Extraocular light perception in photoperiodic responses of the White-crowned sparrow (Zonotrichia leucophrys) and of the Golden-crowned sparrow (Z. atricapilla)

Summary Testicular maturation, migratory restlessness (Zugunruhe) and fattening are induced in many species of birds by long day-lengths in Spring. We have tested the hypothesis that extraocular photoreceptors located in the brain are involved in mediating these photoperiodic responses in White-crowned sparrows (Zonotrichia leucophrys) and Golden-crowned sparrows (Z. atricapilla). Our approach consisted in reducing the amount of light penetrating to the brain with either black India ink injected under the skin of the head (Golden-crowned sparrows) or by covering the entire head (except eyes and beak) with a black collodion hood (White-crowned sparrows). Birds treated in these ways showed significantly less testicular growth, Zugunruhe and premigratory fattening when placed under a 16L-8D photoperiod than control birds which did not have their brains shielded from light. However, even when the bird brains were shielded from light and although light intensity was close to threshold level, some testicular growth, Zugunruhe and fattening did occur. We conclude that extraocular photoreceptors are involved in the control of the three photoperiodic responses studied, but that the eyes are possibly of significance as well.Supported by NASA Headquarters grant NGR-05-020-391 to Dr. C. S. Pittendrigh     

Nonvisual photoreceptors of the deep brain, pineal organs and retina

The role of the nonvisual photoreception is to synchronise periodic functions of living organisms to the environmental light periods in order to help survival of various species in different biotopes. In vertebrates, the so-called deep brain (septal and hypothalamic) photoreceptors, the pineal organs (pineal- and parapineal organs, frontal- and parietal eye) and the retina (of the "lateral" eye) are involved in the light-based entrain of endogenous circadian clocks present in various organs. In humans, photoperiodicity was studied in connection with sleep disturbances in shift work, seasonal depression, and in jet-lag of transmeridional travellers. In the present review, experimental and molecular aspects are discussed, focusing on the histological and histochemical basis of the function of nonvisual photoreceptors. We also offer a view about functional changes of these photoreceptors during pre- and postnatal development as well as about its possible evolution. Our scope in some points is different from the generally accepted views on the nonvisual photoreceptive systems. The deep brain photoreceptors are hypothalamic and septal nuclei of the periventricular cerebrospinal fluid (CSF)-contacting neuronal system. Already present in the lancelet and representing the most ancient type of vertebrate nerve cells ("protoneurons"), CSF-contacting neurons are sensory-type cells sitting in the wall of the brain ventricles that send a ciliated dendritic process into the CSF. Various opsins and other members of the phototransduction cascade have been demonstrated in telencephalic and hypothalamic groups of these neurons. In all species examined so far, deep brain photoreceptors play a role in the circadian and circannual regulation of periodic functions. Mainly called pineal "glands" in the last decades, the pineal organs actually represent a differentiated form of encephalic photoreceptors. Supposed to be intra- and extracranially outgrown groups of deep brain photoreceptors, pineal organs also contain neurons and glial elements. Extracranial pineal organs of submammalians are cone-dominated photoreceptors sensitive to different wavelengths of light, while intracranial pineal organs predominantly contain rod-like photoreceptor cells and thus scotopic light receptors. Vitamin B-based light-sensitive cryptochromes localized immunocytochemically in some pineal cells may take part in both the photoreception and the pacemaker function of the pineal organ. In spite of expressing phototransduction cascade molecules and forming outer segment-like cilia in some species, the mammalian pineal is considered by most of the authors as a light-insensitive organ. Expression of phototransduction cascade molecules, predominantly in young animals, is a photoreceptor-like characteristic of pinealocytes in higher vertebrates that may contribute to a light-percepting task in the perinatal entrainment of rhythmic functions. In adult mammals, adrenergic nerves--mediating daily fluctuation of sympathetic activity rather than retinal light information as generally supposed--may sustain circadian periodicity already entrained by light perinatally. Altogether three phases were supposed to exist in pineal entrainment of internal pacemakers: an embryological synchronization by light and in viviparous vertebrates by maternal effects (1); a light-based, postnatal entrainment (2); and in adults, a maintenance of periodicity by daily sympathetic rhythm of the hypothalamus. In addition to its visual function, the lateral eye retina performs a nonvisual task. Nonvisual retinal light perception primarily entrains genetically-determined periodicity, such as rod-cone dominance, EEG rhythms or retinomotor movements. It also influences the suprachiasmatic nucleus, the primary pacemaker of the brain. As neither rods nor cones seem to represent the nonvisual retinal photoreceptors, the presence of additional photoreceptors has been supposed. Cryptochrome 1, a photosensitive molecule identified in retinal nerve cells and in a subpopulation of retinal photoreceptors, is a good candidate for the nonvisual photoreceptor molecule as well as for a member of pacemaker molecules in the retina. When comparing various visual and nonvisual photoreceptors, transitory, "semi visual" (directional) light-perceptive cells can be detected among them, such as those in the parietal eye of reptiles. Measuring diffuse light intensity of the environment, semivisual photoreceptors also possess some directional light perceptive capacity aided by complementary lens-like structures, and screening pigment cells. Semivisual photoreception in aquatic animals may serve for identifying environmental areas of suitable illumination, or in poikilotermic terrestrial species for measuring direct solar irradiation for thermoregulation. As directional photoreceptors were identified among nonvisual light perceptive cells in the lancelet, but eyes are lacking, an early appearance of semivisual function, prior to a visual one (nonvisual --> semivisual --> visual?) in the vertebrate evolution was supposed.     

Light sensitivity of the photoperiodic response system in higher vertebrates: wavelength and intensity effects

Most species use daily light in one way or the other in regulation of their short and/or long term activities. Light is perceived by pigment(s) present in the retinal (RP) and/or extra-retinal photoreceptors (ERPs). ERPs may be located at various sites in the body but in non-mammalian vertebrates they are found predominantly in the pineal body and hypothalamic region of the brain, Light radiations directly penetrate brain tissues to reach and stimulate the hypothalamic (deep-brain) photoreceptors. How does light information finally reach to the clock is not fully understood in many vertebrate groups? In mammals, however, the light information from the retina to the clock (the hypothalamic suprachiasmatic nuclei, SCN) is relayed through the retino-hypothalamic tract (RHT) which originates from the retinal ganglion cells, and through the geniculo-hypothalamic tract (GHT) which originates from the photically responsive cells of a portion of the lateral geniculate nucleus (LGN), called the intergeniculate leaflet (IGL). A response to light (the photoperiodic response) is the result of the interpretation of light information by the photoperiodic system. Apart from the duration, the animals use the gradual shifts in the intensity and wavelength of daily light to regulate their photoperiodic clock system. The wavelengths to which photoreceptors are maximally sensitive or the wavelengths which have greater access to the photoreceptors can induce a maximal response. There can also be differential effects of wavelength and intensity of light on circadian process(es) involved in the entrainment and induction of the photoperiodic clock. This may have some adaptive implications. Entrainment to daily light-dark (LD) cycle may be achieved at dawn or dusk, depending whether the animal is day- or night-active, when there is relatively low intensity of light. By contrast, photoperiodic induction in many species occurs during long days of spring and summer when plenty of daylight at higher intensity is available later in the day.     

BRAIN PHOTORECEPTOR PATHWAYS CONTRIBUTING TO CIRCADIAN RHYTHMICITY IN CRAYFISH

Freshwater crayfish have three known photoreceptive systems: the compound eyes, extraretinal brain photoreceptors, and caudal photoreceptors. The primary goal of the work described here was to explore the contribution of the brain photoreceptors to circadian locomotory activity and define some of the underlying neural pathways. Immunocytochemical studies of the brain photoreceptors in the parastacid (southern hemisphere) crayfish Cherax destructor reveal their expression of the blue light-sensitive photopigment cryptochrome and the neurotransmitter histamine. The brain photoreceptors project to two small protocerebral neuropils, the brain photoreceptor neuropils (BPNs), where they terminate among fibers expressing the neuropeptide pigment-dispersing hormone (PDH), a signaling molecule in arthropod circadian systems. Comparable pathways are also described in the astacid (northern hemisphere) crayfish Procambarus clarkii. Despite exhibiting markedly different diurnal locomotor activity rhythms, removal of the compound eyes and caudal photoreceptors in both C. destructor and P. clarkii (leaving the brain photoreceptors intact) does not abolish the normal light/dark activity cycle in either species, nor prevent the entrainment of their activity cycles to phase shifts of the light/dark period. These results suggest, therefore, that crayfish brain photoreceptors are sufficient for the entrainment of locomotor activity rhythms to photic stimuli, and that they can act in the absence of the compound eyes and caudal photoreceptors. We also demonstrate that the intensity of PDH expression in the BPNs varies in phase with the locomotor activity rhythm of both crayfish species. Together, these findings suggest that the brain photoreceptor cells can function as extraretinal circadian photoreceptors and that the BPN represents part of an entrainment pathway synchronizing locomotor activity to environmental light/dark cycles, and implicating the neuropeptide PDH in these functions. (Author correspondence: bbeltz@wellesley.edu)     

Modeling the role of mid-wavelength cones in circadian responses to light

Nonvisual responses to light, such as photic entrainment of the circadian clock, involve intrinsically light-sensitive melanopsin-expressing ganglion cells as well as rod and cone photoreceptors. However, previous studies have been unable to demonstrate a specific contribution of cones in the photic control of circadian responses to light. Using a mouse model that specifically lacks mid-wavelength (MW) cones we show that these photoreceptors play a significant role in light entrainment and in phase shifting of the circadian oscillator. The contribution of MW cones is mainly observed for light exposures of short duration and toward the longer wavelength region of the spectrum, consistent with the known properties of this opsin. Modeling the contributions of the various photoreceptors stresses the importance of considering the particular spectral, temporal, and irradiance response domains of the photopigments when assessing their role and contribution in circadian responses to light.     

Emerging Roles and New Paradigms in Signaling Mechanisms of Plant Cryptochromes

Analogous to the opsin-based receptors in animals, plants contain a diverse and elaborate set of photoreceptors to perceive a much wider spectrum of light and adapt to varying light conditions. Cryptochromes (CRYs), the blue/UV-A light sensing receptors, represent one such class of photoreceptors found ubiquitously in plants. Although structurally similar to DNA photolyases which could repair UV-induced DNA damage, photoactivated CRYs, instead, initiate signal transduction pathways, which lead to gene expression changes and eventually more overt photomorphogenic responses. Apart from the well-established roles of CRYs in regulating seedling de-etiolation, flowering time, and circadian clock, recent reports have highlighted their roles in controlling other aspects of plant development as well. This review attempts to describe the novel/atypical roles of CRYs that have emerged in the past few years, and also present an account of the various signaling components involved in CRY signal transduction pathway.     

Light intensity and schooling behaviour in larval gulf menhaden

Schooling behaviour was examined in larval gulf menhaden Brevoortia patronus under decreasing and increasing light intensities and related to movements of retinal photoreceptors and pigment. Nearest neighbour angles and nearest neighbour distances between fish were inversely related to light intensity and were independent of the direction of intensity change. Swimming speeds of individual fish were directly related to light intensity when intensity was decreased but did not increase when intensity was raised. The changes in nearest neighbour angles and distances more closely paralleled movements of retinal pigment cells than cone cell migration, suggesting that rod photoreceptors play a greater role than cones in determining the threshold light intensity for schooling. A general model is developed relating the threshold light intensity for schooling to eye diameter for a number of different species and the implications of this relationship are discussed.     

Angular and spectral sensitivity of fly photoreceptors. III. Dependence on the pupil mechanism in the blowfly<Emphasis Type="Italic"> Calliphora</Emphasis>

A wave optics model for the facet lens-rhabdomere system of fly eyes is used to analyze the dependence of the angular and spectral sensitivity of R1–6 photoreceptors on the pupil mechanism. This assembly of light-absorbing pigment granules in the soma interacts with the waveguide modes propagating in the rhabdomere. A fly rhabdomere carries two modes in the middle wavelength range and four modes at short wavelengths, depending on the rhabdomere diameter and the angle of the incident light flux. The extension of the mode to outside the rhabdomere strongly depends on wavelength, and this dependence plays a determinant role in the light control function of the pupil. The absorbance spectrum of the pigment in the pupil granules is severely depressed at short wavelengths by waveguide effects, resulting in a distinct blue peak. Accordingly, pupil closure suppresses the photoreceptors spectral sensitivity much more in the blue-green than in the UV. The pupil only narrows the angular sensitivity at short wavelengths. The geometrical size of the rhabdomere governs the angular sensitivity of fly photoreceptors in the dark-adapted state, but diffraction takes over in the fully light-adapted state.     

Metarhodopsin control by arrestin,light-filtering screening pigments,and visual pigment turnover in invertebrate microvillar photoreceptors

The visual pigments of most invertebrate photoreceptors have two thermostable photo-interconvertible states, the ground state rhodopsin and photo-activated metarhodopsin, which triggers the phototransduction cascade until it binds arrestin. The ratio of the two states in photoequilibrium is determined by their absorbance spectra and the effective spectral distribution of illumination. Calculations indicate that metarhodopsin levels in fly photoreceptors are maintained below ~35% in normal diurnal environments, due to the combination of a blue-green rhodopsin, an orange-absorbing metarhodopsin and red transparent screening pigments. Slow metarhodopsin degradation and rhodopsin regeneration processes further subserve visual pigment maintenance. In most insect eyes, where the majority of photoreceptors have green-absorbing rhodopsins and blue-absorbing metarhodopsins, natural illuminants are predicted to create metarhodopsin levels greater than 60% at high intensities. However, fast metarhodopsin decay and rhodopsin regeneration also play an important role in controlling metarhodopsin in green receptors, resulting in a high rhodopsin content at low light intensities and a reduced overall visual pigment content in bright light. A simple model for the visual pigment–arrestin cycle is used to illustrate the dependence of the visual pigment population states on light intensity, arrestin levels and pigment turnover.     

Light emitting diode(LED)‐based trapping of the greenhouse whitefly (Trialeurodes vaporariorum)

Visual traps like yellow sticky card traps are used for monitoring and control of the greenhouse whitefly (Trialeurodes vaporariorum). However, reflected intensity (brightness) and hence, attractiveness depend on the ambient light conditions, and the colour (wavelength) might not fit with the sensitivity of whitefly photoreceptors. The use of light emitting diodes (LEDs) is a promising approach to increase the attractiveness, specificity and adaptability of visual traps. We constructed LED‐based visual traps equipped with blue and green high‐power LEDs and ultraviolet (UV) standard LEDs according to the putative spectral sensitivities of the insects' photoreceptors. In a series of small‐scale choice and no‐choice recapture experiments, the factors time of day as well as light intensity and light quality (colour) of LED traps were studied in terms of attractiveness compared to yellow traps without LEDs. Green LED traps (517 nm peak wavelength) were comparably attractive in no‐choice experiments but clearly preferred over yellow traps in all choice experiments. The time of day had a clear effect on the flight activity of the whiteflies and thereby on the trapping success. Blue LEDs (474 nm) suppressed the attractiveness of the light traps when combined with green LEDs suggesting that a yet undetected photoreceptor, sensitive for blue light, and an inhibiting interaction with the green receptor, might exist in T. vaporariorum. In choice experiments between LED traps emitting green light only or in combination with UV (368 nm), the green‐UV combination was preferred. In no‐choice night‐time experiments, UV LEDs considerably increased whitefly flight activity and efficacy of trapping. Most likely, the reason for the modifying effect of UV is the stimulating influence on flight activity. In conclusion, it seems that the use of green LEDs alone or in combination with UV LEDs could be an innovative option for improving attractiveness of visual traps.     

Photosensory input pathways in the medicinal leech

Summary The medicinal leech,Hirudo medicinalis possesses two types of photosensory organs: five bilateral pairs of eyes embedded in two longitudinal rows in the dorsal surface of the head, and seven bilateral pairs of sensilla situated in both the dorsal and the ventral surface of each of the 21 body segments. The photoreceptor cells of each eye or sensillum project their axons centrally via a characteristic cephalic or segmental nerve which carries the photosensory input to the brain or to the segmental ganglion. In response to a pulse of light the photoreceptors produce a train of impulses whose frequency first rises to anearly peak and then declines to asteady state plateau at which it remains until the end of the pulse. The amplitude of the early peak response and the level of the steady state plateau rise linearly with the log of the light pulse intensity, but the dynamic range of the early peak response is much narrower than that of the plateau. Both ocular and sensillar photoreceptors adapt to the intensity of interpulse background illumination; the ocular receptors adapt so completely that their level of background activity is nearly independent of the background light intensity, whereas the ventral sensillar photoreceptors adapt incompletely, so that their background activity rises with the background light intensity. Ocular and sensillar photoreceptors make their maximal response to green light at a wavelength of about 540 nm. They are almost insensitive to red and violet light at both extremes of the visible spectrum. The photosensory response of a single eye is directionally selective, whereas that of a single sensillum has much less directional selectivity. Several higher order sensory neurons were identified in the segmental ganglion that receive photosensory input from the sensilla. One of these neurons has the sensillum in the ipsilateral dorso-medial body wall of the same segment as its receptive field and another neuron the bilateral set of ventral sensilla in the body wall of the next posterior segment.We are indebted to Frank S. Werblin for valuable advice and discussions. We thank Kenneth L. Carlock for designing and constructing much of the special electronic equipment used in this study. We also thank Alexander Petruncola for his helpful suggestions regarding the computational analysis of the experimental results and for writing the computer programs used in the processing of the data.This research was supported by Grant No. GB 31933X from the National Science Foundation, and NIH research grant No. GM 17866 and Training Grant No. GM 00829 from the Institute for General Medical Sciences.     

Evolving visual pigments: Hints from the opsin-based proteins in a phylogenetically old “eyeless” invertebrate

Visual pigments are photosensitive receptor proteins that trigger the transduction process producing the visual excitation once they have absorbed photons. In spite of the molecular and morpho-functional complexity that has characterized the development of animal eyes and eyeless photoreceptive systems, opsin-based protein family appears ubiquous along metazoan visual systems. Moreover, in addition to classic rhodopsin photoreceptors, all Metazoa have supplementary non-visual photosensitive structures, mainly located in the central nervous system, that sense light without forming an image and that rather regulate the organism's temporal physiology. The investigation of novel non-visual photopigments exerting extraretinal photoreception is a challenging field in vision research. Here we propose the cnidarian Hydra as a useful tool of investigation for molecular and functional differences between these pigment families. Hydra is the first metazoan owning a nervous system and it is an eyeless invertebrate showing only an extraocular photoreception, as it has no recognized visual or photosensitive structures. In this paper we provide an overview of the molecular and functional features of the opsin-based protein subfamilies and preliminary evidences in a phylogenetically old species of both image-forming and non-visual opsins. Then we give new insights on the molecular biology of Hydra photoreception and on the evolutionary pathways of visual pigments.     

Extraretinal light perception in the sparrow IV. Further evidence that the eyes do not participate in photoperiodic photoreception

Summary Photoperiodic testicular growth in House Sparrows (Passer domesticus) exposed to long days (16 hrs) of orange-red light ({ie205-01}600 nm) is exclusively controlled by extraretinal photoreceptors in the brain; the eyes are not involved. Careful reconsideration of previously published data from this and other bird species does not support a role for the eyes in photoperiodically significant photoreception.     

Eye size in birds and the timing of song at dawn

Why do different species of birds start their dawn choruses at different times? We test the hypothesis that the times at which different species start singing at dawn are related to their visual capability at low light intensities. Birds with large eyes can achieve greater pupil diameters and hence, all other things being equal, greater visual sensitivity and resolution than birds with small eyes. We estimated the maximum pupil diameter of passerine birds by measuring the diameter of the exposed eye surface, and measured the times of the first songs at dawn of songbirds present in different bird communities, and the light intensities at these times. Using phylogenetic comparative analyses, we found that songbirds with large eyes started to sing at lower light intensities (and therefore earlier) than species with smaller eyes. These relationships were stronger when differences in body size were controlled for statistically, and were consistent between two phylogenies and when species were treated as independent data points. Our results therefore provide robust support for the hypothesis that visual capability at low light levels influences the times at which birds start to sing at dawn.     

Retinal damage induced by commercial light emitting diodes (LEDs)

Spectra of “white LEDs” are characterized by an intense emission in the blue region of the visible spectrum, absent in daylight spectra. This blue component and the high intensity of emission are the main sources of concern about the health risks of LEDs with respect to their toxicity to the eye and the retina. The aim of our study was to elucidate the role of blue light from LEDs in retinal damage. Commercially available white LEDs and four different blue LEDs (507, 473, 467, and 449 nm) were used for exposure experiments on Wistar rats. Immunohistochemical stain, transmission electron microscopy, and Western blot were used to exam the retinas. We evaluated LED-induced retinal cell damage by studying oxidative stress, stress response pathways, and the identification of cell death pathways. LED light caused a state of suffering of the retina with oxidative damage and retinal injury. We observed a loss of photoreceptors and the activation of caspase-independent apoptosis, necroptosis, and necrosis. A wavelength dependence of the effects was observed. Phototoxicity of LEDs on the retina is characterized by a strong damage of photoreceptors and by the induction of necrosis.     

Colour in the eyes of insects

Many insect species have darkly coloured eyes, but distinct colours or patterns are frequently featured. A number of exemplary cases of flies and butterflies are discussed to illustrate our present knowledge of the physical basis of eye colours, their functional background, and the implications for insect colour vision. The screening pigments in the pigment cells commonly determine the eye colour. The red screening pigments of fly eyes and the dorsal eye regions of dragonflies allow stray light to photochemically restore photoconverted visual pigments. A similar role is played by yellow pigment granules inside the photoreceptor cells which function as a light-controlling pupil. Most insect eyes contain black screening pigments which prevent stray light to produce background noise in the photoreceptors. The eyes of tabanid flies are marked by strong metallic colours, due to multilayers in the corneal facet lenses. The corneal multilayers in the gold-green eyes of the deer fly Chrysops relictus reduce the lens transmission in the orange-green, thus narrowing the sensitivity spectrum of photoreceptors having a green absorbing rhodopsin. The tapetum in the eyes of butterflies probably enhances the spectral sensitivity of proximal long-wavelength photoreceptors. Pigment granules lining the rhabdom fine-tune the sensitivity spectra.     

The pineal eye inXenopus laevis embryos and larvae: A photoreceptor with a direct excitatory effect on behaviour

Summary Behavioural experiments demonstrate that embryos and young larval stages ofXenopus laevis when exposed to a sudden drop in light intensity may show locomotor activity. Recordings from motoneurone axons demonstrate that the integrity of the pineal eye photoreceptors are essential for this response to occur. Thus the pineal has a direct excitatory effect on behaviour.The pineal eye arises embryologically as a single dorsal vesicular evagination of the diencephalon and anatomically it is very similar to that of other developing amphibians.Recording from the pineal eye using a suction electrode demonstrates that suddenly lowering the light intensity evokes a burst of impulses followed by a raised firing frequency. Conversely, increasing the light intensity leads to a lowered firing frequency. With prolonged exposure to white light at a range of intensities, the frequency of spike discharge is dependent upon the light intensity. The pineal eye can therefore act as a luminance detector.The pineal photoreceptors are most sensitive to light of a wavelength near 520 nm, this probably enables maximum sensitivity to the wavelengths of light that penetrate the freshwater environment.The possible role of the pineal eye in controlling locomotor activity is discussed.Abbreviations HRP horse raddish peroxidase - SEM scanning electron microscopy     

Evolving visual pigments: hints from the opsin-based proteins in a phylogenetically old "eyeless" invertebrate

Visual pigments are photosensitive receptor proteins that trigger the transduction process producing the visual excitation once they have absorbed photons. In spite of the molecular and morpho-functional complexity that has characterized the development of animal eyes and eyeless photoreceptive systems, opsin-based protein family appears ubiquous along metazoan visual systems. Moreover, in addition to classic rhodopsin photoreceptors, all Metazoa have supplementary non-visual photosensitive structures, mainly located in the central nervous system, that sense light without forming an image and that rather regulate the organism's temporal physiology. The investigation of novel non-visual photopigments exerting extraretinal photoreception is a challenging field in vision research. Here we propose the cnidarian Hydra as a useful tool of investigation for molecular and functional differences between these pigment families. Hydra is the first metazoan owning a nervous system and it is an eyeless invertebrate showing only an extraocular photoreception, as it has no recognized visual or photosensitive structures. In this paper we provide an overview of the molecular and functional features of the opsin-based protein subfamilies and preliminary evidences in a phylogenetically old species of both image-forming and non-visual opsins. Then we give new insights on the molecular biology of Hydra photoreception and on the evolutionary pathways of visual pigments.     

Carotenoid pigments: their possible role in protecting against photooxidation in eyes and photoreceptor cells

The effect of light on animal tissues is ambivalent. Light is necessary for many functions, e.g. for vision and, as in the flagellate halobacterium, to gain energy. But light is potentially dangerous: it is capable of destroying cells or their components by photooxidation, especially in the presence of sensitizing pigments such as haems and cytochromes, which are ubiquitous in aerobic cells. Several different examples are discussed to show how a compromise is achieved in animal tissues that for functional reasons receive high exposure to light. Carotenoid pigments, present in many eyes and photoreceptors, seem especially suited to protect against the deleterious effects of light because they absorb the dangerous short wavelength part of the light spectrum. In plant tissue, carotenoids are also well known to be capable of 'quenching' photoexcited states of sensitizing pigments and of oxygen, a function that they might have also in animal tissue. A consequence of the considerations is that whenever animal tissues are exposed to higher than usual light levels and/or oxygen pressures cellular damage might occur. Examples are discussed; strategies to circumvent the deleterious effects by photooxidation follow directly from the arguments.     

The role of dermal photoreceptors during the sea lamprey (<Emphasis Type="Italic">Petromyzon marinus</Emphasis>) spawning migration

Light avoidance in larval lampreys is mediated by dermal photoreceptors located in the tail. These photoreceptors continue to function in adults, but they seem redundant because post-metamorphic lampreys possess well-developed eyes. This study examined the role of dermal photoreceptors in adult sea lampreys by testing whether temperature-induced changes in refuge-seeking behavior are mediated by a reduction in dermal photosensitivity. In a lighted arena containing a single shaded refuge platform, lampreys at 22°C displayed five times less search activity and were less likely to attach beneath the refuge platform than lampreys at either 7 or 15°C. A behavioral assay for tail photosensitivity (locomotor response to tail illumination) revealed a corresponding reduction in dermal photosensitivity at 22°C. Moreover, the responses to head illumination (eyes and pineal) did not correspond with the observed light avoidance behaviors. The head was less responsive to light than the tail and was not influenced by temperature. These results provide strong evidence that the dermal photoreceptors continue to mediate light avoidance in adult lampreys, even though adults possess fully functional eyes. The fact that the eyes apparently do not take on this role suggests that there is functional specialization between these two light sensing systems.