Phototaxis
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Phototaxis is a kind of
taxis A taxis (; : taxes ) is the motility, movement of an organism in response to a Stimulus (physiology), stimulus such as light or the presence of food. Taxes are innate behavioural responses. A taxis differs from a tropism (turning response, often ...
, or locomotory movement, that occurs when a whole
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
moves towards or away from a stimulus of
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. Phototaxis is called positive if the movement is in the direction of increasing light intensity and negative if the direction is opposite. Phototaxis has been described in microorganisms and algea, insects and other invertebrates, and vertebrates. Typically nocturnal insects can show positive phototaxis, while nocturnal mammals often show negative phototaxis.


Phototaxis in bacteria and archea

Phototaxis can be advantageous for phototrophic
bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
as they can orient themselves most efficiently to receive light for
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. 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
prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s (bacteria and archea). 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 A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
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 Chemotaxis (from ''chemical substance, chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell organism, single-cell or multicellular organisms direct thei ...
except that the attractant is light rather than a chemical. Phototactic responses are observed in a number of bacteria and archae, such as ''
Serratia marcescens ''Serratia marcescens'' () is a species of bacillus (shape), rod-shaped, Gram-negative bacteria in the family Yersiniaceae. It is a facultative anaerobe and an opportunistic pathogen in humans. It was discovered in 1819 by Bartolomeo Bizio in Pa ...
''. Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are bacteriorhodopsin and bacteriophytochromes in some bacteria. See also:
phytochrome Phytochromes are a class of photoreceptor proteins found in plants, bacteria and fungi. They respond to light in the red and far-red regions of the visible spectrum and can be classed as either Type I, which are activated by far-red light, or ...
and phototropism. Most prokaryotes (bacteria and archaea) are unable to sense the direction of light, because at such a small scale it is very difficult to make a detector that can distinguish a single light direction. Still, prokaryotes can measure light intensity and move in a light-intensity gradient. Some gliding filamentous prokaryotes can even sense light direction and make directed turns, but their phototactic movement is very slow. Some bacteria and archaea are phototactic. In most cases the mechanism of phototaxis is a biased random walk, analogous to bacterial chemotaxis.
Halophilic A halophile (from the Greek word for 'salt-loving') is an extremophile that thrives in high salt concentrations. In chemical terms, halophile refers to a Lewis acidic species that has some ability to extract halides from other chemical species. ...
archaea, such as ''
Halobacterium salinarum ''Halobacterium salinarum'', formerly known as ''Halobacterium cutirubrum'' or ''Halobacterium halobium'', is an extremely halophile, halophilic ocean, marine obligate aerobic archaeon. Despite its name, this is not a bacteria, bacterium, but a mem ...
'', use sensory
rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...
s (SRs) for phototaxis. Rhodopsins are 7 transmembrane proteins that bind
retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...
as a
chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...
. Light triggers the
isomerization In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomer ...
of retinal, which leads to phototransductory signalling via a two-component phosphotransfer relay system. ''Halobacterium salinarum'' has two SRs, SRI and SRII, which signal via the transducer proteins Htr1 and Htr2 (halobacterial transducers for SRs I and II), respectively. The downstream signalling in phototactic archaebacteria involves CheA, a histidine kinase, which phosphorylates the response regulator, CheY. Phosphorylated CheY induces swimming reversals. The two SRs in Halobacterium have different functions. SRI acts as an attractant receptor for orange light and, through a two-photon reaction, a repellent receptor for near-UV light, while SRII is a repellent receptor for blue light. Depending on which receptor is expressed, if a cell swims up or down a steep light gradient, the probability of flagellar switch will be low. If light intensity is constant or changes in the wrong direction, a switch in the direction of flagellar rotation will reorient the cell in a new, random direction. As the length of the tracks is longer when the cell follows a light gradient, cells will eventually get closer to or further away from the light source. This strategy does not allow orientation along the light vector and only works if a steep light gradient is present (i.e. not in open water). Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
Some cyanobacteria (e.g. '' Anabaena'', '' Synechocystis'') can slowly orient along a light vector. This orientation occurs in filaments or colonies, but only on surfaces and not in suspension. The filamentous cyanobacterium ''Synechocystis'' is capable of both positive and negative two-dimensional phototactic orientation. The positive response is probably mediated by a bacteriophytochrome photoreceptor, TaxD1. This protein has two chromophore-binding GAF domains, which bind
biliverdin Biliverdin (from the Latin for green bile) is a green tetrapyrrolic bile pigment, and is a product of heme catabolism.Boron W, Boulpaep E. Medical Physiology: a cellular and molecular approach, 2005. 984–986. Elsevier Saunders, United States. ...
chromophore, and a C-terminal domain typical for bacterial taxis receptors ( MCP signal domain). TaxD1 also has two N-terminal transmembrane segments that anchor the protein to the membrane. The photoreceptor and signalling domains are cytoplasmic and signal via a CheA/CheY-type signal transduction system to regulate motility by type IV pili. TaxD1 is localized at the poles of the rod-shaped cells of '' Synechococcus elongatus'', similarly to MCP containing chemosensory receptors in bacteria and archaea. How the steering of the filaments is achieved is not known. The slow steering of these cyanobacterial filaments is the only light-direction sensing behaviour prokaryotes could evolve owing to the difficulty in detecting light direction at this small scale. The ability to link light perception to control of motility is found in a very wide variety of prokaryotes, indicating that this ability must confer a range of physiological advantages. Most directly, the light environment is crucial to phototrophs as their energy source. Phototrophic prokaryotes are extraordinarily diverse, with a likely role for
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
in spreading phototrophy across multiple phyla. Thus, different groups of phototrophic prokaryotes may have little in common apart from their exploitation of light as an energy source, but it should be advantageous for any phototroph to be able to relocate in search of better light environments for photosynthesis. To do this efficiently requires the ability to control motility in response to integrated information on the intensity of light, the spectral quality of light and the physiological status of the cell. A second major reason for light-controlled motility is to avoid light at damaging intensities or wavelengths: this factor is not confined to photosynthetic bacteria since light (especially in the UV region) can be dangerous to all prokaryotes, primarily because of DNA and protein damage and inhibition of the translation machinery by light-generated reactive oxygen species. Finally, light signals potentially contain rich and complex information about the environment, and the possibility should not be excluded that bacteria make sophisticated use of this information to optimize their location and behavior. For example, plant or animal pathogens could use light information to control their location and interaction with their hosts, and in fact light signals are known to regulate development and virulence in several non-phototrophic prokaryotes. Phototrophs could also benefit from sophisticated information processing, since their optimal environment is defined by a complex combination of factors including light intensity, light quality, day and night cycles, the availability of raw materials and alternative energy sources, other beneficial or harmful physical and chemical factors and sometimes the presence of symbiotic partners. Light quality strongly influences specialized developmental pathways in certain filamentous cyanobacteria, including the development of motile hormogonia and nitrogen-fixing heterocysts. Since hormogonia are important for establishing symbiotic partnerships between cyanobacteria and plants, and heterocysts are essential for nitrogen fixation in those partnerships, it is tempting to speculate that the cyanobacteria may be using light signals as one way to detect the proximity of a plant symbiotic partner. Within a complex and heterogeneous environment such as a phototrophic biofilm, many factors crucial for growth could vary dramatically even within the limited region that a single motile cell could explore. We should therefore expect that prokaryotes living in such environments might control their motility in response to a complex signal transduction network linking a range of environmental cues. The photophobic response is a change in the direction of motility in response to a relatively sudden increase in illumination: classically, the response is to a temporal change in light intensity, which the bacterium may experience as it moves into a brightly illuminated region. The directional switch may consist of a random selection of a new direction (‘tumbling’) or it may be a simple reversal in the direction of motility. Either has the effect of repelling cells from a patch of unfavorable light. Photophobic responses have been observed in prokaryotes as diverse as ''
Escherichia coli ''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
'', purple photosynthetic bacteria and haloarchaea. The scotophobic (fear of darkness) response is the converse of the photophobic response described above: a change in direction (tumbling or reversal) is induced when the cell experiences a relatively sudden drop in light intensity. Photophobic and scotophobic responses both cause cells to accumulate in regions of specific (presumably favorable) light intensity and spectral quality. Scotophobic responses have been well documented in purple photosynthetic bacteria, starting with the classic observations of Engelmann in 1883, and in cyanobacteria. Scotophobic/photophobic responses in flagellated bacteria closely resemble the classic ‘biased random walk’ mode of bacterial chemotaxis, which links perception of temporal changes in the concentration of a chemical attractant or repellent to the frequency of tumbling. The only significant distinction is that the scotophobic/photophobic responses involve perception of temporal changes in light intensity rather than the concentration of a chemical. Photokinesis is a light-induced change in the speed (but not direction) of movement. Photokinesis may be negative (light-induced reduction of motility) or positive (light-induced stimulation of motility). Photokinesis can cause cells to accumulate in regions of favorable illumination: they linger in such regions or accelerate out of regions of unfavorable illumination. Photokinesis has been documented in cyanobacteria and purple photosynthetic bacteria. True phototaxis consists of directional movement which may be either towards a light source (positive phototaxis) or away from a light source (negative phototaxis). In contrast to the photophobic/scotophobic responses, true phototaxis is not a response to a temporal change in light intensity. Generally, it seems to involve direct sensing of the direction of illumination rather than a spatial gradient of light intensity. True phototaxis in prokaryotes is sometimes combined with social motility, which involves the concerted movement of an entire colony of cells towards or away from the light source. This phenomenon could also be described as community phototaxis. True phototaxis is widespread in eukaryotic
green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...
, but among the prokaryotes it has been documented only in cyanobacteria, and in social motility of colonies of the purple photosynthetic bacterium '' Rhodocista centenaria''.


Phototaxis in protists

Some
protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...
s (unicellular eukaryotes) can also move toward or away from light, by coupling their locomotion strategy with a light-sensing organ.
Eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s evolved for the first time in the history of life the ability to follow light direction in three dimensions in open water. The strategy of eukaryotic sensory integration, sensory processing and the speed and mechanics of tactic responses is fundamentally different from that found in prokaryotes. Both single-celled and multi-cellular eukaryotic phototactic organisms have a fixed shape, are polarized, swim in a spiral and use cilia for swimming and phototactic steering. Signalling can happen via direct light-triggered ion currents,
adenylyl cyclase Adenylate cyclase (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction: :A ...
s or trimeric G-proteins. The photoreceptors used can also be very different (see below). However, signalling in all cases eventually modifies the beating activity of cilia. The mechanics of phototactic orientation is analogous in all eukaryotes. A photosensor with a restricted view angle rotates to scan the space and signals periodically to the cilia to alter their beating, which will change the direction of the helical swimming trajectory. Three-dimensional phototaxis can be found in five out of the six eukaryotic major groups ( opisthokonts,
Amoebozoa Amoebozoa is a major Taxonomy (biology), taxonomic group containing about 2,400 described species of Amoeba, amoeboid protists, often possessing blunt, fingerlike, Pseudopod#Morphology, lobose pseudopods and tubular mitochondrial cristae. In trad ...
,
plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s, chromalveolates, excavates,
rhizaria The Rhizaria are a diverse and species-rich clade of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus '' Paulinella'' in the phylum Cercozoa, they are all non-photosynthetic, but many Foraminifera ...
). Pelagic phototaxis is present in
green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...
– it is not present in glaucophyte algae or
red algae Red algae, or Rhodophyta (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest Phylum, phyla of algae, containing over 7,000 recognized species within over 900 Genus, genera amidst ongoing taxon ...
. Green algae have a "stigma" located in the outermost portion of the
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
, directly underneath the two chloroplast membranes. The stigma is made of tens to several hundreds of
lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
globules, which often form hexagonal arrays and can be arranged in one or more rows. The lipid globules contain a complex mixture of
carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...
pigments, which provide the screening function and the orange-red colour, as well as proteins that stabilize the globules. The stigma is located laterally, in a fixed plane relative to the cilia, but not directly adjacent to the basal bodies. The fixed position is ensured by the attachment of the chloroplast to one of the ciliary roots. The pigmented stigma is not to be confused with the photoreceptor. The stigma only provides directional shading for the adjacent membrane-inserted photoreceptors (the term "eyespot" is therefore misleading). Stigmata can also reflect and focus light like a concave mirror, thereby enhancing sensitivity. In the best-studied green alga, '' Chlamydomonas reinhardtii'', phototaxis is mediated by a
rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...
pigment, as first demonstrated by the restoration of normal photobehaviour in a blind mutant by analogues of the retinal
chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...
. Two archaebacterial-type rhodopsins, channelrhodopsin-1 and -2, were identified as phototaxis receptors in ''Chlamydomonas''. Both proteins have an N-terminal 7-transmembrane portion, similar to archaebacterial rhodopsins, followed by an approximately 400 residue C-terminal membrane-associated portion. CSRA and CSRB act as light-gated cation channels and trigger depolarizing photocurrents. CSRA was shown to localize to the stigma region using immunofluorescence analysis (Suzuki et al. 2003). Individual RNAi depletion of both CSRA and CSRB modified the light-induced currents and revealed that CSRA mediates a fast, high-saturating current while CSRB a slow, low-saturating one. Both currents are able to trigger photophobic responses and can have a role in phototaxis, although the exact contribution of the two receptors is not yet clear. As in all bikonts (plants, chromalveolates, excavates, rhizaria), green algae have two cilia, which are not identical. The
anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
cilium is always younger than the posterior one. In every cell cycle, one daughter cell receives the anterior cilium and transforms it into a posterior one. The other daughter inherits the posterior, mature cilium. Both daughters then grow a new anterior cilium. As all other ciliary swimmers, green algae always swim in a spiral. The handedness of the spiral is robust and is guaranteed by the chirality of the cilia. The two cilia of green algae have different beat patterns and functions. In Chlamydomonas, the phototransduction cascade alters the stroke pattern and beating speed of the two cilia differentially in a complex pattern. This results in the reorientation of the helical swimming trajectory as long as the helical swimming axis is not aligned with the light vector.


Phototaxis in invertebrates


Jellyfish

Positive and negative phototaxis can be found in several species of
jellyfish Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...
such as those from the genus Polyorchis. Jellyfish use
ocelli A simple eye or ocellus (sometimes called a pigment pit) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most vertebrates. These eyes are called "simple" to distinguish the ...
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 Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...
relationship with
photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...
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 Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...
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.


Marine ragworm

Phototaxis has been well studied in the marine ragworm '' Platynereis dumerilii''. Both ''Platynereis dumerilii''
trochophore A trochophore () is a type of free-swimming planktonic marine larva with several bands of cilia. By moving their cilia rapidly, they make a water eddy to control their movement, and to bring their food closer in order to capture it more easily. ...
and its
metatrochophore A metatrochophore (;) is a type of larva developed from the trochophore larva of a polychaete annelid. The metatrochophore of a deep-sea hydrothermal vent vestimentiferan has a foregut and a midgut. The foregut cells have several microvilli, basa ...
larvae are positively phototactic. Phototaxis is mediated by simple eyespots that consists of a
pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...
cell and a
photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation ...
. The photoreceptor cell synapses directly onto
ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...
d 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'' larvae ( nectochaete) 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 lifestyle of the larva while scanning rotation is more suited for living in the open water column, the lifestyle of the trochophore larva. Phototaxis in the ''Platynereis dumerilii'' larva has a broad spectral range which is at least covered by three
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...
s 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 Gravitaxis (or ''geotaxis'') is a form of taxis characterized by the directional movement of an organism in response to gravity. There are a few different causes for gravitaxis. Many microorganisms have receptors like statocysts that allow them to ...
. 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 A depth gauge is an instrument for measuring depth (coordinate), depth below a vertical reference surface. They include depth gauges for underwater diving and similar applications. A diving depth gauge is a pressure gauge that displays the equ ...
, 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. In the diagram on the right, 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. File:Neuronal-connectome-of-a-sensory-motor-circuit-for-visual-navigation-elife02730v006.ogv, Phototaxis of '' Platynereis dumerilii'' 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. File:Neuronal-connectome-of-a-sensory-motor-circuit-for-visual-navigation-elife02730v007.ogv, ''Phototaxis of Platynereis dumerilii'' 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.


Insects

Positive phototaxis can be found in many flying
insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s such as
moth Moths are a group of insects that includes all members of the order Lepidoptera that are not Butterfly, butterflies. They were previously classified as suborder Heterocera, but the group is Paraphyly, paraphyletic with respect to butterflies (s ...
s,
grasshopper Grasshoppers are a group of insects belonging to the suborder Caelifera. They are amongst what are possibly the most ancient living groups of chewing herbivorous insects, dating back to the early Triassic around 250 million years ago. Grassh ...
s, and
flies Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwin ...
. ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'' 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. 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. 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. Negative phototaxis can be observed in larval ''drosophila melanogaster'' within the first three developmental
instar An instar (, from the Latin '' īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...
stages, despite adult insects 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
pupa A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
tion sites. '' Tenebrio molitor'' by comparison is one species which carries its negative phototaxis into adulthood.


Relation to magnetic fields

Under experimental conditions, organisms that use positive phototaxis have also shown a correlation with light and magnetic fields. Under
homogeneous Homogeneity and heterogeneity are concepts relating to the uniformity of a substance, process or image. A homogeneous feature is uniform in composition or character (i.e., color, shape, size, weight, height, distribution, texture, language, i ...
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. This suggests the larvae can observe a visible pattern in combination with light.


Light based vertical movement

A depth can also be selected based on
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
levels: The
brightness Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and ...
decreases with depth, but depends on the
weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...
(e.g. whether it is sunny or cloudy) and the time of the day. Also the color depends on the water depth and dissolved and suspended matter. The only consistent factor is that at a given place, deeper water is darker. In water, light attenuates differently for each
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
. The UV, violet (> 420 nm), and red (< 500 nm) wavelengths disappear before blue light (470 nm), which penetrates clear water the deepest. The wavelength composition is constant for each depth and is almost independent of time of the day and the
weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...
. To gauge depth, an animal would need two
photopigment Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore Moiety (chemistry), moiety of photosensitive chromoproteins, such as the pigments involved in ph ...
s sensitive to different wavelengths to compare different ranges of the spectrum. Such pigments may be expressed in different structures. Such different structures are found in the
polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...
'' Torrea candida''. Its eyes have a main and two accessory
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
e. The accessory retinae sense UV-light (''λ''max = 400 nm) and the main retina senses blue-green light (''λ''max = 560 nm). If the light sensed from all retinae is compared, the depth can be estimated, and so for ''Torrea candida'' such a ratio-chromatic depth gauge has been proposed. A ratio chromatic depth gauge has been found in larvae of the polychaete '' Platynereis dumerilii''. The larvae have two structures: The rhabdomeric
photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation ...
s of the eyes and in the deep
brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...
the ciliary photoreceptor cells. The ciliary photoreceptor cells express a ciliary
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...
, which is a photopigment maximally sensitive to UV-light (''λ''max = 383 nm). Thus, the ciliary photoreceptor cells react on UV-light and make the larvae swimming down gravitactically. The
gravitaxis Gravitaxis (or ''geotaxis'') is a form of taxis characterized by the directional movement of an organism in response to gravity. There are a few different causes for gravitaxis. Many microorganisms have receptors like statocysts that allow them to ...
here is countered by phototaxis, which makes the larvae swimming up to the light coming from the surface. Phototaxis is mediated by the rhabdomeric eyes. The eyes express at least three opsins (at least in the older larvae), and one of them is maximally sensitive to cyan light (''λ''max = 483 nm) so that the eyes cover a broad wavelength range with phototaxis. When phototaxis and gravitaxis have leveled out, the larvae have found their preferred depth.


See also

* Photokinesis * Phototropism (more relevant to plants and fungi)


References


Further reading

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