The role of extraocular photoreceptors in newt magnetic compass orientation: parallels between light-dependent magnetoreception and polarized light detection in vertebrates

The amphibian magnetic sense(s)

Sensitivity to the earth's magnetic field is the least understood of the major sensory systems, despite being virtually ubiquitous in animals and of widespread interest to investigators in a wide range of fields from behavioral ecology to quantum physics. Although research on the use of magnetic cues by migratory birds, fish, and sea turtles is more widely known, much of our current understanding of the functional properties of vertebrate magnetoreception has come from research on amphibians. Studies of amphibians established the presence of a light-dependent magnetic compass, a second non-light-dependent mechanism involving particles of magnetite and/or maghemite, and an interaction between these two magnetoreception mechanisms that underlies the "map" component of homing. Simulated magnetic displacement experiments demonstrated the use of a high-resolution magnetic map for short-range homing to breeding ponds requiring a sampling strategy to detect weak spatial gradients in the magnetic field despite daily temporal variation at least an order of magnitude greater. Overall, reliance on a magnetic map for short-range homing places greater demands on the underlying sensory detection, processing, and memory mechanisms than comparable mechanisms used by long-distance migrants. Moreover, unlike sea turtles and migratory birds, amphibians are exceptionally well suited to serve as model organisms in which to characterize the molecular and biophysical mechanisms underlying the light-dependent 'quantum compass'.

Effects of non-ionizing electromagnetic fields on flora and fauna, Part 2 impacts: how species interact with natural and man-made EMF

Ambient levels of nonionizing electromagnetic fields (EMF) have risen sharply in the last five decades to become a ubiquitous, continuous, biologically active environmental pollutant, even in rural and remote areas. Many species of flora and fauna, because of unique physiologies and habitats, are sensitive to exogenous EMF in ways that surpass human reactivity. This can lead to complex endogenous reactions that are highly variable, largely unseen, and a possible contributing factor in species extinctions, sometimes localized. Non-human magnetoreception mechanisms are explored. Numerous studies across all frequencies and taxa indicate that current low-level anthropogenic EMF can have myriad adverse and synergistic effects, including on orientation and migration, food finding, reproduction, mating, nest and den building, territorial maintenance and defense, and on vitality, longevity and survivorship itself. Effects have been observed in mammals such as bats, cervids, cetaceans, and pinnipeds among others, and on birds, insects, amphibians, reptiles, microbes and many species of flora. Cyto- and geno-toxic effects have long been observed in laboratory research on animal models that can be extrapolated to wildlife. Unusual multi-system mechanisms can come into play with non-human species - including in aquatic environments - that rely on the Earth's natural geomagnetic fields for critical life-sustaining information. Part 2 of this 3-part series includes four online supplement tables of effects seen in animals from both ELF and RFR at vanishingly low intensities. Taken as a whole, this indicates enough information to raise concerns about ambient exposures to nonionizing radiation at ecosystem levels. Wildlife loss is often unseen and undocumented until tipping points are reached. It is time to recognize ambient EMF as a novel form of pollution and develop rules at regulatory agencies that designate air as 'habitat' so EMF can be regulated like other pollutants. Long-term chronic low-level EMF exposure standards, which do not now exist, should be set accordingly for wildlife, and environmental laws should be strictly enforced - a subject explored in Part 3.

Hydric status influences salinity-dependent water selection in frogs from coastal wetlands

The environment is heterogeneous across spatial and temporal scales, and the behavioural responses required to adjust individuals' needs to resource availability across such variable environments should be under selective pressure. Coastal wetlands are characterized by a diversity of habitats ranging from fresh- to salt water; and individuals occurring in such complex habitats need to adjust their habitat use based on their osmotic status. In this study, we experimentally tested whether an amphibian species (Pelophylax sp.) occurring in coastal wetlands was able to discriminate and select between different salinity concentrations (0, 4, 8 and 12 g.l^-1) and whether hydric status (hydrated versus dehydrated) influenced salinity-dependent water selection. We found that frogs selected water based on salinity differentially between hydrated and dehydrated individuals, with the later favoring lower salinities likely to improve their osmotic status. Interestingly, we highlighted the ability of frogs to select lower salinity before having access to water, suggesting that frogs can assess water salinity without actual contact. In coastal wetlands where salinity of water bodies can dynamically vary through space and time, such behavioural osmoregulation process is potentially a key factor affecting individual movements, habitat choice and thus species distribution. Our study further highlights the importance of salinity-dependent habitat heterogeneity and especially the presence of freshwater environments as structuring factors for the amphibian community.

Effects of non-ionizing electromagnetic fields on flora and fauna, part 1. Rising ambient EMF levels in the environment

Ambient levels of electromagnetic fields (EMF) have risen sharply in the last 80 years, creating a novel energetic exposure that previously did not exist. Most recent decades have seen exponential increases in nearly all environments, including rural/remote areas and lower atmospheric regions. Because of unique physiologies, some species of flora and fauna are sensitive to exogenous EMF in ways that may surpass human reactivity. There is limited, but comprehensive, baseline data in the U.S. from the 1980s against which to compare significant new surveys from different countries. This now provides broader and more precise data on potential transient and chronic exposures to wildlife and habitats. Biological effects have been seen broadly across all taxa and frequencies at vanishingly low intensities comparable to today's ambient exposures. Broad wildlife effects have been seen on orientation and migration, food finding, reproduction, mating, nest and den building, territorial maintenance and defense, and longevity and survivorship. Cyto- and geno-toxic effects have been observed. The above issues are explored in three consecutive parts: Part 1 questions today's ambient EMF capabilities to adversely affect wildlife, with more urgency regarding 5G technologies. Part 2 explores natural and man-made fields, animal magnetoreception mechanisms, and pertinent studies to all wildlife kingdoms. Part 3 examines current exposure standards, applicable laws, and future directions. It is time to recognize ambient EMF as a novel form of pollution and develop rules at regulatory agencies that designate air as 'habitat' so EMF can be regulated like other pollutants. Wildlife loss is often unseen and undocumented until tipping points are reached. Long-term chronic low-level EMF exposure standards, which do not now exist, should be set accordingly for wildlife, and environmental laws should be strictly enforced.

Long-distance navigation and magnetoreception in migratory animals

For centuries, humans have been fascinated by how migratory animals find their way over thousands of kilometres. Here, I review the mechanisms used in animal orientation and navigation with a particular focus on long-distance migrants and magnetoreception. I contend that any long-distance navigational task consists of three phases and that no single cue or mechanism will enable animals to navigate with pinpoint accuracy over thousands of kilometres. Multiscale and multisensory cue integration in the brain is needed. I conclude by raising twenty important mechanistic questions related to long-distance animal navigation that should be solved over the next twenty years.

Insight into shark magnetic field perception from empirical observations

Elasmobranch fishes are among a broad range of taxa believed to gain positional information and navigate using the earth’s magnetic field, yet in sharks, much remains uncertain regarding the sensory receptors and pathways involved, or the exact nature of perceived stimuli. Captive sandbar sharks, Carcharhinus plumbeus were conditioned to respond to presentation of a magnetic stimulus by seeking out a target in anticipation of reward (food). Sharks in the study demonstrated strong responses to magnetic stimuli, making significantly more approaches to the target (p = < 0.01) during stimulus activation (S+) than before or after activation (S−). Sharks exposed to reversible magnetosensory impairment were less capable of discriminating changes to the local magnetic field, with no difference seen in approaches to the target under the S+ and S− conditions (p = 0.375). We provide quantified detection and discrimination thresholds of magnetic stimuli presented, and quantify associated transient electrical artefacts. We show that the likelihood of such artefacts serving as the stimulus for observed behavioural responses was low. These impairment experiments support hypotheses that magnetic field perception in sharks is not solely performed via the electrosensory system, and that putative magnetoreceptor structures may be located in the naso-olfactory capsules of sharks.

The planarian TRPA1 homolog mediates extraocular behavioral responses to near-ultraviolet light

Although light is most commonly thought of as a visual cue, many animals possess mechanisms to detect light outside of the eye for various functions, including predator avoidance, circadian rhythms, phototaxis and migration. Here we confirm that planarians (like Caenorhabditis elegans, leeches and Drosophila larvae) are capable of detecting and responding to light using extraocular photoreception. We found that, when either eyeless or decapitated worms were exposed to near-ultraviolet (near-UV) light, intense wild-type photophobic behaviors were still observed. Our data also revealed that behavioral responses to green wavelengths were mediated by ocular mechanisms, whereas near-UV responses were driven by extraocular mechanisms. As part of a candidate screen to uncover the genetic basis of extraocular photoreception in the planarian species Schmidtea mediterranea, we identified a potential role for a homolog of the transient receptor potential channel A1 (TRPA1) in mediating behavioral responses to extraocular light cues. RNA interference (RNAi) to Smed-TrpA resulted in worms that lacked extraocular photophobic responses to near-UV light, a mechanism previously only identified in Drosophila These data show that the planarian TRPA1 homolog is required for planarian extraocular-light avoidance and may represent a potential ancestral function of this gene. TRPA1 is an evolutionarily conserved detector of temperature and chemical irritants, including reactive oxygen species that are byproducts of UV-light exposure. Our results suggest that planarians possess extraocular photoreception and display an unconventional TRPA1-mediated photophobic response to near-UV light.

Shifted magnetic alignment in vertebrates: Evidence for neural lateralization?

A wealth of evidence provides support for magnetic alignment (MA) behavior in a variety of disparate species within the animal kingdom, in which an animal, or a group of animals, show a tendency to align the body axis in a consistent orientation relative to the geomagnetic field lines. Interestingly, among vertebrates, MA typically coincides with the north-south magnetic axis, however, the mean directional preferences of an individual or group of organisms is often rotated clockwise from the north-south axis. We hypothesize that this shift is not a coincidence, and future studies of this subtle, yet consistent phenomenon may help to reveal some properties of the underlying sensory or processing mechanisms, that, to date, are not well understood. Furthermore, characterizing the fine structure exhibited in MA behaviors may provide key insights to the biophysical substrates mediating magnetoreception in vertebrates. Therefore, in order to determine if a consistent shift is exhibited in taxonomically diverse vertebrates, we performed a meta-analysis on published MA datasets from 23 vertebrate species that exhibited an axial north-south preference. This analysis revealed a significant clockwise shift from the north-south magnetic axis. We summarize and discuss possible competing hypotheses regarding the proximate mechanisms underlying the clockwise shifted MA and conclude that the most likely cause of such a shift would be a lateralization in central processing of magnetic information.

Polarized light modulates light-dependent magnetic compass orientation in birds

Magnetoreception of the light-dependent magnetic compass in birds is suggested to be mediated by a radical-pair mechanism taking place in the avian retina. Biophysical models on magnetic field effects on radical pairs generally assume that the light activating the magnetoreceptor molecules is nondirectional and unpolarized, and that light absorption is isotropic. However, natural skylight enters the avian retina unidirectionally, through the cornea and the lens, and is often partially polarized. In addition, cryptochromes, the putative magnetoreceptor molecules, absorb light anisotropically, i.e., they preferentially absorb light of a specific direction and polarization, implying that the light-dependent magnetic compass is intrinsically polarization sensitive. To test putative interactions between the avian magnetic compass and polarized light, we developed a spatial orientation assay and trained zebra finches to magnetic and/or overhead polarized light cues in a four-arm "plus" maze. The birds did not use overhead polarized light near the zenith for sky compass orientation. Instead, overhead polarized light modulated light-dependent magnetic compass orientation, i.e., how the birds perceive the magnetic field. Birds were well oriented when tested with the polarized light axis aligned parallel to the magnetic field. When the polarized light axis was aligned perpendicular to the magnetic field, the birds became disoriented. These findings are the first behavioral evidence to our knowledge for a direct interaction between polarized light and the light-dependent magnetic compass in an animal. They reveal a fundamentally new property of the radical pair-based magnetoreceptor with key implications for how birds and other animals perceive the Earth's magnetic field.

The Effect of Extremely Low Frequency Alternating Magnetic Field on the Behavior of Animals in the Presence of the Geomagnetic Field

It is known that the geomagnetic field can influence animal migration and homing. The magnetic field detection by animals is known as magnetoreception and it is possible due to two different transduction mechanisms: the first one through magnetic nanoparticles able to respond to the geomagnetic field and the second one through chemical reactions influenced by magnetic fields. Another behavior is the magnetic alignment where animals align their bodies to the geomagnetic field. It has been observed that magnetic alignment of cattle can be disrupted near electric power lines around the world. Experimentally, it is known that alternating magnetic fields can influence living beings, but the exact mechanism is unknown. The parametric resonance model proposes a mechanism to explain that effect on living beings and establishes that, in the presence of a constant magnetic field, molecules associated with biochemical reactions inside cells can absorb resonantly alternating magnetic fields with specific frequencies. In the present paper, a review is made about animal magnetoreception and the effects of alternating magnetic fields in living beings. It is suggested how alternating magnetic fields can interfere in the magnetic alignment of animals and a general conclusion is obtained: alternating magnetic field pollution can affect the magnetic sensibility of animals.

Eumetazoan cryptochrome phylogeny and evolution

Cryptochromes (Crys) are light sensing receptors that are present in all eukaryotes. They mainly absorb light in the UV/blue spectrum. The extant Crys consist of two subfamilies, which are descendants of photolyases but are now involved in the regulation of circadian rhythms. So far, knowledge about the evolution, phylogeny, and expression of cry genes is still scarce. The inclusion of cry sequences from a wide range of bilaterian species allowed us to analyze their phylogeny in detail, identifying six major Cry subgroups. Selective gene inactivations and stabilizations in multiple chordate as well as arthropod lineages suggest several sub- and/or neofunctionalization events. An expression study performed in zebrafish, the model organism harboring the largest amount of crys, showed indeed only partially overlapping expression of paralogous mRNA, supporting gene sub- and/or neofunctionalization. Moreover, the daily cry expression in the adult zebrafish retina indicated varying oscillation patterns in different cell types. Our extensive phylogenetic analysis provides for the first time an overview of cry evolutionary history. Although several, especially parasitic or blind species, have lost all cry genes, crustaceans have retained up to three crys, teleosts possess up to seven, and tetrapods up to four crys. The broad and cyclic expression pattern of all cry transcripts in zebrafish retinal layers implies an involvement in retinal circadian processes and supports the hypothesis of several autonomous circadian clocks present in the vertebrate retina.

Behavioural and physiological mechanisms of polarized light sensitivity in birds

Polarized light (PL) sensitivity is relatively well studied in a large number of invertebrates and some fish species, but in most other vertebrate classes, including birds, the behavioural and physiological mechanism of PL sensitivity remains one of the big mysteries in sensory biology. Many organisms use the skylight polarization pattern as part of a sun compass for orientation, navigation and in spatial orientation tasks. In birds, the available evidence for an involvement of the skylight polarization pattern in sun-compass orientation is very weak. Instead, cue-conflict and cue-calibration experiments have shown that the skylight polarization pattern near the horizon at sunrise and sunset provides birds with a seasonally and latitudinally independent compass calibration reference. Despite convincing evidence that birds use PL cues for orientation, direct experimental evidence for PL sensitivity is still lacking. Avian double cones have been proposed as putative PL receptors, but detailed anatomical and physiological evidence will be needed to conclusively describe the avian PL receptor. Intriguing parallels between the functional and physiological properties of PL reception and light-dependent magnetoreception could point to a common receptor system.

A behavioral perspective on the biophysics of the light-dependent magnetic compass: a link between directional and spatial perception?

In terrestrial organisms, sensitivity to the Earth's magnetic field is mediated by at least two different magnetoreception mechanisms, one involving biogenic ferromagnetic crystals (magnetite/maghemite) and the second involving a photo-induced biochemical reaction that forms long-lasting, spin-coordinated, radical pair intermediates. In some vertebrate groups (amphibians and birds), both mechanisms are present; a light-dependent mechanism provides a directional sense or 'compass', and a non-light-dependent mechanism underlies a geographical-position sense or 'map'. Evidence that both magnetite- and radical pair-based mechanisms are present in the same organisms raises a number of interesting questions. Why has natural selection produced magnetic sensors utilizing two distinct biophysical mechanisms? And, in particular, why has natural selection produced a compass mechanism based on a light-dependent radical pair mechanism (RPM) when a magnetite-based receptor is well suited to perform this function? Answers to these questions depend, to a large degree, on how the properties of the RPM, viewed from a neuroethological rather than a biophysical perspective, differ from those of a magnetite-based magnetic compass. The RPM is expected to produce a light-dependent, 3-D pattern of response that is axially symmetrical and, in some groups of animals, may be perceived as a pattern of light intensity and/or color superimposed on the visual surroundings. We suggest that the light-dependent magnetic compass may serve not only as a source of directional information but also provide a spherical coordinate system that helps to interface metrics of distance, direction and spatial position.

Light-avoidance-mediating photoreceptors tile the Drosophila larval body wall

Photoreceptors for visual perception, phototaxis or light avoidance are typically clustered in eyes or related structures such as the Bolwig organ of Drosophila larvae. Unexpectedly, we found that the class IV dendritic arborization neurons of Drosophila melanogaster larvae respond to ultraviolet, violet and blue light, and are major mediators of light avoidance, particularly at high intensities. These class IV dendritic arborization neurons, which are present in every body segment, have dendrites tiling the larval body wall nearly completely without redundancy. Dendritic illumination activates class IV dendritic arborization neurons. These novel photoreceptors use phototransduction machinery distinct from other photoreceptors in Drosophila and enable larvae to sense light exposure over their entire bodies and move out of danger.

Photoreceptor-based magnetoreception: optimal design of receptor molecules, cells, and neuronal processing

The sensory basis of magnetoreception in animals still remains a mystery. One hypothesis of magnetoreception is that photochemical radical pair reactions can transduce magnetic information in specialized photoreceptor cells, possibly involving the photoreceptor molecule cryptochrome. This hypothesis triggered a considerable amount of research in the past decade. Here, we present an updated picture of the radical-pair photoreceptor hypothesis. In our review, we will focus on insights that can assist biologists in their search for the elusive magnetoreceptors.

Light-dependent magnetic compass orientation in amphibians and insects: candidate receptors and candidate molecular mechanisms

Magnetic compass orientation by amphibians, and some insects, is mediated by a light-dependent magnetoreception mechanism. Cryptochrome photopigments, best known for their role in circadian rhythms, are proposed to mediate such responses. In this paper, we explore light-dependent properties of magnetic sensing at three levels: (i) behavioural (wavelength-dependent effects of light on magnetic compass orientation), (ii) physiological (photoreceptors/photopigment systems with properties suggesting a role in magnetoreception), and (iii) molecular (cryptochrome-based and non-cryptochrome-based signalling pathways that are compatible with behavioural responses). Our goal is to identify photoreceptors and signalling pathways that are likely to play a specialized role in magnetoreception in order to definitively answer the question of whether the effects of light on magnetic compass orientation are mediated by a light-dependent magnetoreception mechanism, or instead are due to input from a non-light-dependent (e.g. magnetite-based) magnetoreception mechanism that secondarily interacts with other light-dependent processes.

Light alters nociceptive effects of magnetic field shielding in mice: intensity and wavelength considerations

Previous experiments with mice have shown that repeated 1 hour daily exposure to an ambient magnetic field-shielded environment induces analgesia (antinociception). The exposures were carried out in the dark (less than 2.0x1016 photonss-1m-2) during the mid-light phase of the diurnal cycle. However, if the mice were exposed in the presence of visible light (2.0x1018 photonss-1m-2, 400-750 nm), then the analgesic effects of shielding were eliminated. Here, we show that this effect of light is intensity and wavelength dependent. Introduction of red light (peak at 635 nm) had little or no effect, presumably because mice do not have photoreceptors sensitive to red light above 600 nm in their eyes. By contrast, introduction of ultraviolet light (peak at 405 nm) abolished the effect, presumably because mice do have ultraviolet A receptors. Blue light exposures (peak at 465 nm) of different intensities demonstrate that the effect has an intensity threshold of approximately 12% of the blue light in the housing facility, corresponding to 5x1016 photonss-1m-2 (integral). This intensity is similar to that associated with photoreceptor-based magnetoreception in birds and in mice stimulates photopic/cone vision. Could the detection mechanism that senses ambient magnetic fields in mice be similar to that in bird navigation?

Magnetic compass orientation by larval Drosophila melanogaster

We report evidence for magnetic compass orientation by larval Drosophila melanogaster. Groups of larvae were exposed from the time of hatching to directional ultraviolet (365nm) light emanating from one of four magnetic directions. Larvae were then tested individually on a circular agar plate under diffuse light in one of four magnetic field alignments. The larvae exhibited magnetic compass orientation in a direction opposite that of the light source in training. Evidence for a well-developed magnetic compass in a larval insect that moves over distances of at most a few tens of centimeters has important implications for understanding the adaptive significance of orientation mechanisms like the magnetic compass. Moreover, the development of an assay for studying magnetic compass orientation in larval D. melanogaster will make it possible to use a wide range of molecular genetic techniques to investigate the neurophysiological, biophysical, and molecular mechanisms underlying the magnetic compass.

Robust hierarchical state-space models reveal diel variation in travel rates of migrating leatherback turtles

1. Biological and statistical complexity are features common to most ecological data that hinder our ability to extract meaningful patterns using conventional tools. Recent work on implementing modern statistical methods for analysis of such ecological data has focused primarily on population dynamics but other types of data, such as animal movement pathways obtained from satellite telemetry, can also benefit from the application of modern statistical tools. 2. We develop a robust hierarchical state-space approach for analysis of multiple satellite telemetry pathways obtained via the Argos system. State-space models are time-series methods that allow unobserved states and biological parameters to be estimated from data observed with error. We show that the approach can reveal important patterns in complex, noisy data where conventional methods cannot. 3. Using the largest Atlantic satellite telemetry data set for critically endangered leatherback turtles, we show that the diel pattern in travel rates of these turtles changes over different phases of their migratory cycle. While foraging in northern waters the turtles show similar travel rates during day and night, but on their southward migration to tropical waters travel rates are markedly faster during the day. These patterns are generally consistent with diving data, and may be related to changes in foraging behaviour. Interestingly, individuals that migrate southward to breed generally show higher daytime travel rates than individuals that migrate southward in a non-breeding year. 4. Our approach is extremely flexible and can be applied to many ecological analyses that use complex, sequential data.

Light-dependent magnetoreception: quantum catches and opponency mechanisms of possible photosensitive molecules

Dozens of experiments on magnetosensitive, migratory birds have shown that their magnetic orientation behavior depends on the spectrum of light under which they are tested. However, it is not certain whether this is due to a direct effect on the magnetoreceptive system and which photosensitive molecules may be involved. We examined 62 experiments of light-dependent magnetoreception in three crepuscular and nocturnal migrants (48 for the European robin Erithacus rubecula, ten for the silvereye Zosterops lateralis, and four on the garden warbler Sylvia borin). For each experiment, we calculated the relative quantum catches of seven of the eight known photosensitive molecules found in the eyes of passerine birds: a short- (SW), medium- (MW) and long-wavelength (LW) cone pigment, rhodopsin, melanopsin, and cryptochrome in its fully-oxidized and semiquinone state. The following five opponency processes were also calculated: LW-SW, LW-MW, MW-SW, LW-(MW+SW), and cryptochrome-semiquinone. While the results do not clearly show which receptor system may be responsible for magnetoreception, it suggests several candidates that may inhibit the process. The two significant inhibitors of magnetoreceptive behavior were overall irradiances (from 400 to 700 nm) higher than those found at sunset and high quantum catch by the LW receptor. The results were also consistent with the hypothesis that high quantum catch by the semiquinone form of cryptochrome inhibits magnetoreception. The opponency mechanism that best separated oriented from non-oriented behavior was LW-MW, where a difference above a certain level inhibited orientation. Certain regions of experimental spectral space have been over-sampled, while large regions have not been sampled at all, including: (1) from 440 to 500 nm at all irradiance levels, (2) for wavelengths longer than 570 nm from 10(12) to 3x10(12) photons s(-1) cm(-2) and (3) for wavelengths less than 560 nm from 10(12) to 3x10(12) photons s(-1) cm(-2) and below 5x10(11) photons s(-1) cm(-2). Experiments under these conditions are needed to draw further conclusions.

The optics of vertebrate photoreceptors: anisotropy and form birefringence

The optics of vertebrate photoreceptors have been investigated with specific reference to the effect of form birefringence. The complex dielectric tensor of the lamellar-like outer segment structure has been derived, allowing the transverse spectral absorbance to be calculated for different incident polarizations. These results were used to calculate the changes in the cellular dichroic ratio as a function of both the volume occupied by the bilayers and the real and complex parts of the intrinsic birefringence of the bilayers. Physiologically realistic values of these parameters show the cellular dichroic ratio to be greater than the bilayer dichroic ratio by a factor of approximately 1.3. Furthermore, the calculations of spectral absorbance indicate that form birefringence may affect measurements of optical density in transversely orientated outer segments.

Occupational magnetic field exposure and melatonin: Interaction with light-at-night

The evidence of magnetic field (MF) effects on melatonin production in humans is limited and inconsistent. Part of the inconsistencies might be explained by findings suggesting interaction with light in pineal responses to MFs. To test this hypothesis, we reanalyzed data from a previously published study on 6-hydroxy melatonin sulfate (6-OHMS) excretion in women occupationally exposed to extremely low-frequency MFs. Based on questionnaire data on exposure to light-at-night (LAN), and measurement-based MF data, the 60 women were classified to four groups: no MF, no LAN; MF, no LAN; no MF, LAN; MF, LAN. The lowest excretion of 6-OHMS was observed in the group of women who were exposed to both MF and LAN, and the differences between the four groups were significant (P < .0001). The result is based on low numbers, but supports the hypothesis that daytime occupational exposure to MF enhances the effects of nighttime light exposure on melatonin production.

Magnetic orientation and magnetoreception in birds and other animals

Animals use the geomagnetic field in many ways: the magnetic vector provides a compass; magnetic intensity and/or inclination play a role as a component of the navigational 'map', and magnetic conditions of certain regions act as 'sign posts' or triggers, eliciting specific responses. A magnetic compass is widespread among animals, magnetic navigation is indicated e.g. in birds, marine turtles and spiny lobsters and the use of magnetic 'sign posts' has been described for birds and marine turtles. For magnetoreception, two hypotheses are currently discussed, one proposing a chemical compass based on a radical pair mechanism, the other postulating processes involving magnetite particles. The available evidence suggests that birds use both mechanisms, with the radical pair mechanism in the right eye providing directional information and a magnetite-based mechanism in the upper beak providing information on position as component of the 'map'. Behavioral data from other animals indicate a light-dependent compass probably based on a radical pair mechanism in amphibians and a possibly magnetite-based mechanism in mammals. Histological and electrophysiological data suggest a magnetite-based mechanism in the nasal cavities of salmonid fish. Little is known about the parts of the brain where the respective information is processed.

Extraocular photoreception and circadian entrainment in nonmammalian vertebrates

In mammals both the regulation of circadian rhythms and photoperiodic responses depend exclusively upon photic information provided by the lateral eyes; however, nonmammalian vertebrates can also rely on multiple extraocular photoreceptors to perform the same tasks. Extraocular photoreceptors include deep brain photoreceptors located in several distinct brain sites and the pineal complex, involving intracranial (pineal and parapineal) and extracranial (frontal organ and parietal eye) components. This review updates the research field of the most recent acquisitions concerning the roles of extraocular photoreceptors on circadian physiology and behavior, particularly photic entrainment and sun compass orientation.

Use of multiple orientation cues by juvenile loggerhead sea turtles Caretta caretta

Although the orientation cues used by hatchling sea turtles have been studied extensively, little is known about the mechanisms of orientation and navigation that guide older turtles. To investigate the orientation cues used by juvenile loggerheads Caretta caretta L., captured turtles were tethered in a water-filled arena located outdoors. Turtles tested under these conditions established and maintained headings in specific directions in the absence of wave cues, familiar landmarks and chemical gradients. Distorting the magnetic field around the anterior part of a turtle's body did not disrupt orientation if vision remained unimpaired. Similarly, eliminating visual cues by attaching frosted goggles did not disrupt orientation if the magnetic environment was undisturbed. However, when turtles experienced a simultaneous disruption of magnetic and visual cues, their orientation was altered. These results imply that sea turtles, like migratory birds and homing pigeons, are able to maintain headings using multiple sources of directional information.

Behavioural investigation of polarisation sensitivity in the Japanese quail (Coturnix coturnix japonica) and the European starling (Sturnus vulgaris)

Many animals have sensitivity to the e-vector of linearly polarised light, which may assist in visually mediated behaviours such as navigation, signalling and foraging. However, it is still controversial as to whether birds possess polarisation sensitivity. Several studies have found that altering the polarisation patterns of the broad visual field surrounding birds alters their intended migratory orientation. However, electrophysiological tests have failed to elicit evidence for polarisation sensitivity in birds, and the mechanism by which birds might perceive polarised light is unknown. In this experiment, we trained Japanese quail and European starlings to discriminate stimuli differing in their polarisation pattern. Although both quail and starlings were able to discriminate stimuli in which the stimulus sub-components either differed or had the same radiant intensity (the control task), they were unable to discriminate stimuli in which the e-vector orientations of the stimulus sub-components either differed by 90 degrees or had the same angle of polarisation. The birds' successful performance on the control task, but failure to complete the polarisation task, demonstrated that they had all the necessary cognitive abilities to make the discrimination except sensitivity to angle of polarisation. We conclude that quail and starlings are unable to use polarisation cues in this foraging task.

`Fixed-axis' magnetic orientation by an amphibian: non-shoreward-directed compass orientation, misdirected homing or positioning a magnetite-based map detector in a consistent alignment relative to the magnetic field?

Experiments were carried out to investigate the earlier prediction that prolonged exposure to long-wavelength (&gt;500 nm) light would eliminate homing orientation by male Eastern red-spotted newts Notophthalmus viridescens. As in previous experiments, controls held in outdoor tanks under natural lighting conditions and tested in a visually uniform indoor arena under full-spectrum light were homeward oriented. As predicted, however,newts held under long-wavelength light and tested under either full-spectrum or long-wavelength light (&gt;500 nm) failed to show consistent homeward orientation. The newts also did not orient with respect to the shore directions in the outdoor tanks in which they were held prior to testing. Unexpectedly, however, the newts exhibited bimodal orientation along a more-or-less `fixed' north-northeast—south-southwest magnetic axis. The orientation exhibited by newts tested under full-spectrum light was indistinguishable from that of newts tested under long-wavelength light,although these two wavelength conditions have previously been shown to differentially affect both shoreward compass orientation and homing orientation. To investigate the possibility that the `fixed-axis' response of the newts was mediated by a magnetoreception mechanism involving single-domain particles of magnetite, natural remanent magnetism (NRM) was measured from a subset of the newts. The distribution of NRM alignments with respect to the head—body axis of the newts was indistinguishable from random. Furthermore, there was no consistent relationship between the NRM of individual newts and their directional response in the overall sample. However, under full-spectrum, but not long-wavelength, light, the alignment of the NRM when the newts reached the 20 cm radius criterion circle in the indoor testing arena (estimated by adding the NRM alignment measured from each newt to its magnetic bearing) was non-randomly distributed. These findings are consistent with the earlier suggestion that homing newts use the light-dependent magnetic compass to align a magnetite-based `map detector'when obtaining the precise measurements necessary to derive map information from the magnetic field. However, aligning the putative map detector does not explain the fixed-axis response of newts tested under long-wavelength light. Preliminary evidence suggests that, in the absence of reliable directional information from the magnetic compass (caused by the 90° rotation of the response of the magnetic compass under long-wavelength light), newts may resort to a systematic sampling strategy to identify alignment(s) of the map detector that yields reliable magnetic field measurements.

Shedding light on vertebrate magnetoreception

We review the challenges and recent progress in elucidating the physiological basis of animal magnetoreception. Behavioral and theoretical studies suggest a link between photoreception and magnetoreception in some animals. Neurophysiological studies have the potential to prove this link and identify the location of and the mechanism underlying the magnetoreception system.