Taxis

A taxis (plural taxes, ) is an innate behavioural response by an organism to a directional stimulus. 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. It also differs from a kinesis, a non-directional change in activity in response to a stimulus that results in the illusion of directed motion due to different rates of activity depending on stimulus intensity.

Examples
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 prefices to specify the stimulus that elicits the response. These include anemotaxis (stimulation by wind), barotaxis (pressure), chemotaxis (chemicals), galvanotaxis (electrical current), geotaxis (gravity), hydrotaxis (moisture), 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.

Geotaxis
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 geotaxis (upward movement). Both positive and negative geotaxes are found in a variety of protozoans.

Phototaxis
Phototaxis is the movement of an organism in response to light. This is advantageous for phototrophic organisms as they can orientate themselves most efficiently to receive light for photosynthesis. 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 phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light.

Chemotaxis
Chemotaxis is a migratory response elicited by chemicals. Unicellular (e.g. protozoa) or multicellular (e.g. worms) organisms are targets of the 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. However, chemotaxis is described in prokaryotic and eukaryotic cells, signalling mechanisms (receptors, intracellular signaling) and effectors are significantly different.

Rheotaxis
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.

Magnetotaxis
Magnetotaxis is the ability of certain motile, aquatic bacteria to sense a magnetic field and coordinate their movement in response. It has been suggested that by orientating themselves toward the Earth's poles, marine bacteria are able to direct their movement downwards, towards the sediment. Furthermore, bacteria that are able to metabolise metal compounds may also use magnetosomes to detect deposits of ferrous compounds.

Galvanotaxis / electrotaxis
Galvanotaxis or electrotaxis is directional movement of motile cells in response to a 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.