phototactic

Phototaxis is directional movement in response to light, toward or away from a light source. It is a behavioral response observed in a wide range of organisms and is distinct from phototropism, the growth-oriented response to light.

Positive phototaxis describes movement toward light, while negative phototaxis describes movement away from light. The behavior can be elicited by gradients in light intensity or changes in light quality, and it may vary with the organism’s physiological state and environment.

In single-celled algae and other protists, such as Euglena gracilis and Chlamydomonas reinhardtii, light is detected by photoreceptors linked to a light-sensing structure on the cell surface, sometimes called an eyespot. The photoreception guides changes in locomotion, biasing swimming toward or away from light to optimize photosynthesis or avoid damage. Bacterial and other microbial phototaxis involve different photoreceptors that influence motility patterns. In many insects, particularly nocturnal species like moths, light is perceived as a cue that attracts behavior, although the strength and direction of phototactic responses can be modulated by context and prior experience.

Ecological and practical relevance includes locating light-rich habitats for photosynthesis, navigating toward or away from predators or damaging irradiation, and shaping interactions with artificial light sources. Phototaxis is also a standard subject in studies of sensory biology, navigation, and microbial ecology.

See also: photoreceptor, chemotaxis, photokinesis.