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The paramecium does this by reversing the direction in which its cilia beat. This results in stopping, spinning or turning, after which point the paramecium resumes swimming forward. If multiple avoidance reactions follow one another, it is possible for a paramecium to swim backward, though not as smoothly as swimming forward. [1]
Paramecium (/ ˌ p ær ə ˈ m iː s (i) ə m / PARR-ə-MEE-s(ee-)əm, /-s i ə m /-see-əm, plural "paramecia" only when used as a vernacular name) [2] is a genus of eukaryotic, unicellular ciliates, widespread in freshwater, brackish, and marine environments. Paramecia are often abundant in stagnant basins and ponds.
Those responses are adaptive, on a timescale matched to the rotational period of the cell body, [44] [45] [46] and allow cells to scan the environment and swim toward light. [47] Multicellular Volvox shows a higher level of complexity, with differentiation between interior germ cells and somatic cells dedicated to propulsion.
A related response ("photoshock" or photophobic response) occurs when cells are briefly exposed to high light intensity, causing the cell to stop, briefly swim backwards, then change swimming direction. Eyespot-mediated light perception helps the cells in finding an environment with optimal light conditions for photosynthesis.
Paramecium, specifically P. caudatum, is capable of learning to associate intense light with stimulus such as electric shocks in its swimming medium; although it appears to be unable to associate darkness with electric shocks. [15] Protozoan ciliate Tetrahymena has the capacity to 'memorize' the geometry of its swimming area. Cells that were ...
Pharotaxis is the movement to a specific location in response to learned or conditioned stimuli, or navigation by means of landmarks. [20] [21] Phonotaxis is the movement of an organism in response to sound. Phototaxis is the movement of an organism in response to light: that is, the response to variation in light intensity and direction.
Middle: photokinesis involving changes in speed induced by changing light intensity. In patchy light environments, positive photokinesis results in accumulation in low light areas (and vice versa for negative photokinesis). Bottom: true phototaxis results in movement towards or away from a light source, but is not a response to a light gradient.
The best-understood contractile vacuoles belong to the protists Paramecium, Amoeba, Dictyostelium and Trypanosoma, and to a lesser extent the green alga Chlamydomonas. Not all species that possess a contractile vacuole are freshwater organisms ; some marine , soil microorganisms and parasites also have a contractile vacuole.