to learned or conditioned stimuli, or navigation by means of landmarks. 
- Phonotaxis is the movement of an organism in response to sound.
- Phototaxis is the movement of an organism in response to light: that is, the response to variation in light intensity and direction. Negative phototaxis, or movement away from a light source, is demonstrated in some insects, such as cockroaches. Positive phototaxis, or movement towards a light source, is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis. Many phytoflagellates, e.g. Euglena, and the chloroplasts of higher plants positively phototactic, moving towards a light source. Two types of positive phototaxis are observed in prokaryotes: scotophototaxis is observable as the movement of a bacterium out of the area illuminated by a microscope, when entering darkness signals the cell to reverse direction and reenter the light; a second type of positive phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light.
- Rheotaxis is a response to a current in a fluid. Positive rheotaxis is shown by fish turning to face against the current. In a flowing stream, this behaviour leads them to hold their position in a stream rather than being swept downstream. Some fish will exhibit negative rheotaxis where they will avoid currents.
- Thermotaxis is a migration along a gradient of temperature. Some slime molds and small nematodes can migrate along amazingly small temperature gradients of less than 0.1 °C/cm. They apparently use this behaviour to move to an optimal level in soil.
- Thigmotaxis is the response of an organism to physical contact or to the proximity of a physical discontinuity in the environment (e.g. rats preferring to swim near the edge of a water maze). Codling moth larvae are believed to used thigmotatic sense to locate fruits to feed on.
Terminology derived from taxis direction
There are five types of taxis based on the movement of organisms.
- Klinotaxis occurs in organisms with receptor cells but no paired receptor organs. The cells for reception are located all over the body, particularly towards the anterior side. The organisms detect the stimuli by turning their head sideways and compare the intensity. When the intensity of stimuli is balanced equally from all sides then the organisms move in a straight line. The movement of blowfly and butterfly larvae clearly demonstrates klinotaxis.
- Tropotaxis is displayed by organisms with paired receptor cells. When the stimuli coming from a source are balanced equally the organisms show movement. Because of this, animals are able to move sideways, unlike klinotaxis where the organisms can move only in a straight line. The movement of grayling butterflies and fish lice clearly demonstrates tropotaxis.
- Telotaxis requires paired receptors. The movement occurs along the direction where the intensity of the stimuli is stronger. Telotaxis is clearly seen in the movement of bees when they leave their hive to look for food. They balance the stimuli from the sun as well as from flowers but land on the flower whose stimulus is most intense for them.
- Menotaxis describes organisms' maintenance of a constant angular orientation. A clear demonstration is shown by bees returning to their hive at night and the movement of ants with respect to the sun.
- Mnemotaxis is the use of memory to follow trails that organisms have left when travelling to or from their home.
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