Phototaxis

Abstract

Phototaxis is a type of taxis response: directed movement steered by the direction of a light gradient, toward the source in positive phototaxis or away from it in negative phototaxis, in contrast to the undirected kinesis. This article covers how an eyespot and a photoreceptor recover the direction of a light field—a rotating cell sampling the light once per body turn, a larva comparing ciliary and rhabdomeric channels—along with the sign of the response, the connectome of a visual circuit, and the behavioural choice that even an insect phototaxis conceals. The contested boundary is where a biased, rotation-gated turn shades from a reflex into a genuine decision. A worked example and three interactive demonstrations accompany the text.

Keywords: phototaxis, positive phototaxis, eyespot, photoreceptor, taxis

A single green alga spirals toward a window; a marine larva rises each dawn toward the lit surface and sinks again at dusk; a fly walks steadily toward a lamp until something it values makes it turn back. Each is performing a phototaxis: converting the geometry of a light field into a heading. The behavior is the luminous case of a directed taxis, and because light can be delivered as a controlled gradient and the swimmer or walker tracked body by body, phototaxis has become one of the reference examples against which accounts of simple directed movement are tested.

This article treats phototaxis as a directed orientation response in the sense fixed by Gottfried Fraenkel and Donald Gunn: a taxis uses the direction of a stimulus to steer, whereas a kinesis uses only its intensity to set a rate (Fraenkel & Gunn, 1961). The questions that follow are mechanical. How does a cell with a single, body-shaded photoreceptor recover the direction of the light at all? How is the sign of the response set, so the same cell approaches a dim light and flees a bright one? And where, in a walking insect, does an apparently reflexive march toward a lamp turn out to hide a decision?

What Phototaxis Is

Phototaxis is a locomotor response that is directed with respect to light: the cell or organism orients its movement along a bearing fixed by the direction from which the light arrives. In MeSH it is classified beneath taxis response, and it is the luminous instance of the general taxis idea. Its defining contrast is with kinesis. A photokinesis discards directional information and registers only how bright the light is, letting that reading govern a rate of movement or turning. A phototaxis does the opposite: it recovers the direction in which light is strongest and steers by it, so that approach to a favorable light, or escape from a harmful one, is aimed rather than statistical (Fraenkel & Gunn, 1961).

Two qualifiers complete the definition. Phototaxis has a sign: a positive phototaxis carries the organism toward the source, and a negative phototaxis carries it away; the same cell may switch sign as light crosses from a tolerable to a damaging intensity. And phototaxis is one modality among several. Movement steered by a chemical is chemotaxis, by gravity is geotaxis, by a current is rheotaxis; phototaxis is the case in which the steering cue is light. Because the response is aimed, it carries the organism to its preferred light far more efficiently than an undirected alternative, which is the first thing a comparison with kinesis makes plain (Jékely, 2009).

Positive and Negative Phototaxis

The sign of a phototaxis is set not by the light but by how the cell reads it. In a positive phototaxis rising brightness marks the favorable direction, so the cell steers up the gradient toward the source; in a negative phototaxis rising brightness marks a direction to avoid, so the cell steers down it. The machinery that produces the two signs is the same—the cell still extracts the light direction and still biases its movement—but the valence attached to increasing intensity is reversed, so the identical sensing apparatus drives approach in one case and retreat in the other.

The sign is a property of a cell at a particular intensity, and it can invert. In Chlamydomonas reinhardtii the eyespot-mediated response flips from positive to negative as light rises past a threshold: a cell swims toward a dim light and away from a bright one, settling where the intensity suits it rather than at the brightest point. Noriko Ueki and colleagues showed that this switch is eyespot-dependent—the sign is set by how the eyespot shades the photoreceptor as the cell rotates, not by a separate decision center (Ueki et al., 2016). This intensity-dependent sign is why a phototaxis is not a fixed label on an organism but a reading the cell makes at the light it currently meets, and it lets a free-swimming population spread itself along a light gradient to its preferred band.

The Eyespot and the Photoreceptor

The problem a rotating single cell faces is acute: it is far too small to compare brightness across its own body at one instant, so it cannot read the light direction in space. It solves the problem temporally, and the solution is a geometric trick. The green alga carries an eyespot, a stack of carotenoid-filled granules that acts as a reflector and a shade, placed beside a membrane patch of light-gated photoreceptors. As the cell swims it rotates about its long axis, so the eyespot alternately exposes and shades the photoreceptor once per rotation. The photoreceptor therefore reads a signal that is modulated in time at the rotation frequency, and the depth of that modulation encodes how far the swimming axis points off the light (Foster & Smyth, 1980; Kreimer, 2009).

The photoreceptor itself is a channelrhodopsin: a light-gated ion channel that opens directly when its retinal chromophore absorbs a photon, depolarizing the cell and modulating the beat of its two flagella in step with the rotation. Georg Nagel, Peter Hegemann, and colleagues identified channelrhodopsin-2 as a directly light-gated cation channel, the molecule that converts the eyespot signal into a membrane current (Nagel et al., 2003). The coupling is phased so that the flagella beat asymmetrically on the side that will turn the cell toward (or, above threshold, away from) the light, and integrating that asymmetry over a rotation produces a net turn. Kenneth Foster and R. D. Smyth had earlier argued, against the prevailing view, that the algal photoreceptor must be a rhodopsin, a prediction the channelrhodopsin work confirmed (Foster & Smyth, 1980). The same channelrhodopsin, expressed in neurons, became the founding tool of optogenetics—a reminder that the molecule evolved to let an alga find the light.

Helical Klinotaxis in Zooplankton

The rotating-cell trick scales up to small swimming animals, and the marine ragworm larva Platynereis dumerilii is where it has been dissected most completely. The larva swims in a helix, driven by a ciliary band, and carries a simple eye of one photoreceptor and one shading pigment cell. Gáspár Jékely and colleagues showed that as the larva rotates, its pigment cell shades the photoreceptor once per turn, and the photoreceptor modulates the beating of the neighboring cilia; the result is that the helix bends toward the light whenever the swimming axis is off-axis to it (Jékely et al., 2008). This is a klinotaxis: the animal does not compare two sensors at one instant, as a two-eyed tropotaxis would, but uses a single sensor sampled across the rotation to recover the light direction over time.

The elegance of the mechanism is that the turning rate is tied directly to the geometry. When the larva already points at the light, the photoreceptor is modulated weakly over a rotation and the net turn is near zero; when the larva swims across the light, the modulation is deepest and the turn is strongest. The heading therefore relaxes toward the light along a smooth curve rather than by a discrete correction—the behavior the Worked Example makes quantitative, and the KlinotaxisDemo and PhotoreceptorTurningDemo both plot. Because the whole circuit is a handful of cells, Platynereis phototaxis became a tractable model of how a sensory signal is converted into steered locomotion with the least possible machinery (Jékely, 2009).

Ciliary and Rhabdomeric Photoreceptors

Animal photoreceptors come in two great families, distinguished by how they expand their membrane to hold opsin: rhabdomeric receptors, which fold their membrane into microvilli and dominate invertebrate eyes, and ciliary receptors, which elaborate a cilium and build the rods and cones of the vertebrate retina. Detlev Arendt and colleagues found ciliary photoreceptors carrying a vertebrate-type opsin in the brain of Platynereis, evidence that both receptor types were already present in the last common ancestor of the two lineages and were later partitioned between brain and eye (Arendt et al., 2004). The larva thus runs two photoreceptor systems at once, and they do different jobs.

Csaba Verasztó, Jékely, and colleagues worked out what the second system is for. The larval eye-spot and its rhabdomeric receptor drive the fast directional phototaxis described above; the brain's ciliary receptors instead form a spectral depth gauge, reading the ratio of wavelengths that penetrate seawater to different depths and driving slow vertical swimming that positions the larva at its preferred depth (Verasztó et al., 2018). Nicolas Randel and colleagues mapped the full synaptic connectome of the visual circuit from photoreceptor to ciliated effector, showing that the sensory-motor loop for visual navigation is short and almost entirely feedforward (Randel et al., 2014). Between them these studies turn phototaxis from a behavior into a wiring diagram: a small, countable set of cells that converts a modulated light signal into a steered course.

Phototaxis as a Decision

A steady walk toward a lamp looks like the most reflexive behavior imaginable, yet even here the reflex framing breaks down. Esteban Gorostiza, Julien Colomb, and Björn Brembs tracked individual Drosophila choosing between light and dark and showed that phototaxis in the fly is not a fixed stimulus-response reflex but the output of a decision: the same fly weighs the light against its internal state and its recent experience, and its choice to approach or avoid is biased by both (Gorostiza et al., 2016). The behavior that textbooks used as the type specimen of a hardwired taxis turns out to carry a graded, modifiable choice inside it.

This matters beyond insects. Prokaryotes face the same directional problem a swimming alga does, and Annegret Wilde and Conrad Mullineaux reviewed how cyanobacteria and other bacteria solve it—some by acting as microscopic lenses that focus the image of the source onto the far side of the cell, recovering a direction from a body far too small to sense a gradient across (Wilde & Mullineaux, 2017). In the zebrafish larva, Harold Burgess and Michael Granato found that light does not simply attract or repel but modulates ongoing locomotion: a sudden change in illumination reshapes the larva's whole pattern of swimming and turning as it adapts, so the phototactic response is embedded in a continuously adjusting motor program rather than triggered as a discrete reflex (Burgess & Granato, 2007). Across these cases the lesson is the same: a phototaxis is rarely the simple reflex it appears, and much of the interest lies in the decision or the adaptation hidden inside it.

Worked Example

Klinotaxis makes a sharp, checkable promise: a rotating swimmer with a single shaded photoreceptor turns toward the light at a rate fixed by one angle, and that rate can be computed from the geometry alone.

Let θ be the angle between the swimming axis and the direction of the light. As the body rotates, the photoreceptor is shaded for part of each turn, and the depth of the light/shade modulation it reads over one rotation is proportional to sin θ: the modulation is deepest when the swimmer crosses the beam (θ = 90°) and vanishes when it points straight at or away from the source (θ = 0° or 180°). The receptor couples that modulation to the cilia so the heading turns toward the light at a rate turn(θ) = k sin θ, where k is the maximal steering gain.

Take the steering gain k = 30 deg/s. When the swimmer crosses the beam at θ = 90°, the turn rate is k sin 90° = 30 × 1 = 30 deg/s—the maximum. When it is already nearly aligned at θ = 30°, the turn rate is k sin 30° = 30 × 0.5 = 15 deg/s, exactly half. The ratio of the two is 30 ÷ 15 = 2.0, so a swimmer crossing the beam turns twice as fast toward the light as one already half-aligned to it.

Because the turn rate is zero at θ = 0° and positive for every heading between 0° and 180°, the light direction is a stable fixed point: any heading relaxes toward it. Integrating dθ∕dt = −k sin θ gives the time to swing from one angle to another in closed form, and a swimmer at k = 30 deg/s takes about 6.0 s to turn from crossing the beam (90°) to within 5° of the light. The KlinotaxisDemo lets the reader set θ and k and watch the turn rate trace the k sin θ curve, and the PhotoreceptorTurningDemo shows the same law emerging from the modulated receptor signal, so the prose, the table, and both demos agree by construction.

Discussion

Phototaxis occupies a specific place in the logic of orientation: it is the directed case in which a single-sensor animal, unable to compare two eyes, still recovers a direction—by spinning its one receptor through the light and reading the modulation. Fraenkel and Gunn's taxonomy was valuable precisely because it forced a mechanistic question—does this response use the direction of the light, or only how bright it is?—and in phototaxis that question has concrete molecular and anatomical answers, from the eyespot and its channelrhodopsin to the mapped connectome of a larval visual circuit (Fraenkel & Gunn, 1961; Nagel et al., 2003; Randel et al., 2014). The answer is rarely all-or-nothing. Real organisms blend a directed phototaxis with whatever undirected photokinesis is cheaper, and much of the research is the work of separating the two in a given animal.

The enduring interest of phototaxis is how little machinery a genuine directedness requires. A reflector, a shade, one light-gated channel, and a body that rotates are enough for a cell to extract the light direction from a signal that is, moment to moment, only a brightness (Foster & Smyth, 1980; Kreimer, 2009). Neither the alga nor the larva needs a map, a compass, or a representation of space—only a way to recover, from the light itself, the direction in which it strengthens, and the machinery to bias movement toward it. That economy is why phototaxis recurs across such distant branches of life, and why the molecule that drives it, channelrhodopsin, could be lifted out of an alga and used to switch neurons on and off at will.

Current Directions

The active work on phototaxis is quantitative and comparative, and much of it concerns how a cell or a circuit converts a light field into a heading. Csaba Verasztó, Gáspár Jékely, and colleagues showed that the Platynereis larva runs two photoreceptor systems in parallel—a rhabdomeric eye for fast directional phototaxis and a ciliary brain receptor that reads the spectral signature of depth—and that the two drive different movements, a fast steered turn and a slow vertical migration (Verasztó et al., 2018). The methodological advance is the ability to tie a behavior to a mapped circuit: Nicolas Randel and colleagues reconstructed the synaptic connectome of the visual sensory-motor loop, so the phototactic turn can now be followed cell by cell from photoreceptor to cilium (Randel et al., 2014).

A second front reopens the question of what a phototaxis is. Esteban Gorostiza, Julien Colomb, and Björn Brembs demonstrated that insect phototaxis is a decision rather than a reflex, modifiable by internal state and experience, which complicates the old reflex-versus-cognition dichotomy at its simplest supposed case (Gorostiza et al., 2016). Noriko Ueki and colleagues, working the molecular end, pinned the positive-versus-negative sign switch to the eyespot itself, showing that even the valence of the response is set by the sensor geometry rather than a downstream choice (Ueki et al., 2016). The open questions are shared across scales: how a cell sets its steering gain to a particular light, how directional and depth-reading systems are combined, and where exactly a directed phototaxis gives way to an undirected photokinesis.

Common Misconceptions

Phototaxis and photokinesis are two words for the same thing.

They are the two halves of a deliberate distinction. A phototaxis is directed—it steers using the direction of the light—whereas a photokinesis is undirected, modulating only a rate from the local brightness. The direction-versus-intensity test is what separates them (Fraenkel & Gunn, 1961).

A single cell senses which way the light points and swims up it.

A swimming alga is far too small to compare brightness across its own body, so it cannot read the light direction in space at one instant. It rotates, lets its eyespot shade the photoreceptor once per turn, and reads the direction from the timing of that modulation (Foster & Smyth, 1980).

The sign of a phototaxis is a fixed property of the organism.

The sign can invert with intensity. In Chlamydomonas the eyespot-mediated response switches from positive to negative as light rises past a threshold, so the same cell approaches a dim light and flees a bright one (Ueki et al., 2016).

Walking toward a light is a pure reflex with no choice in it.

Even insect phototaxis is a decision, not a fixed stimulus-response reflex. A fly weighs the light against its internal state and recent experience, and its approach or avoidance is biased by both (Gorostiza et al., 2016).

Glossary

Animal behavior.

The broad study of what animals do and the mechanisms behind it; one of the MeSH categories under which the taxis family that includes phototaxis is classified.

Channelrhodopsin.

A light-gated ion channel of the green-algal eyespot that opens when its retinal chromophore absorbs a photon; the photoreceptor that converts the light signal into a membrane current driving phototaxis.

Chemotaxis.

Directed movement along a chemical concentration gradient; a sibling modality of phototaxis under the same taxis parent.

Ciliary photoreceptor.

A photoreceptor that expands its light-sensing membrane on a cilium; the type that builds the vertebrate rods and cones and, in the larval brain, a spectral depth gauge.

Connectome.

The complete map of synaptic connections in a circuit; for larval phototaxis, the short feedforward loop from photoreceptor to ciliated effector.

Eyespot.

A pigmented reflector-and-shade beside the photoreceptor of a green alga that shades it once per body rotation, giving the cell the time-varying signal from which it recovers the light direction.

Kinesis.

An undirected locomotor response in which stimulus intensity sets the rate of movement or turning, with no steering relative to the stimulus direction; the counterpart to taxis.

Klinotaxis.

Directed orientation recovered by a single sensor sampled across time, as a rotating swimmer sweeps one shaded photoreceptor through the light, rather than by comparing two sensors at one instant.

Negative phototaxis.

Phototaxis directed away from the light, as when a cell flees an intensity it treats as harmful.

Photoreceptor.

A cell or molecule that converts absorbed light into a physiological signal; in phototaxis, the sensor whose modulation across a rotation encodes the light direction.

Positive phototaxis.

Phototaxis directed toward the light, steering up the gradient to the source.

Rhabdomeric photoreceptor.

A photoreceptor that expands its membrane into microvilli; the type that dominates invertebrate eyes and drives the larval eye-spot that steers fast directional phototaxis.

Taxis response.

A directed orientation response that uses the direction of a stimulus to steer the body toward or away from its source; the MeSH parent kind under which phototaxis is classified.

Tropotaxis.

Directed orientation recovered by comparing two sensors at one instant, such as two eyes; contrasted with the single-sensor klinotaxis of a rotating phototactic swimmer.

Key Researchers

Detlev Arendt

(EMBL Heidelberg; honorary professor, Heidelberg University). Evolutionary developmental biologist who found ciliary photoreceptors with a vertebrate-type opsin in an invertebrate brain, evidence that the two photoreceptor families predate the split between invertebrate and vertebrate eyes. ORCID - Faculty - Wikidata

Björn Brembs

(University of Regensburg, Institute of Zoology). Neurogeneticist who showed that insect phototaxis is a behavioural choice modifiable by internal state and experience, reframing a textbook reflex as a genuine decision. ORCID - Wikidata

Kenneth W. Foster

(Syracuse University, Professor Emeritus of Physics and Biophysical Science). Biophysicist who argued, before the molecule was in hand, that the algal photoreceptor driving phototaxis must be a rhodopsin, a prediction the channelrhodopsin work later confirmed. Faculty

Michael Granato

(University of Pennsylvania, Perelman School of Medicine). Developmental neuroscientist who showed that light modulates the whole locomotor program of the zebrafish larva, embedding the phototactic response in a continuously adapting motor output. ORCID - Faculty

Peter Hegemann

(Humboldt University of Berlin, Experimental Biophysics). Biophysicist who, with Georg Nagel and colleagues, identified channelrhodopsin as the directly light-gated channel of the algal eyespot, the photoreceptor of phototaxis and the founding tool of optogenetics. ORCID - Wikipedia - Wikidata

Gáspár Jékely

(Heidelberg University, Centre for Organismal Studies; University of Exeter). Neurobiologist who dissected the mechanism and evolution of phototaxis in the Platynereis larva, from the single-sensor klinotaxis to the connectome of its visual circuit. ORCID - Faculty

Frequently Asked Questions

What is phototaxis?

Phototaxis is the directed movement of a cell or organism along a light gradient, toward the source in positive phototaxis or away from it in negative phototaxis. It is classified in MeSH as a type of taxis response, the luminous modality of a directed taxis, and it contrasts with undirected kinesis (Fraenkel & Gunn, 1961).

What is the difference between positive and negative phototaxis?

Positive phototaxis steers toward the light and negative phototaxis steers away from it. The sensing machinery is the same; only the valence attached to rising intensity differs, and the same cell can switch sign as light crosses from a tolerable to a harmful level (Ueki et al., 2016).

How does a single cell sense the direction of light if it is too small to compare its two sides?

It senses the light across time rather than across its body. The cell rotates as it swims, its eyespot shades the photoreceptor once per rotation, and the depth of that light and shade modulation encodes how far the swimming axis points off the light (Foster & Smyth, 1980).

What is the eyespot and what does it do?

The eyespot is a stack of pigment granules beside the photoreceptor of a green alga that acts as a reflector and a shade. As the cell rotates it alternately exposes and shades the photoreceptor, giving the cell the time-varying signal from which it recovers the light direction (Kreimer, 2009).

What is channelrhodopsin and why is it famous?

Channelrhodopsin is the light-gated ion channel that serves as the algal eyespot photoreceptor, opening directly when it absorbs light. Expressed in neurons, the same molecule lets researchers switch cells on and off with light, which founded the field of optogenetics (Nagel et al., 2003).

What is klinotaxis?

Klinotaxis is directed orientation recovered by a single sensor sampled across time, as a rotating swimmer sweeps one shaded photoreceptor through the light. It differs from tropotaxis, which compares two sensors at one instant, and it is how a one-eyed larva still steers toward the light (Jékely et al., 2008).

Is phototaxis a reflex or a decision?

Even in an insect it is a decision rather than a fixed reflex. A fly weighs the light against its internal state and recent experience, so the same stimulus can yield approach or avoidance, which makes phototaxis a genuine behavioural choice (Gorostiza et al., 2016).

How does phototaxis relate to cognition?

Phototaxis is a largely non-cognitive strategy, and that is its value to cognitive science: it shows that genuinely goal-directed, steered movement can arise with almost no machinery, so it is the null model that any claim of map-based spatial cognition must outperform (Jékely, 2009).

References