A taxis (plural taxes, //), from Ancient Greek τάξις meaning "arrangement," is an innate behavioral response by an organism (or organelles) to a directional stimulus or gradient of stimulus intensity. A taxis differs from a tropism (turning response, often growth towards or away from a stimulus) in that 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.
- 1 Examples
- 2 Terminology regarding direction of taxis
- 3 See also
- 4 References
- 5 External links
For example, flagellate protozoans of the genus Euglena move towards a light source. Here the directional stimulus is light, and the orientation movement is towards the light. This reaction or behaviour is a positive one to light and specifically termed "positive phototaxis", since phototaxis is a response to a light stimulus, and the organism is moving towards the stimulus. If the organism moves away from the stimulus, then the taxis is negative. Many types of taxis have been identified and named using prefixes to specify the stimulus that elicits the response. These include aerotaxis (stimulation by oxygen) anemotaxis (wind), barotaxis (pressure), chemotaxis (chemicals), galvanotaxis (electrical current), gravitaxis (gravity), hydrotaxis (moisture), magnetotaxis (magnetic field), phototaxis (light), rheotaxis (fluid flow), thermotaxis (temperature changes) and thigmotaxis (physical contact).
Depending on the type of sensory organs present, taxes can be classified as klinotaxes, where an organism continuously samples the environment to determine the direction of a stimulus, tropotaxes, where bilateral sense organs are used to determine the stimulus direction, and telotaxes, which are similar to tropotaxes but where a single organ suffices to establish the orientation movement.
Aerotaxis is the response of an organism to variation in oxygen concentration, and is mainly found in aerobic bacteria.
Chemotaxis is a migratory response that is 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.
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.
- 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. 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.
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 0.1C/cm. They apparently use this behavior to move to an optimal level in soil. 
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 .
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 behavior 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.
Logically, magnetotaxis is 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 "behavior" has nothing to do with sensation, and the bacteria are more accurately described as "magnetic bacteria".
Galvanotaxis or electrotaxis
Galvanotaxis or electrotaxis is directional movement of motile cells in response to an electric field. It has been suggested that by detecting and orientating 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 1) the existence of measurable electric fields that naturally occur during wound healing, development and regeneration; and 2) cells in cultures respond to applied electric fields by directional cell migration – electrotaxis / galvanotaxis.
Phonotaxis is the movement of an organism in response to sound.
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).
Terminology regarding direction of taxis
Based on classification of finding path the taxis may be classified into following 5 types:
Klinotaxis occur 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. Examples: movement of larva of blowfly and butterfly.
Tropotaxis are displayed by organisms with paired receptor cells. When the stimuli coming from a source is balanced equally the organisms show movement. In this animals are capable of showing sideways movement unlike klinotaxis where the organisms show movement in a straight line. Example: movement of Greyling butterfly, fish louse
Telotaxis require paired receptors. The movement occurs along the direction where the intensity of the stimuli is stronger. For example: when bees move from their hive for food they balance the stimuli from the sun as well as flower but reside on the flower whose intensity is higher for them.
Menotaxis In this constant angular orientation of the organisms takes place. Example: Bees returning to their hive at night, movement of ant with response to the sun.
Mnemotaxis are a complex type of stimuli. In this the organisms pick up the trails left by them when traveling back to their home. Thus this is a memory response of an organisms.
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