Sense-cell structure and interspecies comparisons of polarized-light absorption in arthropod compound eyes

Polarized object detection in crabs: a two-channel system

Many animal species take advantage of polarization vision for vital tasks such as orientation, communication and contrast enhancement. Previous studies have suggested that decapod crustaceans use a two-channel polarization system for contrast enhancement. Here, we characterize the polarization contrast sensitivity in a grapsid crab. We estimated the polarization contrast sensitivity of the animals by quantifying both their escape response and changes in heart rate when presented with polarized motion stimuli. The motion stimulus consisted of an expanding disk with an 82 deg polarization difference between the object and the background. More than 90% of animals responded by freezing or trying to avoid the polarized stimulus. In addition, we co-rotated the electric vector (e-vector) orientation of the light from the object and background by increments of 30 deg and found that the animals' escape response varied periodically with a 90 deg period. Maximum escape responses were obtained for object and background e-vectors near the vertical and horizontal orientations. Changes in cardiac response showed parallel results but also a minimum response when e-vectors of object and background were shifted by 45 deg with respect to the maxima. These results are consistent with an orthogonal receptor arrangement for the detection of polarized light, in which two channels are aligned with the vertical and horizontal orientations. It has been hypothesized that animals with object-based polarization vision rely on a two-channel detection system analogous to that of color processing in dichromats. Our results, obtained by systematically varying the e-vectors of object and background, provide strong empirical support for this theoretical model of polarized object detection.

Null point of discrimination in crustacean polarisation vision

The polarisation of light is used by many species of cephalopods and crustaceans to discriminate objects or to communicate. Most visual systems with this ability, such as that of the fiddler crab, include receptors with photopigments that are oriented horizontally and vertically relative to the outside world. Photoreceptors in such an orthogonal array are maximally sensitive to polarised light with the same fixed e-vector orientation. Using opponent neural connections, this two-channel system may produce a single value of polarisation contrast and, consequently, it may suffer from null points of discrimination. Stomatopod crustaceans use a different system for polarisation vision, comprising at least four types of polarisation-sensitive photoreceptor arranged at 0°, 45°, 90° and 135° relative to each other, in conjunction with extensive rotational eye movements. This anatomical arrangement should not suffer from equivalent null points of discrimination. To test whether these two systems were vulnerable to null points, we presented the fiddler crab Uca heteropleura and the stomatopod Haptosquilla trispinosa with polarised looming stimuli on a modified LCD monitor. The fiddler crab was less sensitive to differences in the degree of polarised light when the e-vector was at -45°, than when the e-vector was horizontal. In comparison, stomatopods showed no difference in sensitivity between the two stimulus types. The results suggest that fiddler crabs suffer from a null point of sensitivity, while stomatopods do not.

Evolutionary tinkering with visual photoreception

Eyes have evolved many times, and arthropods and vertebrates share transcription factors for early development. Moreover, the photochemistry of vision in all eyes employs an opsin and the isomerization of a retinoid from the 11-cis to the all-trans configuration. The opsins, however, have associated with several different G proteins, initiating hyperpolarizing and depolarizing conductance changes at the photoreceptor membrane. Beyond these obvious instances of homology, much of the evolutionary story is one of tinkering, producing a great variety of morphological forms and variation within functional themes. This outcome poses a central issue in the convergence of evolutionary and developmental biology: what are the heritable features in the later stages of development that give natural selection traction in altering phenotypic outcomes? This paper discusses some results of evolutionary tinkering where this question arises and, in some cases, where the reasons for particular outcomes and the role of adaptation may not be understood. Phenotypic features include: the exploitation of microvilli in rhabdomeric photoreceptors for detecting the plane of polarized light; different instances of retinoid in the visual pigment; examples of the many uses of accessory pigments in tuning the spectral sensitivity of photoreceptors; selection of opsins in tuning sensitivity in aquatic environments; employing either reflection or refraction in the optics of compound eyes; the multiple ways of constructing images in compound eyes; and the various ways of regenerating 11-cis retinals to maintain visual sensitivity. Evolution is an irreversible process, but tinkering may recover some lost functions, albeit by new mutational routes. There is both elegance and intellectual coherence to the natural processes that produce such variety and functional complexity. But marginalizing the teaching of evolution in public education is a continuing social and political problem that contributes to the reckless capacity of humans to alter the planet without trying to understand how nature works.

Electrophysiological evidence for polarization sensitivity in the camera-type eyes of the aquatic predacious insect larva, Thermonectus marmoratus (Coleoptera: Dytiscidae)

Polarization sensitivity has most often been studied in mature insects, yet it is likely that larvae also make use of this visual modality. The aquatic larvae of the predacious diving beetle Thermonectus marmoratus are highly successful visually guided predators, with a UV-sensitive proximal retina that, according to its ultrastructure, has three distinct cell types with anatomical attributes that are consistent with polarization sensitivity. In the present study we used electrophysiological methods and single-cell staining to confirm polarization sensitivity in the proximal retinas of both principal eyes of these larvae. As expected from their microvillar orientation, cells of type T1 are most sensitive to vertically polarized light, while cells of type T2 are most sensitive to horizontally polarized light. In addition, T3 cells likely constitute a second population of cells that are most sensitive to light with vertical e-vector orientation, characterized by shallower polarization modulations, and smaller polarization sensitivity (PS) values than are typical for T1 cells. The level of PS values found in this study suggests that polarization sensitivity likely plays an important role in the visual system of these larvae. Based on their natural history and behavior, possible functions are: (1) finding water after hatching, (2) finding the shore before pupation, and (3) making prey more visible, by filtering out horizontally polarized haze, and/or using polarization features for prey detection.

Electrophysiological evidence for linear polarization sensitivity in the compound eyes of the stomatopod crustacean Gonodactylus chiragra

Gonodactyloid stomatopod crustaceans possess polarization vision, which enables them to discriminate light of different e-vector angle. Their unusual apposition compound eyes are divided by an equatorial band of six rows of enlarged, structurally modified ommatidia, the mid-band (MB). The rhabdoms of the two most ventral MB rows 5 and 6 are structurally designed for polarization vision. Here we show, with electrophysiological recordings, that the photoreceptors R1-R7 within these two MB rows in Gonodactylus chiragra are highly sensitive to linear polarized light of two orthogonal directions (PS=6.1). They possess a narrow spectral sensitivity peaking at 565 nm. Unexpectedly, photoreceptors within the distal rhabdomal tier of MB row 2 also possess highly sensitive linear polarization receptors, which are in their spectral and polarization characteristics similar to the receptors of MB rows 5 and 6. Photoreceptors R1-R7 within the remainder of the MB exhibit low polarization sensitivity (PS=2.3). Outside the MB, in the two hemispheres, R1-R7 possess medium linear polarization sensitivity (PS=3.8) and a broad spectral sensitivity peaking at around 500 nm, typical for most crustaceans. Throughout the retina the most distally situated UV-sensitive R8 cells are not sensitive to linear polarized light.

Two-channel polarization analyzer in the sustaining fiber-dimming fiber ensemble of crayfish visual system

Polarization sensitivity (PS) was examined in two classes of neurons, sustaining fibers and dimming fibers, in the medulla externa (second optic neuropile) of the crayfish, Pacifasticus leniusculus. Visual responses were recorded intracellularly and extracellularly. The influence of e-vector orientation (theta) was probed in steady-state responses, with brief flashes and with a rotating polarizer. The results indicate that the entire sustaining fiber population appears to be maximally sensitive to vertically polarized light. Although the evidence is less complete for dimming fibers, they appear to be maximally inhibited by vertically polarized light and excited by horizontally polarized light. Thus the sustaining fibers and dimming fibers form a two-channel polarization analyzer that captures the main features of the polarization system established in photoreceptors and lamina monopolar cells. The available evidence suggests that this two-channel system has the same characteristics across most or all of the retinula. Lateral inhibition in sustaining fibers is differentially sensitive to theta. Inhibition is substantial at theta = 90 degrees (horizontal) and essentially absent at theta = 0 degrees. The details of the sustaining fiber polarization response closely follow features established in more peripheral neurons, including the magnitude of PS, enhanced responsiveness to a changing e-vector, and modest directionality to a changing e-vector in approximately 40% of the cells.

Double-cone internal reflection as a basis for polarization detection in fish

Some species of fish are able to discriminate, in addition to intensity and wavelength (color), the direction of polarization of visible light. Optical experiments on axially oriented retinal cones from trout and sunfish with use of two types of polarization microscope indicate anisotropic light transmission through paired cones. The measured linear birefringence of paired cone ellipsoids is consistent with the presence of membranous partitions. It is proposed that the partition between the two members of a paired cone, which often appears extensive and flat, functions as a dielectric mirror and that polarization-dependent reflection and refraction at this partition constitutes the underlying mechanism in the transduction of polarization into intensity variation at the photoreceptor's outer segments. We support this hypothesis with linear birefringence and linear dichroism measurements, histological evidence, large-scale optical model measurements, and theoretical calculations based on Fresnel's formulas.

The fine structure of the compound eye of the African armyworm moth, Spodoptera exempta Walk. (Lepidoptera, Noctuidae)

The morphology of the compound eye of the noctuid moth Spodoptera exempta was investigated by electron microscopy. This eucone superposition eye is composed of about 8000 ommatidia. Each ommatidium is surrounded by six secondary pigment cells showing pigment movement according to the state of adaptation. It contains four crystalline cone cells forming together a crystalline cone and tract, two primary pigment cells, which encompass the crystalline cone, and usually eight retinula cells. On the basis of their rhabdomeric structure, three types of retinula cells can be distinguished. According to the structure of the rhabdom, two types of ommatidia are found in different regions of the eye. The rhabdom of the lobed type, providing more than 80% of ommatidia, is composed of V-shaped rhabdomeres with fanwise arranged microvilli. The rhabdom of the square type, found in a small area in the dorsal region of the eye, consists of triangular rhabdomeres with parallel microvilli. The functional significance of this difference is discussed.

Types and arrangements of neurons in the crayfish optic lamina

The neural arrangements in the optic lamina of the crayfish Pacifastacus leniusculus Dana have been studied by light microscopy by means of silver impregnation techniques. The lamina is composed of columnar synaptic compartments (cartridges). Each cartridge is composed of seven receptor terminals distributed in two layers and second-order monopolar neurons connecting the lamina with the second synaptic region, the medulla externa. The neurons found in the lamina consist of five classes: monopolar neurons, centrifugal small-field neurons, tangential neurons, multipolar cells (possibly of a glial nature) and photoreceptor axons (fig. 13). Among the monopolar cells, five types are classified (M1-M5) according to their lamina arborizations. Two types are stratified (M3 and M5) corresponding to the photoreceptor terminal strata. On this basis, the lamina plexiform layer is subdivided into two layers (epl1 and epl2). The remaining monopolar neurons have lateral processes in both layers, two of them within one cartridge (M1 and M2) and one over several cartridges (M5). There is one type of small-field centrifugal neuron (C1) and two types of tangential medulla to lamina neurons (Tan1 and Tan2), both having processes covering a large number of cartridges. Multipolar cells with cell bodies distal (MP1) or proximal (MP2) to the plexiform layer send processes to several cartridges. The receptor axons consit of three types. One has terminals in epl1 or epl2, the second has its terminal in epl1 and a thin process to epl2, and the third (corresponding to the 8th retinular cell) bypasses the lamina and has a terminal in the medulla externa. A brief comparison is made with the neural arrangements in the lamina of the Norway lobster Nephrops norvegicus L.

Ultrastructural and molecular characteristics of crayfish photoreceptor membranes

The ultrastructure of photoreceptor cells of the crayfish (P. clarkii) has been examined by means of thin sections and freeze-fracturing. The study reveals that in the photoreceptor membranes there are particles associated primarily with the A faces of freeze-fracture preparations which have a mean diameter of 80-84 A and a density of 6,600 per per micrometer2. Treatment of the retina with digitonin (a substance capable of extracting visual photopigments) in Ringer's causes marked disruption of the hexagonal arrangement of the microvilli, breakdown of the microvilli into smaller segments, and gradual removal of the particles. The estimated photopigment concentration in the microvillus is 4,000 per micrometer. It is suggested that the observed particles represent the photopigment in situ.

Photoreceptor processes: some problems and perspectives

Visual photoreceptors from both vertebrates and invertebrates are characterized by extensive elaboration of membrane which contains visual pigment (rhodopsin). Visual pigments in all phyla examined are chemically similar: the chromophore is 11-cis retinaldehyde attached by an aldimine linkage (Schiff base) to a membrane protein, opsin. The effect of light is to isomerize the chromophore to the all-trans configuration. Beyond these fundamental similarities, several specific areas are discussed in which variations and differences appear. (1) Light causes vertebrate visual pigments to bleach, liberating the chromophore. Most invertebrate visual pigments do not bleach in the light, but instead form a thermally stable metarhodopsin, with the chromophore in the all-trans configuration still attached to the opsin. (2) In the disk membranes of vertebrate rod and cone outer segments, the rhodopsin molecules are oriented with their chromophores nearly coplanar with the disks. Within this plane, however, both rotational and translational diffusion are possible. In the microvillar membranes of arthropod and cephalopod rhabdoms, on the other hand, the situation is less clear. There is evidence for some preferential orientation of chromophores that implies restrictions on Brownian rotation. (3) In the outer segments of vertebrate receptors, absorption of light by rhodopsin causes the plasma membrane to hyperpolarize due to a decrease in sodium conductance, possibly mediated by calcium ions. In most invertebrate photoreceptors, light causes a depolarization due to an increase in conductance, principally to sodium ions. A subsequent entry of calcium causes a partial repolarization of the membrane, due to a decrease in sodium conductance. (4) For vertebrate receptors, log threshold is directly proportional to the fraction of rhodopsin bleached (Dowling-Rushton relationship). The proportionality constant varies in different preparations from less than four to more than 30, and the physical basis for the relationship is unknown. For invertebrates, by contrast, the dependence of sensitivity on rhodopsin concentration is much less dramatic and may well depend simply on the probability of quantum catch. (5) In most species, vertebrate and invertebrate, the accumulation of photoproduct probably has no effect on membrane conductance, but several possible exceptions exist. (6) Photoregeneration of rhodopsin from metarhodopsin is likely an important mechanism of recovery in certain arthropods such as diurnal insects, but dark mechanisms of recovery also exist in all phyla. In no single case are they adequately understood.

Localization of the violet and yellow receptor cells in the crayfish retinula

Cellular identification of color receptors in crayfish compound eyes has been made by selective adaptation at 450 nm and 570 nm, wavelengths near the lambda(max)'s of the two retinular cell classes previously demonstrated. By utilizing earlier evidence, the concentration of lysosome-related bodies (LRB) was used to measure relative light adaptation and thus wavelength sensitivity in 665 retinular cells from six eyes. The observed particle distributions demonstrate the following. Both violet and yellow receptors occur ordinarily in each retinula. Of the seven regular retinular cells two (R(3) and R(4) using Eguchi's numbering [1965]) have mean sensitivities significantly greater to violet and less to yellow than the other five. The latter apparently comprise "pure" yellow receptors (R(1) and R(7)) and mixed yellow and violet receptors (R(2), R(5), and R(6)). Explanations of such ambiguity requiring two visual pigments in single retinular cells or intercellular coupling of adjacent neuroreceptors are apparently precluded by previous evidence. Present data imply alternatively some positional variability in the violet pair's location in individual retinulas. Thus R(3) and R(4) are predominantly the violet receptors but in some retinulas R(2) and R(3) or R(4) and R(5) (or rarely some other cell pairs) may be. The retinal distribution of such variations has yet to be determined. In agreement with intracellular recordings the blue and yellow cells here identified belong to both the vertical and horizontal e-vector sensitive channels.

Photoreceptors in the crayfish compound eye: electrical interactions between cells as related to polarized-light sensitivity

1. The sensitivity to plane-polarized light and the electrical interactions of photoreceptors were examined with intracellular and extracellular micro-electrodes in excised compound eyes of the crayfish.2. There are two types of photoreceptor: each photoreceptor cell responds best to polarized light when the electric-vector of the light is oriented in one of two orthogonal directions. Seven cells, representing each type, are grouped together to form ommatidia.3. In each ommatidium, cells that are sensitive to the same orientation of the electric-vector of polarized light are coupled electrically. Cells having orthogonal polarized-light sensitivities are not coupled.4. Nearly all cells studied were sensitive to orange light. A few cells of both types were found that were sensitive to blue light. Blue-sensitive cells were not coupled to orange-sensitive cells.5. The photocurrents of both cell types produce negative extracellular potentials which can be greater than 10 mV when measured near the photoreceptive membranes within ommatidia. Evidence suggests that the extracellular potentials produced by one type of cell can effectively reduce the receptor potentials recorded in the other cell type. It is proposed that such a mutual non-synaptic interaction can make a cell more sensitive to the orientation of polarized-light than would be predicted from the cell's differential absorption of polarized light (i.e. its dichroic ratio).