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Moths are positively phototactic

Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light.[1] This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis. Phototaxis is called positive if the movement is in the direction of increasing light intensity and negative if the direction is opposite.[2]

Two types of positive phototaxis are observed in prokaryotes. The first is called scotophobotaxis (from the word "scotophobia"), which is observed only under a microscope. This occurs when a bacterium swims by chance out of the area illuminated by the microscope. Entering darkness signals the cell to reverse flagella rotation direction and reenter the light. The second type of phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. This is analogous to positive chemotaxis except that the attractant is light rather than a chemical.

Phototactic responses are observed in many organisms such as Serratia marcescens, Tetrahymena, and Euglena. Each organism has its own specific biological cause for a phototactic response, many of which are incidental and serve no end purpose.

Phototaxis in zooplankton[edit]

Phototaxis and UV-avoidance of Platynereis dumerilii nectochaete larvae responding to UV-light (380 nm) from above: The larvae start swimming upwards when UV-light switched on (marked by the violet square). But later, they are swimming downward. The larval tracks are color coded: Red for upward and blue for downward swimming larvae. The video runs at double speed.[3]
Phototaxis of Platynereis dumerilii nectochaete larvae: Some larvae show positive phototaxis by swimming towards the light. Other larvae show negative phototaxis by swimming away from the light. First, the light comes from left and then from the right side. When the light direction is switched the larvae turn. The side where the light is coming from is indicated by a white bar. The larvae display mixed phototaxis, some negatively phototactic larvae are tracked. The scale bar represents 2 mm.[4]
Phototaxis of Platynereis dumerilii nectochaete larvae: The larvae turn when the light coming from the left is switched on. While the larvae turn they bend their body with their longitudinal muscles. The larvae show two dots on the head, which are the shading pigment of their adult cup eyes that mediate phototaxis. The direction where the light is coming from is indicated by white bars.[4]

Phototaxis in zooplankton is well studied in the marine annelid Platynereis dumerilii:

Platynereis dumerilii trochophore and metatrochophore larvae are positively phototactic. Phototaxis there is mediated by simple eyespots that consists of a pigment cell and a photoreceptor cell. The photoreceptor cell synapses directly onto ciliated cells, which are used for swimming. The eyespots do not give spatial resolution, therefore the larvae are rotating to scan their environment for the direction where the light is coming from.[5]

Platynereis dumerilii nectochaete larvae can switch between positive and negative phototaxis. Phototaxis there is mediated by two pairs of more complex pigment cup eyes. These eyes contain more photoreceptor cells that are shaded by pigment cells forming a cup. The photoreceptor cells do not synapse directly onto ciliated cells or muscle cells but onto inter-neurons of a processing center. This way the information of all four eye cups can be compared and a low resolution image of four pixels can be created telling the larvae where the light is coming from. This way the larva does not need to scan its environment by rotating.[4] This is an adaption for living on the bottom of the sea the life style of the nectochaete larva while scanning rotation is more suited for living in the open water column, the life style of the trochophore larva. Phototaxis in the Platynereis dumerilii nectochaete larva has a broad spectral range which is at least covered by three opsins that are expressed by the cup eyes:[6] Two rhabdomeric opsins[7] and a Go-opsin.[6]

However, not every behavior that looks like phototaxis is phototaxis: Platynereis dumerilii nechtochate and metatrochophore larvae swim up first when they are stimulated with UV-light from above. But after a while, they change the direction and avoid the UV-light by swimming down. This looks like a change from positive to negative phototaxis (see video left), but the larvae also swim down if UV-light comes non-directionally from the side. And so they do not swim to or away from the light, but swim down,[3] this means to the center of gravity. Thus this is a UV-induced positive gravitaxis. Positive phototaxis (swimming to the light from the surface) and positive gravitaxis (swimming to the center of gravity) are induced by different ranges of wavelengths and cancel out each other at a certain ratio of wavelengths.[3] Since the wavelengths compositions change in water with depth: Short (UV, violet) and long (red) wavelengths are lost first,[6] phototaxis and gravitaxis form a ratio-chromatic depth gauge, which allows the larvae to determine their depth by the color of the surrounding water. This has the advantage over a brightness based depth gauge that the color stays almost constant independent of the time of the day or whether it is cloudy.[8][9]

Phototaxis in jellyfish[edit]

Positive and negative phototaxis can be found in several species of jellyfish such as those from the genus Polyorchis. Jellyfish use ocelli to detect the presence and absence of light, which is then translated into anti-predatory behaviour in the case of a shadow being cast over the ocelli, or feeding behaviour in the case of the presence of light.[10] Many tropical jellyfish have a symbiotic relationship with photosynthetic zooxanthellae that they harbor within their cells.[11] The zooxanthellae nourish the jellyfish, while the jellyfish protects them, and moves them toward light sources such as the sun to maximize their light-exposure for efficient photosynthesis. In a shadow, the jellyfish can either remain still, or quickly move away in bursts to avoid predation and also re-adjust toward a new light source.[12]

This motor response to light and absence of light is facilitated by a chemical response from the ocelli, which results in a motor response causing the organism to swim toward a light source.[12]

Phototaxis in insects[edit]

Positive phototaxis can be found in many flying insects such as moths, grasshoppers, and flies. Drosophila melanogaster has been studied extensively for its innate positive phototactic response to light sources, using controlled experiments to help understand the connection between airborne locomotion toward a light source.[13] This innate response is common among insects that fly primarily during the night utilizing transverse orientation vis-à-vis the light of the moon for orientation.[14] Artificial lighting in cities and populated areas results in a more pronounced positive response compared to that with the distant light of the moon, resulting in the organism repeatedly responding to this new supernormal stimulus and innately flying toward it.

Evidence for the innate response of positive phototaxis in Drosophila melanogaster was carried out by altering the wings of several individual specimens, both physically (via removal) and genetically (via mutation). In both cases there was a noticeable lack of positive phototaxis, demonstrating that flying toward light sources is an innate response to the organisms' photoreceptors receiving a positive response.[13]

Negative phototaxis can be observed in larval drosophila melanogaster within the first three developmental instar stages, despite adult insects displaying positive phototaxis.[15] This behaviour is common among other species of insects which possess a flightless larval and adult stage in their life cycles, only switching to positive phototaxis when searching for pupation sites. Tenebrio molitor by comparison is one species which carries its negative phototaxis into adulthood.[15]

Relation to magnetic fields[edit]

Under experimental conditions organisms that use positive phototaxis have also shown correlation with light and magnetic fields. Under homogeneous light conditions with a shifting magnetic field, Drosophila melanogaster larvae reorient themselves toward predicted directions of greater or lesser light intensities as expected by a rotating magnetic field. In complete darkness, the larvae orient randomly without any notable preference.[15] This suggests, the larvae can observe a visible pattern in combination with light.

See also[edit]

Further reading[edit]

  • Madigan, Michael T.; Martinko, John M. (2006). Brock Biology of Microorganisms (11th ed.). Upper Saddle River, N.J.: Pearson/Prentice Hall. ISBN 978-0131443297.
  • Jékely, G. (31 August 2009). "Evolution of phototaxis". Philosophical Transactions of the Royal Society B: Biological Sciences. 364 (1531): 2795–2808. doi:10.1098/rstb.2009.0072. PMC 2781859. PMID 19720645.
  • Randel, Nadine; Jékely, Gáspár (23 November 2015). "Phototaxis and the origin of visual eyes". Philosophical Transactions of the Royal Society B: Biological Sciences. 371 (1685): 20150042. doi:10.1098/rstb.2015.0042. PMC 4685581. PMID 26598725.


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  2. ^ Menzel, Randolf (1979), "Spectral Sensitivity and Color Vision in Invertebrates", in H. Autrum (ed.), Comparative Physiology and Evolution of Vision in Invertebrates- A: Invertebrate Photoreceptors, Handbook of Sensory Physiology, VII/6A, New York: Springer-Verlag, pp. 503–580. See section D: Wavelength–Specific Behavior and Color Vision, ISBN 978-3-540-08837-0
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  6. ^ a b c Gühmann, Martin; Jia, Huiyong; Randel, Nadine; Verasztó, Csaba; Bezares-Calderón, Luis A.; Michiels, Nico K.; Yokoyama, Shozo; Jékely, Gáspár (August 2015). "Spectral Tuning of Phototaxis by a Go-Opsin in the Rhabdomeric Eyes of Platynereis". Current Biology. 25 (17): 2265–2271. doi:10.1016/j.cub.2015.07.017. PMID 26255845.
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  10. ^ Katsuki, Takeo; Greenspan, Ralph J. (July 2013). "Jellyfish nervous systems". Current Biology. 23 (14): R592–R594. doi:10.1016/j.cub.2013.03.057. ISSN 0960-9822. PMID 23885868.
  11. ^ E., Ruppert, Edward (2004). Invertebrate zoology : a functional evolutionary approach. Barnes, Robert D.,, Fox, Richard S. (Seventh ed.). Delhi, India. ISBN 9788131501047. OCLC 970002268.
  12. ^ a b Anderson, P.; Mackie, G. (1977-07-08). "Electrically coupled, photosensitive neurons control swimming in a jellyfish". Science. 197 (4299): 186–188. doi:10.1126/science.17918. ISSN 0036-8075.
  13. ^ a b Gorostiza, E. Axel; Colomb, Julien; Brembs, Bjoern (2015-08-03). "A decision underlies phototaxis in an insect". doi:10.1101/023846. PMC 5204122. Cite journal requires |journal= (help)
  14. ^ Reynolds, Andy M.; Reynolds, Don R.; Sane, Sanjay P.; Hu, Gao; Chapman, Jason W. (2016-08-15). "Orientation in high-flying migrant insects in relation to flows: mechanisms and strategies". Philosophical Transactions of the Royal Society B: Biological Sciences. 371 (1704): 20150392. doi:10.1098/rstb.2015.0392. ISSN 0962-8436. PMC 4992716. PMID 27528782.
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