Gravitaxis (or geotaxis[1]) is a form of taxis characterized by the directional movement of an organism in response to gravity.[2]
There are a few different causes for gravitaxis. 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.[3]
Gravitaxis is different from gravitropism in a way that the latter is more about the growth response of an organism to gravity.