thumb|A moth exhibiting [[phototaxis, moving towards a source of light]]

A taxis (; : taxes ) 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. Taxis can be positive (moving towards the stimulus) or negative (moving away from the stimulus).

Classification

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 are positive, while if it moves away the taxis are negative. For example, flagellate protozoans of the genus Euglena move towards a light source. This reaction or behavior is called positive phototaxis since phototaxis refers to a response to light and the organism is moving towards the stimulus.

Terminology derived from type of stimulus

Many types of taxis have been identified, including:

  • Aerotaxis (stimulation by oxygen)
  • Anemotaxis (by wind)
  • Durotaxis (by stiffness)
  • Electrotaxis or galvanotaxis (by electric current)
  • Gravitaxis or geotaxis (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 the stimulus.

Terminology derived from taxis direction

There are five types of taxes based on the movement of organisms.

  • Klinotaxis occurs in organisms with receptor cells but not paired receptor organs. The cells for reception may be located all over the body, but often towards the anterior side. The organism detects the stimuli by turning its head sideways and comparing the intensity of the stimulus. Their direction of movement is then based on the stronger stimulus, either moving toward a desirable stimulus or away from an undesired one.
  • 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 or pheromones. Rats have specialized supra-orbital whiskers that detect wind and cause anemotactic turning.
  • 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.
  • Gravitaxis (known historically as geotaxis) is the directional movement (along the vector of gravity) to the center of gravity. The planktonic larvae of a king crab, Lithodes aequispinus, combine positive phototaxis (movement towards the light) and negative gravitaxis (upward movement). Also the larvae of a polychaete, Platynereis dumerilii, combine positive phototaxis (movement to the light coming from the water surface) and UV-induced positive gravitaxis (downward movement) to form a ratio-chromatic depth-gauge. 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 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. 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 use thigmotactic sense to locate fruits to feed on. Mice and rats, when inhabiting human-made structures, tend to stick close to vertical surfaces; this primarily manifests as running along the floor/wall juncture. Whiskers (vibrissae) are often used to detect the presence of a wall or surface in the absence of sufficient light in rodents and felines to aid in thigmotaxis.

See also

;Biology

  • Animal locomotion
  • Haptotaxis
  • Mechanotaxis
  • Optomotor response
  • Tropism

;Different, wider context

  • Taxonomy, science of categorisation or classification

References