Fiddler crabs (Afruca tangeri) detect second-order motion in both intensity and polarization

Motion vision is vital for a wide range of animal behaviors. Fiddler crabs, for example, rely heavily on motion to detect the movement of avian predators. They are known to detect first-order motion using both intensity (defined by spatiotemporal correlations in luminance) and polarization information (defined separately as spatiotemporal correlations in the degree and/or angle of polarization). However, little is known about their ability to detect second-order motion, another important form of motion information; defined separately by spatiotemporal correlations in higher-order image properties. In this work we used behavioral experiments to test how fiddler crabs (Afruca tangeri) responded to both second-order intensity and polarization stimuli. Fiddler crabs responded to a number of different intensity based second-order stimuli. Furthermore, the crabs also responded to second-order polarization stimuli, a behaviorally relevant stimulus applicable to an unpolarized flying bird when viewed against a polarized sky. The detection of second-order motion in polarization is, to the best of our knowledge, the first demonstration of this ability in any animal. This discovery therefore opens a new dimension in our understanding of how animals use polarization vision for target detection and the broader importance of second-order motion detection for animal behavior.

Polarization vision in terrestrial hermit crabs

Polarization vision is used by a wide range of animals for navigating, orienting, and detecting objects or areas of interest. Shallow marine and semi-terrestrial crustaceans are particularly well known for their abilities to detect predator-like or conspecific-like objects based on their polarization properties. On land, some terrestrial invertebrates use polarization vision for detecting suitable habitats, oviposition sites or conspecifics, but examples of threat detection in the polarization domain are less well known. To test whether this also applies to crustaceans that have evolved to occupy terrestrial habitats, we determined the sensitivity of two species of land and one species of marine hermit crab to predator-like visual stimuli varying in the degree of polarization. All three species showed an ability to detect these cues based on polarization contrasts alone. One terrestrial species, Coenobita rugosus, showed an increased sensitivity to objects with a higher degree of polarization than the background. This is the inverse of most animals studied to date, suggesting that the ecological drivers for polarization vision may be different in the terrestrial environment.

Views from 'crabworld': the spatial distribution of light in a tropical mudflat

Natural scene analysis has been extensively used to understand how the invariant structure of the visual environment may have shaped biological image processing strategies. This paper deals with four crucial, but hitherto largely neglected aspects of natural scenes: (1) the viewpoint of specific animals; (2) the fact that image statistics are not independent of the position within the visual field; (3) the influence of the direction of illumination on luminance, spectral and polarization contrast in a scene; and (4) the biologically relevant information content of natural scenes. To address these issues, I recorded the spatial distribution of light in a tropical mudflat with a spectrographic imager equipped with a polarizing filter in an attempt to describe quantitatively the visual environment of fiddler crabs. The environment viewed by the crabs has a distinct structure. Depending on the position of the sun, the luminance, the spectral composition, and the polarization characteristics of horizontal light distribution are not uniform. This is true for both skylight and for reflections from the mudflat surface. The high-contrast feature of the line of horizon dominates the vertical distribution of light and is a discontinuity in terms of luminance, spectral distribution and of image statistics. On a clear day, skylight intensity increases towards the horizon due to multiple scattering, and its spectral composition increasingly resembles that of sunlight. Sky-substratum contrast is highest at short wavelengths. I discuss the consequences of this extreme example of the topography of vision for extracting biologically relevant information from natural scenes.

Terrestrial crustaceans (Arthropoda, Crustacea): taxonomic diversity, terrestrial adaptations, and ecological functions

Terrestrial crustaceans are represented by approximately 4,900 species from six main lineages. The diversity of terrestrial taxa ranges from a few genera in Cladocera and Ostracoda to about a third of the known species in Isopoda. Crustaceans are among the smallest as well as the largest terrestrial arthropods. Tiny microcrustaceans (Branchiopoda, Ostracoda, Copepoda) are always associated with water films, while adult stages of macrocrustaceans (Isopoda, Amphipoda, Decapoda) spend most of their lives in terrestrial habitats, being independent of liquid water. Various adaptations in morphology, physiology, reproduction, and behavior allow them to thrive in virtually all geographic areas, including extremely arid habitats. The most derived terrestrial crustaceans have acquired highly developed visual and olfactory systems. The density of soil copepods is sometimes comparable to that of mites and springtails, while the total biomass of decapods on tropical islands can exceed that of mammals in tropical rainforests. During migrations, land crabs create record-breaking aggregations and biomass flows for terrestrial invertebrates. The ecological role of terrestrial microcrustaceans remains poorly studied, while omnivorous macrocrustaceans are important litter transformers and soil bioturbators, occasionally occupying the position of the top predators. Notably, crustaceans are the only group among terrestrial saprotrophic animals widely used by humans as food. Despite the great diversity and ecological impact, terrestrial crustaceans, except for woodlice, are often neglected by terrestrial ecologists. This review aims to narrow this gap discussing the diversity, abundance, adaptations to terrestrial lifestyle, trophic relationships and ecological functions, as well as the main methods used for sampling terrestrial crustaceans.

Fiddler crabs are unique in timing their escape responses based on speed-dependent visual cues

Predation risk imposes strong selection pressures on visual systems to quickly and accurately identify the position and movement of potential predators.^1,2 Many invertebrates and other small animals, however, have limited capacity for distance perception due to their low spatial resolution and closely situated eyes.^3,4 Consequently, they often rely on simplified decision criteria, essentially heuristics or "rules of thumb", to make decisions. The visual cues animals use to make escape decisions are surprisingly consistent, especially among arthropods, with the timing of escape commonly triggered by size-dependent visual cues such as angular size or angular size increment.^5,6,7,8,9,10 Angular size, however, confuses predator size and distance and provides no information about the speed of the attack. Here, we show that fiddler crabs (Gelasimus dampieri) are unique among the arthropods studied to date as they timed their escape response based on the speed of an object's angular expansion. The crabs responded reliably by running away from visual stimuli that expanded at approximately 1.7 degrees/s, irrespective of stimulus size, speed, or its initial distance from the crabs. Though the threshold expansion speed was consistent across different stimulus conditions, we found that the escape timing was modulated by the elevation at which the stimulus approached, suggesting that other risk factors can bias the expansion speed threshold. The results suggest that the visual escape cues used by arthropods are less conserved than previously thought and that lifestyle and environment are significant drivers determining the escape cues used by different species.

Polarization vision mitigates visual noise from flickering light underwater

In shallow water, downwelling light is refracted from surface waves onto the substrate creating bands of light that fluctuate in both time and space, known as caustics. This dynamic illumination can be a visual hindrance for animals in shallow underwater environments. Animals in such habitats may have evolved to use polarization vision for discriminating objects while ignoring the variations in illumination caused by caustics. To explore this possibility, crabs (Carcinus maenas) and cuttlefish (Sepia officinalis), both of which have polarization vision, were presented with moving stimuli overlaid with caustics. Dynamic caustics inhibited the detection of an intensity-based stimulus but not when these stimuli were polarized. This study is the first to demonstrate that polarization vision reduces the negative impacts that dynamic illumination can have on visual perception.

Behavioural and neural responses of crabs show evidence for selective attention in predator avoidance

Selective attention, the ability to focus on a specific stimulus and suppress distractions, plays a fundamental role for animals in many contexts, such as mating, feeding, and predation. Within natural environments, animals are often confronted with multiple stimuli of potential importance. Such a situation significantly complicates the decision-making process and imposes conflicting information on neural systems. In the context of predation, selectively attending to one of multiple threats is one possible solution. However, how animals make such escape decisions is rarely studied. A previous field study on the fiddler crab, Gelasimus dampieri, provided evidence of selective attention in the context of escape decisions. To identify the underlying mechanisms that guide their escape decisions, we measured the crabs' behavioural and neural responses to either a single, or two simultaneously approaching looming stimuli. The two stimuli were either identical or differed in contrast to represent different levels of threat certainty. Although our behavioural data provides some evidence that crabs perceive signals from both stimuli, we show that both the crabs and their looming-sensitive neurons almost exclusively respond to only one of two simultaneous threats. The crabs' body orientation played an important role in their decision about which stimulus to run away from. When faced with two stimuli of differing contrasts, both neurons and crabs were much more likely to respond to the stimulus with the higher contrast. Our data provides evidence that the crabs' looming-sensitive neurons play an important part in the mechanism that drives their selective attention in the context of predation. Our results support previous suggestions that the crabs' escape direction is calculated downstream of their looming-sensitive neurons by means of a population vector of the looming sensitive neuronal ensemble.

Angular dependence of polarisation contrast sensitivity in octopus

Cephalopod photoreceptors are polarisation-sensitive, giving them an ability to discriminate between lights of different angle and degree of polarisation. While colour vision is achieved by comparison of signals of photoreceptors tuned to different parts of light spectra, polarisation vision is achieved by comparison of signals of photoreceptors tuned to different orientations of e-vector. Therefore, from a theoretical point of view, polarisation vision is similar to colour vision. In particular, detection of polarised light against an unpolarised background is analogous to detection of chromatic light against grey. The dependence of polarisation contrast sensitivity on the angle of polarisation can be theoretically predicted using a receptor noise limited model in much the same way as it has been done for predicting the shape of the increment threshold spectral sensitivity in animals with colour vision. Here we report angular dependence of polarisation contrast sensitivity in octopus (O. tetricus, Gould 1852) and compare the theoretical predictions of polarisation contrast with the experimental results. Polarisation gratings were generated using LCD screens with removed polarisers and the orientation of polarisation was changed by rotating the screen. Reaction to the stimulus was recorded using a fixation reflex. We show that, in agreement with the theoretical predictions, the maximum contrast sensitivity is achieved at horizontal and vertical orientations of polarisation. Our results demonstrate that the dependence of polarisation contrast sensitivity on the angle of polarisation can be analysed in the same way as the dependence of colour thresholds on wavelength of monochromatic light added to a grey background.

Thresholds of polarization vision in octopuses

Polarization vision is widespread in nature, mainly among invertebrates, and is used for a range of tasks including navigation, habitat localization and communication. In marine environments, some species such as those from the Crustacea and Cephalopoda that are principally monochromatic, have evolved to use this adaptation to discriminate objects across the whole visual field, an ability similar to our own use of colour vision. The performance of these polarization vision systems varies, and the few cephalopod species tested so far have notably acute thresholds of discrimination. However, most studies to date have used artificial sources of polarized light that produce levels of polarization much higher than found in nature. In this study, the ability of octopuses to detect polarization contrasts varying in angle of polarization (AoP) was investigated over a range of different degrees of linear polarization (DoLP) to better judge their visual ability in more ecologically relevant conditions. The ‘just-noticeable-differences’ (JND) of AoP contrasts varied consistently with DoLP. These JND thresholds could be largely explained by their ‘polarization distance’, a neurophysical model that effectively calculates the level of activity in opposing horizontally and vertically oriented polarization channels in the cephalopod visual system. Imaging polarimetry from the animals’ natural environment was then used to illustrate the functional advantage that these polarization thresholds may confer in behaviourally relevant contexts.

Summary: Octopuses are highly sensitive to small changes in the angle of polarization (<1 deg contrast), even when the degree of polarization is low, which may confer a functional advantage in behaviourally relevant contexts.

Polarization contrasts and their effect on the gaze stabilization of crustaceans

Many animals go to great lengths to stabilize their eyes relative to the visual scene and do so to enhance the localization of moving objects and to functionally partition the visual system relative to the outside world. An important cue that is used to control these stabilization movements is contrast within the visual surround. Previous studies on insects, spiders and fish have shown that gaze stabilization is achromatic (‘colour blind’), meaning that chromatic contrast alone (in the absence of apparent intensity contrasts) does not contribute to gaze stabilization. Following the assumption that polarization vision is analogous in many ways to colour vision, the present study shows that five different crustacean species do not use the polarization of light alone for gaze stabilization, despite being able to use this modality for detecting predator-like objects. This work therefore suggests that the gaze stabilization in many crustaceans cannot be elicited by the polarization of light alone.

Summary: Five different crustacean species do not use the polarization of light alone for gaze stabilization, despite being able to use this modality for detecting predator-like objects.

Photoreceptors and diurnal variation in spectral sensitivity in the fiddler crab Gelasimus dampieri

Colour signals, and the ability to detect them, are important for many animals and can be vital to their survival and fitness. Fiddler crabs use colour information to detect and recognise conspecifics, but their colour vision capabilities remain unclear. Many studies have attempted to measure their spectral sensitivity and identify contributing retinular cells, but the existing evidence is inconclusive. We used electroretinogram (ERG) measurements and intracellular recordings from retinular cells to estimate the spectral sensitivity of Gelasimus dampieri and to track diurnal changes in spectral sensitivity. G. dampieri has a broad spectral sensitivity and is most sensitive to wavelengths between 420 and 460 nm. Selective adaptation experiments uncovered an ultraviolet (UV) retinular cell with a peak sensitivity shorter than 360 nm. The species' spectral sensitivity above 400 nm is too broad to be fitted by a single visual pigment and using optical modelling, we provide evidence that at least two medium-wavelength sensitive (MWS) visual pigments are contained within a second blue-green sensitive retinular cell. We also found a ∼25 nm diurnal shift in spectral sensitivity towards longer wavelengths in the evening in both ERG and intracellular recordings. Whether the shift is caused by screening pigment migration or changes in opsin expression remains unclear, but the observation shows the diel dynamism of colour vision in this species. Together, these findings support the notion that G. dampieri possesses the minimum requirement for colour vision, with UV and blue/green receptors, and help to explain some of the inconsistent results of previous research.

Visual metamorphoses in insects and malacostracans: Transitions between an aquatic and terrestrial life

Arthropods operate in an outrageous diversity of environments. From the deep sea to dense tropical forests, to wide open arctic tundra, they have colonized almost every possible habitat. Within these environments, the presence of light is nearly ubiquitous, varying in intensity, wavelength, and polarization. Light provides critical information about the environment, such as time of day or where food sources may be located. Animals take advantage of this prevalent and informative cue to make behavioral choices. However, the types of choices animals face depend greatly on their environments and needs at any given time. In particular, animals that undergo metamorphosis, with arthropods being the prime example, experience dramatic changes in both behavior and ecology, which in turn may require altering the structure and function of sensory systems such as vision. Amphibiotic organisms maintain aquatic lifestyles as juveniles before transitioning to terrestrial lifestyles as adults. However, light behaves differently in water than in air, resulting in distinct aquatic and terrestrial optical environments. Visual changes in response to these optical differences can occur on multiple levels, from corneal structure down to neural organization. In this review, we summarize examples of alterations in the visual systems of amphibiotic larval and adult insects and malacostracan crustaceans, specifically those attributed to environmental differences between metamorphic phases.

Light adaptation mechanisms in the eye of the fiddler crab Afruca tangeri

A great diversity of adaptations is found among animals with compound eyes and even closely related taxa can show variation in their light-adaptation strategies. A prime example of a visual system evolved to function in specific light environments is the fiddler crab, used widely as a model to research aspects of crustacean vision and neural pathways. However, questions remain regarding how their eyes respond to the changes in brightness spanning many orders of magnitude, associated with their habitat and ecology. The fiddler crab Afruca tangeri forages at low tide on tropical and semi-tropical mudflats, under bright sunlight and on moonless nights, suggesting that their eyes undergo effective light adaptation. Using synchrotron X-ray tomography, light and transmission electron microscopy and in vivo ophthalmoscopy, we describe the ultrastructural changes in the eye between day and night. Dark adaptation at dusk triggered extensive widening of the rhabdoms and crystalline cone tips. This doubled the ommatidial acceptance angles and increased microvillar surface area for light capture in the rhabdom, theoretically boosting optical sensitivity 7.4 times. During daytime, only partial dark-adaptation was achieved and rhabdoms remained narrow, indicating strong circadian control on the process. Bright light did not evoke changes in screening pigment distributions, suggesting a structural inability to adapt rapidly to the light level fluctuations frequently experienced when entering their burrow to escape predators. This should enable fiddler crabs to shelter for several minutes without undergoing significant dark-adaptation, their vision remaining effectively adapted for predator detection when surfacing again in bright light.

Spatial Contrast Sensitivity to Polarization and Luminance in Octopus

While color vision is achieved by comparison of signals of photoreceptors tuned to different parts of light spectra, polarization vision is achieved by comparison of signals of photoreceptors tuned to different orientations of the electric field component of visible light. Therefore, it has been suggested that polarization vision is similar to color vision. In most animals that have color vision, the shape of luminance contrast sensitivity curve differs from the shape of chromatic contrast sensitivity curve. While luminance contrast sensitivity typically decreases at low spatial frequency due to lateral inhibition, chromatic contrast sensitivity generally remains high at low spatial frequency. To find out if the processing of polarization signals is similar to the processing of chromatic signals, we measured the polarization and luminance contrast sensitivity dependence in a color-blind animal with well-developed polarization vision, Octopus tetricus. We demonstrate that, in Octopus tetricus, both luminance and polarization contrast sensitivity decrease at low spatial frequency and peak at the same spatial frequency (0.3 cpd). These results suggest that, in octopus, polarization and luminance signals are processed via similar pathways.

Von Uexküll Revisited: Addressing Human Biases in the Study of Animal Perception

More than 100 years ago, the biologist Jakob von Uexküll suggested that, because sensory systems are diverse, animals likely inhabit different sensory worlds (umwelten) than we do. Since von Uexküll, work across sensory modalities has confirmed that animals sometimes perceive sensory information that humans cannot, and it is now well-established that one must account for this fact when studying an animal's behavior. We are less adept, however, at recognizing cases in which non-human animals may not detect or perceive stimuli the same way we do, which is our focus here. In particular, we discuss three ways in which our own perception can result in misinformed hypotheses about the function of various stimuli. In particular, we may (1) make untested assumptions about how sensory information is perceived, based on how we perceive or measure it, (2) attribute undue significance to stimuli that we perceive as complex or striking, and (3) assume that animals divide the sensory world in the same way that we as scientists do. We discuss each of these biases and provide examples of cases where animals cannot perceive or are not attending to stimuli in the same way that we do, and how this may lead us to mistaken assumptions. Because what an animal perceives affects its behavior, we argue that these biases are especially important for researchers in sensory ecology, cognition, and animal behavior and communication to consider. We suggest that studying animal umwelten requires integrative approaches that combine knowledge of sensory physiology with behavioral assays.

Parallel processing of polarization and intensity information in fiddler crab vision

Many crustaceans are sensitive to the polarization of light and use this information for object-based visually guided behaviors. For these tasks, it is unknown whether polarization and intensity information are integrated into a single-contrast channel, whereby polarization directly contributes to perceived intensity, or whether they are processed separately and in parallel. Using a novel type of visual display that allowed polarization and intensity properties of visual stimuli to be adjusted independently and simultaneously, we conducted behavioral experiments with fiddler crabs to test which of these two models of visual processing occurs. We found that, for a loom detection task, fiddler crabs process polarization and intensity information independently and in parallel. The crab's response depended on whichever contrast was the most salient. By contributing independent measures of visual contrast, polarization and intensity provide a greater range of detectable contrast information for the receiver, increasing the chance of detecting a potential threat.

Polarisation signals: a new currency for communication

Most polarisation vision studies reveal elegant examples of how animals, mainly the invertebrates, use polarised light cues for navigation, course-control or habitat selection. Within the past two decades it has been recognised that polarised light, reflected, blocked or transmitted by some animal and plant tissues, may also provide signals that are received or sent between or within species. Much as animals use colour and colour signalling in behaviour and survival, other species additionally make use of polarisation signalling, or indeed may rely on polarisation-based signals instead. It is possible that the degree (or percentage) of polarisation provides a more reliable currency of information than the angle or orientation of the polarised light electric vector (e-vector). Alternatively, signals with specific e-vector angles may be important for some behaviours. Mixed messages, making use of polarisation and colour signals, also exist. While our knowledge of the physics of polarised reflections and sensory systems has increased, the observational and behavioural biology side of the story needs more (and more careful) attention. This Review aims to critically examine recent ideas and findings, and suggests ways forward to reveal the use of light that we cannot see.

A different view: sensory drive in the polarized-light realm

Sensory drive, the concept that sensory systems primarily evolve under the influence of environmental features and that animal signals are evolutionarily shaped and tuned by these previously existing sensory systems, has been thoroughly studied regarding visual signals across many animals. Much of this work has focused on spectral aspects of vision and signals. Here, I review work on polarized-light signals of animals and relate these to what is known of polarization visual systems, polarized-light aspects of visual scenes, and polarization-related behavior (e.g., orientation, habitat-finding, contrast enhancement). Other than the broad patterns of scattered polarized light in the sky, most polarization in both terrestrial and aquatic environments results from either reflection or scattering in the horizontal plane. With overhead illumination, horizontal features such as the surfaces of many leaves or of air: water interfaces reflect horizontal polarization, and water scatters horizontally polarized light under most conditions. Several animal species have been demonstrated to use horizontally polarized light fields or features in critical aspects of their biology. Significantly, most biological signals are also horizontally polarized. Here, I present relevant polarization-related behavior and discuss the hypothesis that sensory drive has evolutionarily influenced the structure of polarization signals. The paper also considers the evolutionary origin of circular polarization vision and circularly polarized signals. It appears that this class of signals did not evolve under the influence of sensory drive. The study of signals based on polarized light is becoming a mature field of research.

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.

Polarisation vision: overcoming challenges of working with a property of light we barely see

In recent years, the study of polarisation vision in animals has seen numerous breakthroughs, not just in terms of what is known about the function of this sensory ability, but also in the experimental methods by which polarisation can be controlled, presented and measured. Once thought to be limited to only a few animal species, polarisation sensitivity is now known to be widespread across many taxonomic groups, and advances in experimental techniques are, in part, responsible for these discoveries. Nevertheless, its study remains challenging, perhaps because of our own poor sensitivity to the polarisation of light, but equally as a result of the slow spread of new practices and methodological innovations within the field. In this review, we introduce the most important steps in designing and calibrating polarised stimuli, within the broader context of areas of current research and the applications of new techniques to key questions. Our aim is to provide a constructive guide to help researchers, particularly those with no background in the physics of polarisation, to design robust experiments that are free from confounding factors.

The spectral, spatial and contrast sensitivity of human polarization pattern perception

It is generally believed that humans perceive linear polarized light following its conversion into a luminance signal by diattenuating macular structures. Measures of polarization sensitivity may therefore allow a targeted assessment of macular function. Our aim here was to quantify psychophysical characteristics of human polarization perception using grating and optotype stimuli defined solely by their state of linear polarization. We show: (i) sensitivity to polarization patterns follows the spectral sensitivity of macular pigment; (ii) the change in sensitivity across the central field follows macular pigment density; (iii) polarization patterns are identifiable across a range of contrasts and scales, and can be resolved with an acuity of 15.4 cycles/degree (0.29 logMAR); and (iv) the human eye can discriminate between areas of linear polarization differing in electric field vector orientation by as little as 4.4°. These findings, which support the macular diattenuator model of polarization sensitivity, are unique for vertebrates and approach those of some invertebrates with a well-developed polarization sense. We conclude that this sensory modality extends beyond Haidinger's brushes to the recognition of quantifiable spatial polarization-modulated patterns. Furthermore, the macular origin and sensitivity of human polarization pattern perception makes it potentially suitable for the detection and quantification of macular dysfunction.

A Novel Display System Reveals Anisotropic Polarization Perception in the Motion Vision of the Butterfly Papilio xuthus

While the linear polarization of light is virtually invisible to humans, many invertebrates' eyes can detect it. How this information is processed in the nervous system, and what behavioral function it serves, are in many cases unclear. One reason for this is the technical difficulty involved in presenting images or video containing polarization contrast, particularly if intensity and/or color contrast is also required. In this primarily methods-focused article, we present a novel technique based on projecting video through a synchronously rotating linear polarizer. This approach allows the intensity, angle of polarization, degree of linear polarization, and potentially also color of individual pixels to be controlled independently. We characterize the performance of our system, and then use it to investigate the relationship between polarization and motion vision in the swallowtail butterfly Papilio xuthus. Although this animal has photoreceptors sensitive to four different polarization angles, we find that its motion vision cannot distinguish between diagonally-polarized and unpolarized light. Furthermore, it responds more strongly to vertically-polarized moving objects than horizontally-polarized ones. This implies that Papilio's polarization-based motion detection employs either an unbalanced two-channel (dipolatic) opponent architecture, or possibly a single-channel (monopolatic) scheme without opponent mechanisms.

Bio-Optics and Bio-Inspired Optical Materials

Through the use of the limited materials palette, optimally designed micro- and nanostructures, and tightly regulated processes, nature demonstrates exquisite control of light-matter interactions at various length scales. In fact, control of light-matter interactions is an important element in the evolutionary arms race and has led to highly engineered optical materials and systems. In this review, we present a detailed summary of various optical effects found in nature with a particular emphasis on the materials and optical design aspects responsible for their optical functionality. Using several representative examples, we discuss various optical phenomena, including absorption and transparency, diffraction, interference, reflection and antireflection, scattering, light harvesting, wave guiding and lensing, camouflage, and bioluminescence, that are responsible for the unique optical properties of materials and structures found in nature and biology. Great strides in understanding the design principles adapted by nature have led to a tremendous progress in realizing biomimetic and bioinspired optical materials and photonic devices. We discuss the various micro- and nanofabrication techniques that have been employed for realizing advanced biomimetic optical structures.

Can invertebrates see the e-vector of polarization as a separate modality of light?

The visual world is rich in linearly polarized light stimuli, which are hidden from the human eye. But many invertebrate species make use of polarized light as a source of valuable visual information. However, exploiting light polarization does not necessarily imply that the electric (e)-vector orientation of polarized light can be perceived as a separate modality of light. In this Review, I address the question of whether invertebrates can detect specific e-vector orientations in a manner similar to that of humans perceiving spectral stimuli as specific hues. To analyze e-vector orientation, the signals of at least three polarization-sensitive sensors (analyzer channels) with different e-vector tuning axes must be compared. The object-based, imaging polarization vision systems of cephalopods and crustaceans, as well as the water-surface detectors of flying backswimmers, use just two analyzer channels. Although this excludes the perception of specific e-vector orientations, a two-channel system does provide a coarse, categoric analysis of polarized light stimuli, comparable to the limited color sense of dichromatic, 'color-blind' humans. The celestial compass of insects employs three or more analyzer channels. However, that compass is multimodal, i.e. e-vector information merges with directional information from other celestial cues, such as the solar azimuth and the spectral gradient in the sky, masking e-vector information. It seems that invertebrate organisms take no interest in the polarization details of visual stimuli, but polarization vision grants more practical benefits, such as improved object detection and visual communication for cephalopods and crustaceans, compass readings to traveling insects, or the alert 'water below!' to water-seeking bugs.

Dynamic polarization vision in mantis shrimps

Gaze stabilization is an almost ubiquitous animal behaviour, one that is required to see the world clearly and without blur. Stomatopods, however, only fix their eyes on scenes or objects of interest occasionally. Almost uniquely among animals they explore their visual environment with a series pitch, yaw and torsional (roll) rotations of their eyes, where each eye may also move largely independently of the other. In this work, we demonstrate that the torsional rotations are used to actively enhance their ability to see the polarization of light. Both Gonodactylus smithii and Odontodactylus scyllarus rotate their eyes to align particular photoreceptors relative to the angle of polarization of a linearly polarized visual stimulus, thereby maximizing the polarization contrast between an object of interest and its background. This is the first documented example of any animal displaying dynamic polarization vision, in which the polarization information is actively maximized through rotational eye movements.

Mantis shrimps are known to display large pitch, yaw and torsional eye rotations. Here, the authors show that these eye movements allow mantis shrimp to orientate particular photoreceptors in order to better discriminate the polarization of light.

Perceiving polarization with the naked eye: characterization of human polarization sensitivity

Like many animals, humans are sensitive to the polarization of light. We can detect the angle of polarization using an entoptic phenomenon called Haidinger's brushes, which is mediated by dichroic carotenoids in the macula lutea. While previous studies have characterized the spectral sensitivity of Haidinger's brushes, other aspects remain unexplored. We developed a novel methodology for presenting gratings in polarization-only contrast at varying degrees of polarization in order to measure the lower limits of human polarized light detection. Participants were, on average, able to perform the task down to a threshold of 56%, with some able to go as low as 23%. This makes humans the most sensitive vertebrate tested to date. Additionally, we quantified a nonlinear relationship between presented and perceived polarization angle when an observer is presented with a rotatable polarized light field. This result confirms a previous theoretical prediction of how uniaxial corneal birefringence impacts the perception of Haidinger's brushes. The rotational dynamics of Haidinger's brushes were then used to calculate corneal retardance.We suggest that psychophysical experiments, based upon the perception of polarized light, are amenable to the production of affordable technologies for self-assessment and longitudinal monitoring of visual dysfunctions such as age-related macular degeneration.

Introduction to the Symposium: Towards a General Framework for Predicting Animal Movement Speeds in Nature

Speed of movement is fundamental to animal behavior-defining the intensity of a task, the time needed to complete it, and the likelihood of success-but how does an animal decide how fast to move? Most studies of animal performance measure maximum capabilities, but animals rarely move at their maximum in the wild. It was the goal of our symposium to develop a conceptual framework to explore the choices of speed in nature. A major difference between our approach and previous work is our move toward understanding optimal rather than maximal speeds. In the following series of papers, we provide a starting point for future work on animal movement speeds, including a conceptual framework, a simple optimality model, an evolutionary context, and an exploration of the various biomechanical and energetic constraints on speed. By applying a cross-disciplinary approach to the study of the choice of speed-as we have done here-we can reveal much about the way animals use habitats, interact with conspecifics, avoid predators, obtain food, and negotiate human-modified landscapes.

Out of the blue: the evolution of horizontally polarized signals in Haptosquilla (Crustacea, Stomatopoda, Protosquillidae)

The polarization of light provides information that is used by many animals for a number of different visually guided behaviours. Several marine species, such as stomatopod crustaceans and cephalopod molluscs, communicate using visual signals that contain polarized information, content that is often part of a more complex multi-dimensional visual signal. In this work, we investigate the evolution of polarized signals in species of Haptosquilla, a widespread genus of stomatopod, as well as related protosquillids. We present evidence for a pre-existing bias towards horizontally polarized signal content and demonstrate that the properties of the polarization vision system in these animals increase the signal-to-noise ratio of the signal. Combining these results with the increase in efficacy that polarization provides over intensity and hue in a shallow marine environment, we propose a joint framework for the evolution of the polarized form of these complex signals based on both efficacy-driven (proximate) and content-driven (ultimate) selection pressures.

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.

Polarization distance: a framework for modelling object detection by polarization vision systems

The discrimination of polarized light is widespread in the natural world. Its use for specific, large-field tasks, such as navigation and the detection of water bodies, has been well documented. Some species of cephalopod and crustacean have polarization receptors distributed across the whole visual field and are thought to use polarized light cues for object detection. Both object-based polarization vision systems and large field detectors rely, at least initially, on an orthogonal, two-channel receptor organization. This may increase to three-directional analysis at subsequent interneuronal levels. In object-based and some of the large-field tasks, the dominant e-vector detection axes are often aligned (through eye, head and body stabilization mechanisms) horizontally and vertically relative to the outside world. We develop Bernard and Wehner's 1977 model of polarization receptor dynamics to apply it to the detection and discrimination of polarized objects against differently polarized backgrounds. We propose a measure of 'polarization distance' (roughly analogous to 'colour distance') for estimating the discriminability of objects in polarized light, and conclude that horizontal/vertical arrays are optimally designed for detecting differences in the degree, and not the e-vector axis, of polarized light under natural conditions.