Extraocular perception of polarized light by orienting salamanders

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'.

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.

No response to linear polarization cues in operant conditioning experiments with zebra finches

Many animals can use the polarization of light in various behavioural contexts. Birds are well known to use information from the skylight polarization pattern for orientation and compass calibration. Still, there are few controlled studies of polarization vision in birds, and the majority of them have not been successful in convincingly demonstrating polarization vision. We used a two-alternative forced choice conditioning approach to assess linear polarization vision in male zebra finches in the “visible” spectral range (wavelengths>400 nm). The birds were trained to discriminate colour, brightness, and polarization stimuli presented on either one of two LCD-screens. All birds were able to discriminate the colour and brightness stimuli, but they were unable to discriminate the polarization stimuli. Our results suggest that in the behavioural context studied here, zebra finches are not able to discriminate polarized light stimuli.

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.

Physiological, behavioral, and ecological aspects of migration in reptiles

Seasonal movements between foraging, breeding, and overwintering sites occur in a wide variety of reptile species. Terrestrial snakes, lizards, and turtles migrate short distances (\20 km) between seasonal habitats, whereas fully aquatic marine turtles migrate hundreds to thousands of kilometers between foraging and breeding areas. The purpose of this article is to summarize aspects of migratory physiology and behavior in reptiles, particularly with regards to energetics and sensory mechanisms for navigation and orientation. We discuss the influence of aerobic scope, endurance, and cost of transport on migratory capacity, the effects of temperature and circulating hormones on activity and behavior, and mechanisms of detecting and transducing environmental cues to successfully navigate and orient toward a goal during migration. Topics worthy of further research are highlighted in the text, and we conclude with a discussion of how information on migration patterns of reptiles may be used to manage and conserve threatened populations.

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.

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

Theoretical models implicating specialized photoreceptors in the detection of the geomagnetic field have been the impetus for studying the effects of light on magnetic compass orientation. Magnetic orientation in flies, amphibians and birds has been found to be influenced by light, and in all these groups a shift of approximately 90° in the direction of magnetic compass orientation has been observed under certain wavelengths and/or intensities of light. In the eastern red-spotted newt Notophthalmus viridescens, wavelength-dependent effects of light on magnetic compass orientation appear to result from an antagonistic interaction between short-wavelength (≤450nm) and long-wavelength (≥500nm) photoreception mechanisms. We have demonstrated that at least the short-wavelength input to the newt’s magnetic compass is mediated by extraocular photoreceptors located in or near the pineal organ, and here we present new findings that indicate that the putative long-wavelength mechanism is also associated with pineal photoreceptors. Interestingly, the amphibian pineal organ mediates orientation to both the e-vector of plane-polarized light and the magnetic field. Although the wavelength-dependence of the polarized light orientation in amphibians has not been studied, polarization sensitivity in fishes appears to be mediated by two antagonistic photoreception mechanisms that have similar spectral characteristics to those of the newts’ magnetic compass response. These parallels, along with similarities in the types of receptors that are expected to be involved in light-dependent magnetoreception and polarized light detection, suggest that similar photoreception mechanisms may mediate the light-dependent magnetic and polarized light compasses.