Photokinesis

Definition

A non-directional change in locomotor speed in response to changes in light intensity, independent of the light's direction. Positive photokinesis denotes increased velocity with greater illumination; negative photokinesis denotes decreased velocity. Unlike phototaxis, which involves oriented movement toward or away from a light source, photokinesis alters only the rate of undirected or randomly directed movement, potentially producing aggregation in shaded or illuminated microhabitats through differential speed rather than steering.

Etymology

Greek phōs, phōtos (light) + kīnēsis (movement)

Example

Aquatic midge larvae (Chironomidae) often exhibit negative photokinesis, slowing their swimming in bright surface light and thereby accumulating in darker benthic refuges where predatory fish are less active.

Related Terms

• Phototaxis
• klinokinesis
• orthokinesis
• photophobic response
• Rheotaxis
• thigmokinesis

Usage Notes

Distinguished from phototaxis by the absence of directional steering; the organism does not 'know' where the light is coming from. Photokinetic responses are measured as speed changes, not turning angles. In field studies, positive photokinesis can create apparent 'avoidance' of light patches simply because organisms pass through them faster, not because they turn away. The term is sometimes loosely applied to any light-modulated activity level, but strict usage reserves it for locomotor velocity effects.