Photokinesis

{{Short description|Change in the velocity of movement of an organism as a result of changes in light intensity}}

{{About|the biological term|the fictional superhuman ability to manipulate light|:Category:Fictional characters who can manipulate light{{!}}Fictional characters who can manipulate light|the related psychic ability|Telekinesis}}

Photokinesis is a change in the velocity of movement of an organism as a result of changes in light intensity.{{cite book|author1=Häder, D.-P.|author2=Lebert, M.|title=Photomovement|url=https://books.google.com/books?id=2nevsljDiCYC&pg=PA305|year= 2001 |publisher=Elsevier |isbn=978-0-08-053886-0 |page=305}} 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.{{cite book|author1=Häder, Donat-Peter |author2=Hemmersbach, Ruth |author3=Lebert, Michael |title=Gravity and the Behavior of Unicellular Organisms |url=https://books.google.com/books?id=EQvzwpAWEy0C&pg=PA115 |year=2005 |publisher=Cambridge University Press|isbn=978-0-521-82052-3 |page=115}} 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.{{cite book|author=Smith, Kendric C. |title=The Science of Photobiology |url=https://books.google.com/books?id=EGfSBwAAQBAJ&pg=PA308 |year=2013 |publisher=Springer Science & Business Media|isbn=978-1-4615-8061-4 |pages=308–309}} This may be beneficial for the organisms if it is unfavourable to their predators, or it may be propitious to them in their quest for prey.{{cite book|author1=Häder, D.-P.|author2=Lebert, M.|title=Photomovement |url=https://books.google.com/books?id=2nevsljDiCYC&pg=PA477 |year=2001 |publisher=Elsevier |isbn=978-0-08-053886-0 |page=477}}

In photosynthetic prokaryotes, the mechanism for photokinesis appears to be an energetic process. In cyanobacteria, 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 microorganisms, including those like Astasia longa which are not photosynthetic, and in these, the mechanism is not fully understood. In Euglena gracilis, 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.{{cite journal |author=Wolken, J.J. |author2=Shin, E. |year=1958 |title=Photomotion in Euglena gracilis * I. Photokinesis II. Phototaxis |journal=The Journal of Protozoology |volume=5 |issue=1 |pages=39–46 |doi=10.1111/j.1550-7408.1958.tb02525.x }}

The sea slug Discodoris boholiensis 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.{{cite web |url=http://www.seaslugforum.net/find/7223 |title=Discodoris boholiensis from Malaysia |author=Rudman, Bill |date=8 July 2002 |work=Sea Slug Forum |publisher=Australian Museum, Sydney |accessdate=12 July 2020 }} Photokinesis is common in tunicate larvae, which accumulate in areas with low light intensity just before settlement,{{cite book|author=Barnes, Harold |title=Oceanography and Marine Biology|url=https://books.google.com/books?id=CH_tJjVK94QC&pg=PA42|year=1989 |publisher=CRC Press |isbn=978-0-08-037718-6 |page=42}} and the behaviour is also present in juvenile fish such as sockeye salmon smolts.{{cite book|title=Advances in Marine Biology|url=https://books.google.com/books?id=EQl_EopIp7UC&pg=PA174|year=1962|publisher=Academic Press|isbn=978-0-08-057924-5|page=174}}