Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from stimulus of light. 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.
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
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.
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. 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: Two rhabdomeric opsins and a Go-opsin.
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, 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. Since the wavelengths compositions change in water with depth: Short (UV, violet) and long (red) wavelengths are lost first, 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.
Phototaxis in jellyfish
Positive and negative phototaxis can be found in several species of jellyfish such as species from the polyorchis genus. 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. Many tropical jellyfish have a symbiotic relationship with photosynthetic zooxanthellae that they harbor within their cells. 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.
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.
Phototaxis in insects
Positive phototaxis can be found in many insects that can fly such as moths, grasshoppers, and flies. Drosophila melanogaster has been studied extensively on its innate positive phototactic response to light sources using controlled experiments to help understand the connection between airborne locomotion toward a light source. This innate response is common among insects that fly primarily during the night utilizing transverse orientation with the light of the moon to fly. Artificial lights in cities and populated areas result in a more bright and positive response compared to 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 individuals both physically via removal, or genetically via mutation. In all cases the organisms with wings removed and wings mutated there was a noticeable lack of positive phototaxis demonstrating flying toward light is an innate response to the organisms photoreceptors receiving a positive response.
Negative phototaxis can be observed in larval drosophila melanogaster within the first three developmental instars despite the adult insect displaying positive phototaxis. 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 is one such species which carries its negative phototaxis into adulthood.
Relation to magnetic fields
Under experimental conditions organisms that utilize positive phototactic behaviour have also shown correlation with light and magnetic fields. Under homogeneous light conditions with a shifting magnetic field drosophila melanogaster larva reorient themselves toward predicted directions of greater or lesser light intensities as expected by a rotating magnetic field. In complete darkness the same organism orients randomly without any notable preference. This suggests a visible pattern can be observed by the organism in combination with light.
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