Photokinesis is a change in the velocity of movement of an

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

as a result of changes in light intensity. The alteration in speed is independent of the direction from which the light is shining. Photokinesis is described as positive if the velocity of travel is greater with an increase in light intensity and negative if the velocity is slower. If a group of organisms with a positive photokinetic response is swimming in a partially shaded environment, there will be fewer organisms per unit of volume in the sunlit portion than in the shaded parts. This may be beneficial for the organisms if it is unfavourable to their

predators Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

, or it may be propitious to them in their quest for prey. In

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...

s, the mechanism for photokinesis appears to be an energetic process. In

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

, for example, an increase in illumination results in an increase of photophosphorylation which enables an increase in metabolic activity. However the behaviour is also found among

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

microorganisms, including those like ''

Astasia longa Astasis is a lack of motor coordination marked by an inability to stand, walk or even sit without assistance due to disruption of muscle coordination. The term ''astasia'' is interchangeable with ''astasis'' and is most commonly referred to as '' ...

'' which are not photosynthetic, and in these, the mechanism is not fully understood. In ''

Euglena gracilis ''Euglena gracilis'' is a freshwater species of euglenid, a microscopic type of algae, in the genus ''Euglena''. It has secondary chloroplasts, and is a mixotroph able to feed by photosynthesis or phagocytosis. It has a highly flexible cell surf ...

'', the rate of swimming has been shown to speed up with increased light intensity until the light reaches a certain saturation level, beyond which the swimming rate declines. The sea slug ''

Discodoris boholiensis ''Discodoris boholiensis'', known commonly as the Bohol discodoris,Le Bris, S., Müller, Y., Pean, M. in: DORIS, 25/10/2014 ''Discodoris boholiensis'' Bergh, 1877 Données d'Observations pour la Reconnaissance et l'Identification de la faune et l ...

'' also displays positive photokinesis; it is nocturnal and moves slowly at night, but much faster when caught in the open during daylight hours. Moving faster in the exposed environment should reduce predation and enable it to conceal itself as soon as possible, but its brain is quite incapable of working this out. Photokinesis is common in

tunicate Tunicates are marine invertebrates belonging to the subphylum Tunicata ( ). This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

larvae, which accumulate in areas with low light intensity just before settlement, and the behaviour is also present in juvenile fish such as

sockeye salmon The sockeye salmon (''Oncorhynchus nerka''), also called red salmon, kokanee salmon, blueback salmon, or simply sockeye, is an anadromous species of salmon found in the Northern Pacific Ocean and rivers discharging into it. This species is a ...

smolts.

References

{{Reflist Biology terminology

See also

References