{{Short description|Directed movement of a cell or organism in response to gravity}} {{Use dmy dates|date=April 2022}} '''Gravitaxis''' (or ''geotaxis''<ref name="King Crab">{{cite journal|last1=Adams|first1=C. F.|last2=Paul|first2=A. J.|title=Phototaxis and geotaxis of light-adapted zoeae of the golden king crab ''Lithodes aequispinus'' (Anomura: Lithodidae) in the laboratory|journal=Journal of Crustacean Biology|date=February 1999|volume=19|issue=1|pages=106|doi=10.2307/1549552|jstor=1549552}}</ref>) is a form of taxis characterized by the directional movement of an organism in response to gravity.<ref>{{cite web|url=https://www.biology-online.org/dictionary/Gravitaxis|accessdate=25 March 2018|title=Gravitaxis - Biology-Online Dictionary}}</ref>
Organisms can perform gravitaxis through different mechanisms, either actively or passively. Many microorganisms have receptors like statocysts that allow them to sense the direction of gravity and to adjust their orientation accordingly. However, gravitaxis can result also from a purely physical mechanism so that organs for sensing the direction of gravity are not necessary. An example is given by microorganisms with a center of mass that is shifted to one end of the organism. Similar to a buoy, such mass-anisotropic microorganisms orient upwards under gravity. It has been shown that even an asymmetry in the shape of microorganisms can be sufficient to cause gravitaxis.<ref>{{Cite journal|last1=ten Hagen|first1=B.|last2=Kümmel|first2=F.|last3=Wittkowski|first3=R.|last4=Takagi|first4=D.|author5-link=Hartmut Löwen|last5=Löwen|first5=H.|last6=Bechinger|first6=C.|date=2014|title=Gravitaxis of asymmetric self-propelled colloidal particles|journal=Nature Communications|volume=5|issue=1|pages=4829|doi=10.1038/ncomms5829|pmid=25234416 |arxiv=1409.6882|s2cid=16768325 }}</ref>
Gravitaxis is different from gravitropism in a way that the latter is more about the growth response of an organism to gravity.
==Taxis== {{main|Taxis}} Taxis is a behavioral response of a cell or an organism to an external stimulus. The movement is characteristically directional. The movement may be positive or negative. A positive taxis is one in which the organism or a cell gravitates towards the source of stimulation (attraction). A negative taxis is when the organism or a cell moves away from the source of stimulation (repulsion).
==Examples== Many microorganisms show gravitaxis, including Euglena.<ref>{{cite book|last1=Häder|first1=DP|last2=Hemmersbach|first2=R|title=Euglena: Biochemistry, Cell and Molecular Biology |chapter=Gravitaxis in Euglena |series=Advances in Experimental Medicine and Biology |date=2017|volume=979|pages=237–266|doi=10.1007/978-3-319-54910-1_12|pmid=28429325|isbn=978-3-319-54908-8}}</ref> King crab planktonic larvae (''Lithodes aequispinus'') show negative gravitaxis, but positive phototaxis.<ref name="King Crab" /> Gravitaxis can also be observed in Drosophila.<ref>{{cite journal|last1=Armstrong|first1=JD|last2=Texada|first2=MJ|last3=Munjaal|first3=R|last4=Baker|first4=DA|last5=Beckingham|first5=KM|title=Gravitaxis in Drosophila melanogaster: a forward genetic screen.|journal=Genes, Brain and Behavior|date=April 2006|volume=5|issue=3|pages=222–39|doi=10.1111/j.1601-183X.2005.00154.x|pmid=16594976|doi-access=free}}</ref> The larval stages of a number of trematodes parasites also show gravitaxis. This gravitaxis helps them locate hosts in the water column. They can either show positive, or negative gravitaxis, depending on the type of host species.<ref name="Loy 2001">{{cite journal |last1=Loy |first1=C. |last2=Motzel |first2=W. |last3=Haas |first3=W. |title=Photo- and geo-orientation by echinostome cercariae results in habitat selection |journal=Journal of Parasitology |date=2001 |volume=87 |issue=3 |page=505-509 |doi=10.1645/0022-3395(2001)087[0505:PAGOBE]2.0.CO;2}}</ref> <ref name="Platt 2010">{{cite journal |last1=Platt |first1=T.R. |last2=Greenlee |first2=H. |last3=Zelmer |first3=D.A. |title=The interaction of light and gravity on the transmission of ''Echinostoma caproni'' (Digenea: Echinostomatidae) cercariae to the second intermediate host, ''Biomphalaria glabrata'' (Gastropoda: Pulmonata) |journal=Journal of Parasitology |date=2010 |volume=96 |issue=2 |page=325-328 |doi=10.1645/GE-2327.1}}</ref>
==Etymology== The term is coined from ''gravi-'' meaning gravity, and ''taxis'' or the movement of an organism in response to a stimulus.
== See also == * Animal locomotion * Haptotaxis * Mechanotaxis * Optomotor response * Tropism
==References== {{reflist}}
==External links== {{Wiktionary-inline}}
{{Taxes (Biology)}}
Category:Taxes (biology) Category:Perception