Reviving a neglected celestial underwater polarization compass for aquatic animals

Polarized vision in the eyes of the most effective predators: dragonflies and damselflies (Odonata)

Polarization is a property of light that describes the oscillation of the electric field vector. Polarized light can be detected by many invertebrate animals, and this visual channel is widely used in nature. Insects rely on light polarization for various purposes, such as water detection, improving contrast, breaking camouflage, navigation, and signaling during mating. Dragonflies and damselflies (Odonata) are highly visual insects with polarization sensitivity for water detection and likely also navigation. Thus, odonates can serve as ideal models for investigating the ecology and evolution of polarized light perception. We provide an overview of the current state of knowledge concerning polarized light sensitivity in these insects. Specifically, we review recent findings related to the ecological, morphological, and physiological causes that enable these insects to perceive polarized light and discuss the optical properties responsible for the reflection of polarized light by their bodies and wings. Finally, we identify gaps in the current research and suggest future directions that can help to further advance our knowledge of polarization sensitivity in odonates.

Learning a global underwater geolocalization model with sectoral transformer

Polarization-based underwater geolocalization presents an innovative method for positioning unmanned autonomous devices beneath the water surface, in environments where GPS signals are ineffective. While the state-of-the-art deep neural network (DNN) method achieves high-precision geolocalization based on sun polarization patterns in same-site tasks, its learning-based nature limits its generalizability to unseen sites and subsequently impairs its performance on cross-site tasks, where an unavoidable domain gap between training and test data exists. In this paper, we present an advanced Deep Neural Network (DNN) methodology, which includes a neural network built on a Transformer architecture, similar to the core of large language models such as ChatGPT, and integrates an unscented Kalman filter (UKF) for estimating underwater geolocation using polarization-based images. This combination effectively simulates the sun's daily trajectory, yielding enhanced performance across different locations and quicker inference speeds compared to current benchmarks. Following thorough analysis of over 10 million polarization images from four global locations, we conclude that our proposed technique significantly boosts cross-site geolocalization accuracy by around 28% when contrasted with traditional DNN methods.

Comparative transcriptomics revealed the ecological trap effect of linearly polarized light on Oratosquilla oratoria

Although polarized light can assist many animals in performing special visual tasks, current polarized light pollution (PLP) caused by urban construction has been shown to induce maladaptive behaviors of PL-sensitive animals and change ecological interactions. However, the underlying mechanisms remain unclear. Our previous work hypothesized that linearly polarized light (LPL) is an ecological trap for Oratosquilla oratoria, a common Stomatopoda species in the China Sea. Here we explored the underlying negative effects of artificially LPL on O. oratoria based on comparative transcriptomics. We identified 3616 differentially expressed genes (DEGs) in O. oratoria compound eyes continuous exposed to natural light (NL) and LPL scenarios. In comparison with the NL scenario, a total of 1972 up- and 1644 down- regulated genes were obtained from the O. oratoria compound eyes under LPL scenario, respectively. Furthermore, we performed functional annotation of those DEGs described above and identified 65 DEGs related to phototransduction, reproduction, immunity, and synapse. Based on the functional information, we suspected that continuous LPL exposure could block the light transmission, disrupt the reproductive process, and lead to the progressive failure of the immune response of O. oratoria. In conclusion, this study is the first to systematically describe the negative effects of artificial LPL exposure on O. oratoria at the genetic level, and it can improve the biological conservation theory behind PLP.

Passive Polarized Vision for Autonomous Vehicles: A Review

This review article aims to address common research questions in passive polarized vision for robotics. What kind of polarization sensing can we embed into robots? Can we find our geolocation and true north heading by detecting light scattering from the sky as animals do? How should polarization images be related to the physical properties of reflecting surfaces in the context of scene understanding? This review article is divided into three main sections to address these questions, as well as to assist roboticists in identifying future directions in passive polarized vision for robotics. After an introduction, three key interconnected areas will be covered in the following sections: embedded polarization imaging; polarized vision for robotics navigation; and polarized vision for scene understanding. We will then discuss how polarized vision, a type of vision commonly used in the animal kingdom, should be implemented in robotics; this type of vision has not yet been exploited in robotics service. Passive polarized vision could be a supplemental perceptive modality of localization techniques to complement and reinforce more conventional ones.

Underwater biomimetic orientation method using imaging polarization sensor based on direct sunlight compensation

This paper addresses the challenge of significant interference caused by direct sunlight, which adversely affects the orientation accuracy of underwater imaging polarization sensors (IPS). A novel underwater polarization orientation method is proposed based on direct sunlight compensation. Firstly, based on the polarization transmission model at the water-air interface, the interference mechanism of the underwater direct sunlight polarization detection model was analyzed. The underwater IPS detection model based on direct sunlight compensation is constructed, which uses the weight coefficient of underwater direct sunlight to compensate for the interference on the polarization channel and improve the accuracy of underwater polarization detection models. Furthermore, the analytical solution method for the polarization state information of underwater IPS is proposed, employing the augmented Stokes vectors to construct a linear equation for solving the weight coefficients of direct sunlight and improving the computational efficiency. Finally, an underwater polarization orientation experimental platform is established, and both simulation and actual underwater experiments are conducted. Compared with the traditional methods, the proposed method reduces heading error by an average of 92.53% at different solar altitudes.

Juvenile Atlantic herring (Clupea harengus) use a time-compensated sun compass for orientation

Atlantic herring (Clupea harengus), an ecologically and economically important species in the Northern Hemisphere, shows pronounced seasonal migratory behaviour. To follow distinctive migration patterns over hundreds of kilometers between feeding, overwintering, and spawning grounds, they are probably guided by orientation mechanisms. We tested whether juvenile Western Baltic Spring-Spawning Herring use a sun compass for orientation just before they start leaving their hatching area. Fish were randomly divided into two groups, one of them clock-shifted 6 h backwards, to investigate if they shift their orientation direction accordingly. Individual fish were placed in a circular bowl and their orientation was tested multiple times with the sun as a sole visual orientational cue. Our results show for the first time that juvenile Atlantic herring use a time-compensated sun compass during their migration. Their swimming direction was impaired, but still present, even when the sky was very cloudy, indicating additional orientation capabilities.

Active swimming and transphort by currents observed in Japanese eels (Anguilla japonica) acoustically tracked in the western North Pacific

The mechanisms of oceanic animal migration remain enigmatic. Adult Japanese eels start their long-distance oceanic migration from coastal areas to breed near the West Mariana Ridge. We tracked acoustically tagged eels released in the Kuroshio Current (KC) area near Japan (five silver-phase eels, three of which had impaired swim bladders) and a tropical/subtropical (TS) area near/in the spawning area (two yellow-phase and three silver-phase eels). We analyzed their active swimming and transport by water currents. The strong flow of the KC dominated the eels’ movements in the north, and TS area; their swimming influenced their movements. In the KC area, greater distances were covered at night than during the day, because eels swam in shallower layers with strong currents at night. Three and one eel in the TS and KC area in the upper 400 m showed counterclockwise and clockwise movements around the time of solar culmination, respectively. The meta-analysis showed that eels released at middle latitudes (20°–34° N) generally swam southward through currents, whereas those released at low latitudes (12°–13° N) generally swam northward through currents. Our study suggests the influence of the surrounding current and a potential effect of solar cues on the movements of Japanese eels.

Evidence for the use of a high-resolution magnetic map by a short-distance migrant, the Alpine newt (Ichthyosaura alpestris)

Newts can use spatial variation in the magnetic field (MF) to derive geographic position, but it is unclear how they detect the 'spatial signal', which, over the distances that newts move in a day, is an order of magnitude lower than temporal variation in the MF. Previous work has shown that newts take map readings using their light-dependent magnetic compass to align a magnetite-based 'map detector' relative to the MF. In this study, time of day, location and light exposure (required by the magnetic compass) were varied to determine when newts obtain map information. Newts were displaced from breeding ponds without access to route-based cues to sites where they were held and/or tested under diffuse natural illumination. We found that: (1) newts held overnight at the testing site exhibited accurate homing orientation, but not if transported to the testing site on the day of testing; (2) newts held overnight under diffuse lighting at a 'false testing site' and then tested at a site located in a different direction from their home pond oriented in the home direction from the holding site, not from the site where they were tested; and (3) newts held overnight in total darkness (except for light exposure for specific periods) only exhibited homing orientation the following day if exposed to diffuse illumination during the preceding evening twilight in the ambient MF. These findings demonstrate that, to determine the home direction, newts require access to light and the ambient MF during evening twilight when temporal variation in the MF is minimal.

Bioinspired polarization vision enables underwater geolocalization

With its never-ending blue color, the underwater environment often seems monotonic and featureless. However, to an animal with polarization-sensitive vision, it is anything but bland. The rich repertoire of underwater polarization patterns-a consequence of light's air-to-water transmission and in-water scattering-can be exploited both as a compass and for geolocalization purposes. We demonstrate that, by using a bioinspired polarization-sensitive imager, we can determine the geolocation of an observer based on radial underwater polarization patterns. Our experimental data, recorded at various locations around the world, at different depths and times of day, indicate that the average accuracy of our geolocalization is 61 km, or 6 m of error for every 1 km traveled. This proof-of-concept study of our bioinspired technique opens new possibilities in long-distance underwater navigation and suggests additional mechanisms by which marine animals with polarization-sensitive vision might perform both local and long-distance navigation.

The hydrographic features of anguillid spawning areas: potential signposts for migrating eels

Catadromous anguillid eels (Genus Anguilla) migrate from their freshwater or estuarine habitats to marine spawning areas. Evidence from satellite tagging studies indicates that tropical and temperate eel species exhibit pronounced diel vertical migrations between 150 to 300 m nighttime depths to 600 to 800 m during the day. Collections of eggs and larvae of Japanese eels (A. japonica) show they may spawn at these upper nighttime migration depths. How anguillid eels navigate through the ocean and find their spawning areas remains unknown, so this study describes the salinity, temperature and geostrophic currents between 0 and 800 m depths within two confirmed and three hypothetical anguillid spawning areas during likely spawning seasons. Within all four ocean gyres many eels would encounter subducted 'Subtropical Underwater' during their nighttime ascents that could provide odor plumes as signposts. Four spawning areas are located near the western margins of where subducted water masses form cores of elevated salinities (~35.0 to 36.8) around 150 m depths, while one is found near the center of subduction. Low salinity surface waters and fronts are present in some of the areas above the high-salinity cores. Spawning may occur at temperatures between 16 to 24°C where the thermocline locally deepens. At spawning depths, weak westward currents (~0 to 0.1 m s^-1) prevail, and eastward surface countercurrents are present. Anguillid eels possess acute sensory capabilities to detect these hydrographic features as potential signposts guiding them to where they spawn.

Broadband and polarization reflectors in the lookdown, Selene vomer

Predator evasion in the open ocean is difficult because there are no objects to hide behind. The silvery surface of fish plays an important role in open water camouflage. Various models have been proposed to account for the broadband reflectance by the fish skin that involve one-dimensional variations in the arrangement of guanine crystal reflectors, yet the three-dimensional organization of these guanine platelets have not been well characterized. Here, we report the three-dimensional organization and the optical properties of integumentary guanine platelets in a silvery marine fish, the lookdown (Selene vomer). Our structural analysis and computational modelling show that stacks of guanine platelets with random yaw angles in the fish skin produce broadband reflectance via colour mixing. Optical axes of the guanine platelets and the collagen layer are aligned closely and provide bulk birefringence properties that influence the polarization reflectance by the skin. These data demonstrate how the lookdown preserves or alters polarization states at different incident polarization angles. These optical properties resulted from the organization of these guanine platelets and the collagen layer may have implications for open ocean camouflage in varying light fields.

Contribution of Raman scattering to polarized radiation field in ocean waters

We have implemented Raman scattering in a vector radiative transfer model for coupled atmosphere and ocean systems. A sensitivity study shows that the Raman scattering contribution is greatest in clear waters and at longer wavelengths. The Raman scattering contribution may surpass the elastic scattering contribution by several orders of magnitude at depth. The degree of linear polarization in water is smaller when Raman scattering is included. The orientation of the polarization ellipse shows similar patterns for both elastic and inelastic scattering contributions. As polarimeters and multipolarization-state lidars are planned for future Earth observing missions, our model can serve as a valuable tool for the simulation and interpretation of these planned observations.

Polarization sensitivity as a visual contrast enhancer in the Emperor dragonfly larva, Anax imperator (Leach, 1815)

Polarization sensitivity (PS) is a common feature of invertebrate visual systems. In insects, PS is well known for its use in several different visually guided behaviours, particularly navigation and habitat search. Adult dragonflies use the polarization of light to find water but a role for PS in aquatic dragonfly larvae, a stage that inhabits a very different photic environment to the adults, has not been investigated. The optomotor response of the larvae of the Emperor dragonfly, Anax imperator, was used to determine whether these larvae use PS to enhance visual contrast underwater. Two different light scattering conditions were used to surround the larval animals: a naturalistic horizontally polarized light field and non-naturalistic weakly polarized light field. In both cases these scattering light fields obscured moving intensity stimuli that provoke an optokinetic response in the larvae. Animals were shown to track the movement of a square-wave grating more closely when it was viewed through the horizontally polarized light field, equivalent to a similar increase in tracking ability observed in response to an 8% increase in the intensity contrast of the stimuli. Our results suggest that larval PS enhances the intensity contrast of a visual scene under partially polarized lighting conditions that occur naturally in freshwater environments.

Polarization signaling in swordtails alters female mate preference

Polarization of light, and visual sensitivity to it, is pervasive across aquatic and terrestrial environments. Documentation of invertebrate use of polarized light is widespread from navigation and foraging to species recognition. However, studies demonstrating that polarization body patterning serves as a communication signal (e.g., with evidence of changes in receiver behavior) are rare among invertebrate taxa and conspicuously absent among vertebrates. Here, we investigate polarization-mediated communication by northern swordtails, Xiphophorus nigrensis, using a custom-built videopolarimeter to measure polarization signals and an experimental paradigm that manipulates polarization signals without modifying their brightness or color. We conducted mate choice trials in an experimental tank that illuminates a pair of males with light passed through a polarization filter and a diffusion filter. By alternating the order of these filters between males, we presented females with live males that differed in polarization reflectance by >200% but with intensity and color differences below detection thresholds (∼5%). Combining videopolarimetry and polarization-manipulated mate choice trials, we found sexually dimorphic polarized reflectance and polarization-dependent female mate choice behavior with no polarization-dependent courtship behavior by males. Male swordtails exhibit greater within-body and body-to-background polarization contrast than females, and females preferentially associate with high-polarization-reflecting males. We also found limited support that males increase polarization contrast in social conditions over asocial conditions. Polarization cues in mate choice contexts may provide aquatic vertebrates with enhanced detection of specific display features (e.g., movements, angular information), as well as a signaling mechanism that may enhance detection by intended viewers while minimizing detection by others.

Polarized light sensitivity and orientation in coral reef fish post-larvae

Recent studies of the larvae of coral-reef fishes reveal that these tiny vertebrates possess remarkable swimming capabilities, as well as the ability to orient to olfactory, auditory, and visual cues. While navigation according to reef-generated chemicals and sounds can significantly affect dispersal, the effect is limited to the vicinity of the reef. Effective long-distance navigation requires at least one other capacity–the ability to maintain a bearing using, for example, a sun compass. Directional information in the sun’s position can take the form of polarized-light related cues (i.e., e-vector orientation and percent polarization) and/or non-polarized-light related cues (i.e., the direct image of the sun, and the brightness and spectral gradients). We examined the response to both types of cues using commercially-reared post-larvae of the spine-cheeked anemonefish Premnas biaculeatus. Initial optomotor trials indicated that the post-larval stages are sensitive to linearly polarized light. Swimming directionality was then tested using a Drifting In-Situ Chamber (DISC), which allowed us to examine the response of the post-larvae to natural variation in light conditions and to manipulated levels of light polarization. Under natural light conditions, 28 of 29 post-larvae showed significant directional swimming (Rayleigh’s test p<0.05, R = 0.74±0.23), but to no particular direction. Swimming directionality was positively affected by sky clarity (absence of clouds and haze), which explained 38% of the observed variation. Moreover, post-larvae swimming under fully polarized light exhibited a distinct behavior of tracking the polarization axis, as it rotated along with the DISC. This behavior was not observed under partially-polarized illumination. We view these findings as an indication for the use of sun-related cues, and polarized light signal in specific, by orienting coral-reef fish larvae.

Polaro-cryptic mirror of the lookdown as a biological model for open ocean camouflage

With no object to hide behind in 3D space, the open ocean represents a challenging environment for camouflage. Conventional strategies for reflective crypsis (e.g., standard mirror) are effective against axially symmetric radiance fields associated with high solar altitudes, yet ineffective against asymmetric polarized radiance fields associated with low solar inclinations. Here we identify a biological model for polaro-crypsis. We measured the surface-reflectance Mueller matrix of live open ocean fish (lookdown, Selene vomer) and seagrass-dwelling fish (pinfish, Lagodon rhomboides) using polarization-imaging and modeling polarization camouflage for the open ocean. Lookdowns occupy the minimization basin of our polarization-contrast space, while pinfish and standard mirror measurements exhibit higher contrast values than optimal. The lookdown reflective strategy achieves significant gains in polaro-crypsis (up to 80%) in comparison with nonpolarization sensitive strategies, such as a vertical mirror. Lookdowns achieve polaro-crypsis across solar altitudes by varying reflective properties (described by 16 Mueller matrix elements m(ij)) with incident illumination. Lookdowns preserve reflected polarization aligned with principle axes (dorsal-ventral and anterior-posterior, m(22) = 0.64), while randomizing incident polarization 45° from principle axes (m(33) = -0.05). These reflectance properties allow lookdowns to reflect the uniform degree and angle of polarization associated with high-noon conditions due to alignment of the principle axes and the sun, and reflect a more complex polarization pattern at asymmetrical light fields associated with lower solar elevations. Our results suggest that polaro-cryptic strategies vary by habitat, and require context-specific depolarization and angle alteration for effective concealment in the complex open ocean environment.

Measurements and simulations of polarization states of underwater light in clear oceanic waters

Polarization states of the underwater light field were measured by a hyperspectral and multiangular polarimeter and a video polarimeter under various atmospheric, surface, and water conditions, as well as solar and viewing geometries, in clear oceanic waters near Port Aransas, Texas. Some of the first comprehensive comparisons were made between the measured polarized light, including the degree and angle of linear polarization and linear Stokes parameters (Q and U), and those from Monte Carlo simulations that used concurrently measured water inherent optical properties and particle volume scattering functions as input. For selected wavelengths in the visible spectrum, measured and model-simulated polarization characteristics were found to be consistent in most cases. Measured degree and angle of linear polarization are found to be largely determined by an in-water single-scattering model. Model simulations suggest that the degree of linear polarization (DoLP) at horizontal viewing directions is highly dependent on the viewing azimuth angle for a low solar elevation. This implies that animals can use the DoLP signal for orientation.

Navigation by light polarization in clear and turbid waters

Certain terrestrial animals use sky polarization for navigation. Certain aquatic species have also been shown to orient according to a polarization stimulus, but the correlation between underwater polarization and Sun position and hence the ability to use underwater polarization as a compass for navigation is still under debate. To examine this issue, we use theoretical equations for per cent polarization and electric vector (e-vector) orientation that account for the position of the Sun, refraction at the air-water interface and Rayleigh single scattering. The polarization patterns predicted by these theoretical equations are compared with measurements conducted in clear and semi-turbid coastal sea waters at 2 m and 5 m depth over sea floors of 6 m and 28 m depth. We find that the per cent polarization is correlated with the Sun's elevation only in clear waters. We furthermore find that the maximum value of the e-vector orientation angle equals the angle of refraction only in clear waters, in the horizontal viewing direction, over the deeper sea floor. We conclude that navigation by use of underwater polarization is possible under restricted conditions, i.e. in clear waters, primarily near the horizontal viewing direction, and in locations where the sea floor has limited effects on the light's polarization.

Underwater linear polarization: physical limitations to biological functions

Polarization sensitivity is documented in a range of marine animals. The variety of tasks for which animals can use this sensitivity, and the range over which they do so, are confined by the visual systems of these animals and by the propagation of the polarization information in the aquatic environment. We examine the environmental physical constraints in an attempt to reveal the depth, range and other limitations to the use of polarization sensitivity by marine animals. In clear oceanic waters, navigation that is based on the polarization pattern of the sky appears to be limited to shallow waters, while solar-based navigation is possible down to 200-400 m. When combined with intensity difference, polarization sensitivity allows an increase in target detection range by 70-80% with an upper limit of 15 m for large-eyed animals. This distance will be significantly smaller for small animals, such as plankton, and in turbid waters. Polarization-contrast detection, which is relevant to object detection and communication, is strongly affected by water conditions and in clear waters its range limit may reach 15 m as well. We show that polarization sensitivity may also serve for target distance estimation, when examining point source bioluminescent objects in the photic mesopelagic depth range.

Underwater and surface behavior of homing juvenile northern elephant seals

Northern elephant seals, Mirounga angustirostris, travel between colonies along the west coast of North America and foraging areas in the North Pacific. They also have the ability to return to their home colony after being experimentally translocated. However, the mechanisms of this navigation are not known. Visual information could serve an important role in navigation, either primary or supplementary. We examined the role of visual cues in elephant seal navigation by translocating three seals and recording their heading direction continuously using GPS, and acceleration and geomagnetic data loggers while they returned to the colony. The seals first reached the coast and then proceeded to the colony by swimming along the coast. While underwater the animals exhibited a horizontally straight course (mean net-to-gross displacement ratio=0.94±0.02). In contrast, while at the surface they changed their headings up to 360 deg. These results are consistent with the use of visual cues for navigation to the colony. The seals may visually orient by using landmarks as they swim along the coast. We further assessed whether the seals could maintain a consistent heading while underwater during drift dives where one might expect that passive spiraling during drift dives could cause disorientation. However, seals were able to maintain the initial course heading even while underwater during drift dives where there was spiral motion (to within 20 deg). This behavior may imply the use of non-visual cues such as acoustic signals or magnetic fields for underwater orientation.

The molecular basis of mechanisms underlying polarization vision

The underlying mechanisms of polarization sensitivity (PS) have long remained elusive. For rhabdomeric photoreceptors, questions remain over the high levels of PS measured experimentally. In ciliary photoreceptors, and specifically cones, little direct evidence supports any type of mechanism. In order to promote a greater interest in these fundamental aspects of polarization vision, we examined a varied collection of studies linking membrane biochemistry, protein-protein interactions, molecular ordering and membrane phase behaviour. While initially these studies may seem unrelated to polarization vision, a common narrative emerges. A surprising amount of evidence exists demonstrating the importance of protein-protein interactions in both rhabdomeric and ciliary photoreceptors, indicating the possible long-range ordering of the opsin protein for increased PS. Moreover, we extend this direction by considering how such protein paracrystalline organization arises in all cell types from controlled membrane phase behaviour and propose a universal pathway for PS to occur in both rhabdomeric and cone photoreceptors.

Potential ultraviolet vision in pre-settlement larvae and settled reef fish—A comparison across 23 families

After hatching, larvae of coral reef fishes experience a pelagic phase during which they are diurnal planktivores. It has been suggested that ultraviolet (UV) vision is beneficial for the detection of planktonic prey. Aims were therefore to investigate whether ocular media of pre-settlement reef fish differ from those of respective adults, and whether larvae have UV-transparent ocular media required for UV vision. The ocular media of 84 pre-settlement and 98 adult species belonging to the same families were measured and compared. We suggest that adult lifestyle rather than planktivory in general shapes the ocular media properties of pre-settlement larvae.