Demonstration of a rhodopsin-retinochrome system in the stalk eye of a marine gastropod, Onchidium, by immunohistochemistry

Expression of proteins supporting visual function in heterobranch gastropods

To sense light, animals often utilize mechanisms that rely on visual pigments composed of opsin and retinal. The photon-induced isomerization of 11-cis-retinal to the all-trans configuration triggers phototransduction cascades, resulting in a change in the membrane potential of the photoreceptor. In mollusks, the most abundant opsin in the eye is Gq-coupled rhodopsin (Gq-rhodopsin). The Gq-rhodopsin-based visual pigment is bistable, with the regeneration of 11-cis-retinal occurring in a light-dependent manner without leaving the opsin moiety. 11-cis-retinal is also regenerated by the action of retinochrome in the cell bodies. Retinal binding protein (RALBP) mediates retinal transport between Gq-rhodopsin and retinochrome in the cytoplasm. However, recent studies have identified additional bistable opsins in mollusks, including Opn5 and xenopsin. It is unknown whether these bistable opsins require RALBP and retinochrome for the continuous regeneration of 11-cis-retinal. In the present study, we examined the expression of RALBP and retinochrome in the photoreceptors expressing Opn5 or Xenopsin in the heterobranch gastropods Limax and Peronia. Our findings revealed that retinochrome, but not RALBP, was present in some of the Opn5A-positive brain photosensory neurons of Limax. The ciliary cells in the dorsal eye of Peronia, which express Xenopsin2, lacked both retinochrome and RALBP. Therefore, bistable opsins do not necessarily depend on the RALBP-retinochrome system in a cell. We also examined the expression of other proteins that support visual function, such as β-arrestin, Gq, and Go, in all types of photoreceptors in these animals, and uncovered differences in the molecular composition among the photoreceptors.

Visual afferents from an eye in the terrestrial slug Limax valentianus

Terrestrial gastropods have a lens-bearing eye on the tip of their tentacles. There are two morphologically distinct photoreceptors, called Type-I and Type-II photoreceptors, in the retina. Type-I photoreceptors are equipped with highly developed photoreceptive microvilli in their outer rhabdomeric segment, whereas Type-II photoreceptors have short and fewer microvilli. Although both types of photoreceptors send afferent projections directly to the brain, their destinations in the brain, called optic neuropiles, have not been sufficiently investigated. Our recent studies revealed that there are commissural fibers in the cerebral ganglia that transmit photic information acquired by bilateral eyes. Moreover, some of the retinal photoreceptors are connected by gap junctions to the photosensitive brain neurons, suggesting the functional interaction of the photic information between the eye and brain photoreceptors, as well as between bilateral eyes. However, it has not been clarified which type of retinal photoreceptors send commissural projections to the contralateral hemiganglion nor interact with the brain photoreceptors. In the present study, we demonstrated by molecular histological analyses and tracer injections that (1) Type-I and Type-II photoreceptors send glutamatergic afferent projections to the medial and lateral lobes of the ipsilateral optic neuropile, respectively, (2) direct synaptic interaction between bilateral optic nerves occurs in the medial lobe of the optic neuropile, and (3) brain photosensory neurons form gap junctions with the medial lobe of the contralateral optic neuropile. These results reveal an ordered pattern of afferent projections from the retina and provide insight into the different functional roles of retinal photoreceptors.

Eyeless razor clam Sinonovacula constricta discriminates light spectra through opsins to guide Ca2+ and cAMP signaling pathways

Phototransduction is based on opsins that drive distinct types of Gα cascades. Although nonvisual photosensitivity has long been known in marine bivalves, the underlying molecular basis and phototransduction mechanism are poorly understood. Here, we introduced the eyeless razor clam Sinonovacula constricta as a model to clarify this issue. First, we showed that S. constricta was highly diverse in opsin family members, with a significant expansion in xenopsins. Second, the expression of putative S. constricta opsins was highly temporal-spatio specific, indicating their potential roles in S. constricta development and its peripheral photosensitivity. Third, by cloning four S. constricta opsins with relatively higher expression (Sc_opsin1, 5, 7, and 12), we found that they exhibited different expression levels in response to different light environments. Moreover, we demonstrated that these opsins (excluding Sc_opsin7) couple with Gαq and Gαi cascades to mediate the light-dependent Ca2+ (Sc_opsin1 and 5) and cAMP (Sc_opsin12) signaling pathways. The results indicated that Sc_opsin1 and 5 belonged to Gq-opsins, Sc_opsin12 belonged to Gi-opsins, while Sc_opsin7 might act as a photo-isomerase. Furthermore, we found that the phototransduction function of S. constricta Gq-opsins was dependent on the lysine at the seventh transmembrane domain, and greatly influenced by the external light spectra in a complementary way. Thus, a synergistic photosensitive system mediated by opsins might exist in S. constricta to rapidly respond to the transient or subtle changes of the external light environment. Collectively, our findings provide valuable insights into the evolution of opsins in marine bivalves and their potential functions in nonvisual photosensitivity.

Functional characterization of four opsins and two G alpha subtypes co-expressed in the molluscan rhabdomeric photoreceptor

BACKGROUND

Rhabdomeric photoreceptors of eyes in the terrestrial slug Limax are the typical invertebrate-type but unique in that three visual opsins (Gq-coupled rhodopsin, xenopsin, Opn5A) and one retinochrome, all belonging to different groups, are co-expressed. However, molecular properties including spectral sensitivity and G protein selectivity of any of them are not determined, which prevents us from understanding an advantage of multiplicity of opsin properties in a single rhabdomeric photoreceptor. To gain insight into the functional role of the co-expression of multiple opsin species in a photoreceptor, we investigated the molecular properties of the visual opsins in the present study.

RESULTS

First, we found that the fourth member of visual opsins, Opn5B, is also co-expressed in the rhabdomere of the photoreceptor together with previously identified three opsins. The photoreceptors were also demonstrated to express Gq and Go alpha subunits. We then determined the spectral sensitivity of the four visual opsins using biochemical and spectroscopic methods. Gq-coupled rhodopsin and xenopsin exhibit maximum sensitivity at ~ 456 and 475 nm, respectively, and Opn5A and Opn5B exhibit maximum sensitivity at ~ 500 and 470 nm, respectively, with significant UV sensitivity. Notably, in vitro experiments revealed that Go alpha was activated by all four visual opsins, in contrast to the specific activation of Gq alpha by Gq-coupled rhodopsin, suggesting that the eye photoreceptor of Limax uses complex G protein signaling pathways.

CONCLUSIONS

The eye photoreceptor in Limax expresses as many as four different visual opsin species belonging to three distinct classes. The combination of opsins with different spectral sensitivities and G protein selectivities may underlie physiological properties of the ocular photoreception, such as a shift in spectral sensitivity between dark- and light-adapted states. This may be allowed by adjustment of the relative contribution of the four opsins without neural networks, enabling a simple strategy for fine-tuning of vision.

Opsins in the Cephalic and Extracephalic Photoreceptors in the Marine Gastropod Onchidium verruculatum

AbstractThe marine gastropod Onchidium verruculatum has a pair of ocular photoreceptors, the stalk eyes, on the tip of its stalk near the head, as well as several extracephalic photosensory organs. The retinas of the stalk eye consist of two morphologically distinct visual cells, namely, the type I cells equipped with well-developed microvilli and the type II cells with less developed microvilli. The extracephalic photosensors comprise the dorsal eye, dermal photoreceptor, and brain photosensitive neurons. The characteristics of these cephalic and extracephalic photosensory organs have been studied from morphological and electrophysiological perspectives. However, little is known about the visual pigment molecules responsible for light detection in these organs. In the present study, we searched for opsin molecules that are expressed in the neural tissues of Onchidium and identified six putative signaling-competent opsin species, including Xenopsin1, Xenopsin2, Gq-coupled rhodopsin1, Gq-coupled rhodopsin2, Opsin-5B, and Gq-coupled rhodopsin-like. Immunohistochemical staining of four of the six opsins revealed that Xenopsin1, Gq-coupled rhodopsin1, and Gq-coupled rhodopsin2 are expressed in the rhabdomere of the stalk eye and in the dermal photoreceptor. Xenopsin2 was expressed in the type II photoreceptors of the stalk eye and in the ciliary photoreceptors of the dorsal eye. These immunohistochemical data were consistent with the results of the expression analysis, revealed by quantitative reverse transcription polymerase chain reaction. This study clarified the identities of opsins expressed in the extracephalic photosensory organs of Onchidium and the distinct molecular compositions among the photoreceptors.

Effects of artificial light at night on the feeding behaviour of three marine benthic grazers from the Adriatic Sea are species-specific and mostly short-lived

Artificial light at night (ALAN) has the potential to change the day-night activity of marine benthic grazers, and can therefore alter the top-down control they exert on macroalgal communities. In laboratory experiments, we investigated the influence of three realistic ALAN regimes on food consumption and feeding rhythmicity in the sea urchins Arbacia lixula and Paracentrotus lividus as well as in the snail Cerithium spp. from the Adriatic Sea. Food consumption was assessed in assays with algal pellets, while feeding rhythms were documented with 24 h time-lapse photography. Both was done in ALAN-acclimated and in non-acclimated animals. We observed temporary and potential long-term changes in the feeding rhythms of Cerithium spp. and Paracentrotus lividus, respectively, but found no lasting influence of ALAN on consumption rates. Effects were weaker when ALAN was applied only part-night, which suggests a possible mitigation measure to reduce the impact of nighttime lighting on coastal ecosystems.

The marine gastropod Conomurex luhuanus (Strombidae) has high-resolution spatial vision and eyes with complex retinas

All species within the conch snail family Strombidae possess large camera-type eyes that are surprisingly well-developed compared with those found in most other gastropods. Although these eyes are known to be structurally complex, very little research on their visual function has been conducted. Here, we use isoluminant expanding visual stimuli to measure the spatial resolution and contrast sensitivity of a strombid, Conomurex luhuanus. Using these stimuli, we show that this species responds to objects as small as 1.06 deg in its visual field. We also show that C. luhuanus responds to Michelson contrasts of 0.07, a low contrast sensitivity between object and background. The defensive withdrawal response elicited by visual stimuli of such small angular size and low contrast suggests that conch snails may use spatial vision for the early detection of potential predators. We support these findings with morphological estimations of spatial resolution of 1.04 deg. These anatomical data therefore agree with the behavioural measures and highlight the benefits of integrating behavioural and morphological approaches in animal vision studies. Using contemporary imaging techniques [serial block-face scanning electron microscopy (SBF-SEM), in conjunction with transmission electron microscopy (TEM)], we found that C. luhuanus have more complex retinas, in terms of cell type diversity, than expected based on previous studies of the group using TEM alone. We find the C. luhuanus retina comprises six cell types, including a newly identified ganglion cell and accessory photoreceptor, rather than the previously described four cell types.

Summary: Behavioural trials indicate the eyes of conch snail Conomurex luhuanus provide high-resolution spatial vision, and morphological examination reveals the retina contains more cell types than those of other gastropods.

Regional expression of neuropeptides in the retina of the terrestrial slug Limax valentianus (Gastropoda, Stylommatophora, Limacidae)

Gastropods use lens-bearing eyes to detect ambient light. The retina contains photoreceptors that directly project to the brain. Here we identified the neurotransmitters that the retinal cells use for projection to the brain in the terrestrial slug Limax. We identified 12 genes encoding neuropeptides as well as a novel vesicular glutamate transporter, a marker of glutamatergic neuron, expressed in the retinal cells. Spatial expression profiles of the neuropeptide genes were determined by in situ hybridization. WWamide/MIP1/Pedal peptide2 were co-expressed in the neurons of the accessory retina. In the main retina, prohormone-4 was expressed in the ventro-lateral region. Clionin was expressed in the ventro-medial region. Pedal peptide was expressed in the anterior region of the main retina and in the accessory retina. Enterin was expressed in many neurons, including the accessory retina, but not in the dorsal region. FxRIamide1 and 2 were co-expressed in the posterior region. Prohormone-4 variant was uniformly expressed in many neurons but scarcely in the accessory retina. MIP2 was widely expressed throughout the dorso-ventral axis in the posterio-lateral region of the main retina. Myo1 was expressed in many neurons of the main retina but predominantly in the dorsal region. These expression patterns were confirmed by immunohistochemistry with specific antibodies against the neuropeptides. Projections of these peptidergic retinal neurons were confirmed by immunostaining of the optic nerve. Our present study revealed regional differentiation of the retina with respect to the neurotransmitters that the retinal cells use. neuropeptides, retina, neurotransmitter, gastropod, Lehmannia This article is protected by copyright. All rights reserved.

The rhodopsin-retinochrome system for retinal re-isomerization predates the origin of cephalopod eyes

Background

The process of photoreception in most animals depends on the light induced isomerization of the chromophore retinal, bound to rhodopsin. To re-use retinal, the all-trans-retinal form needs to be re-isomerized to 11-cis-retinal, which can be achieved in different ways. In vertebrates, this mostly includes a stepwise enzymatic process called the visual cycle. The best studied re-isomerization system in protostomes is the rhodopsin-retinochrome system of cephalopods, which consists of rhodopsin, the photoisomerase retinochrome and the protein RALBP functioning as shuttle for retinal. In this study we investigate the expression of the rhodopsin-retinochrome system and functional components of the vertebrate visual cycle in a polyplacophoran mollusk, Leptochiton asellus, and examine the phylogenetic distribution of the individual components in other protostome animals.

Results

Tree-based orthology assignments revealed that orthologs of the cephalopod retinochrome and RALBP are present in mollusks outside of cephalopods. By mining our dataset for vertebrate visual cycle components, we also found orthologs of the retinoid binding protein RLBP1, in polyplacophoran mollusks, cephalopods and a phoronid. In situ hybridization and antibody staining revealed that L. asellus retinochrome is co-expressed in the larval chiton photoreceptor cells (PRCs) with the visual rhodopsin, RALBP and RLBP1. In addition, multiple retinal dehydrogenases are expressed in the PRCs, which might also contribute to the rhodopsin-retinochrome system.

Conclusions

We conclude that the rhodopsin-retinochrome system is a common feature of mollusk PRCs and predates the origin of cephalopod eyes. Our results show that this system has to be extended by adding further components, which surprisingly, are shared with vertebrates.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01939-x.

Integration of ocular and non-ocular photosensory information in the brain of the terrestrial slug Limax

In the terrestrial slugs Limax, most of the photosensory information is thought to be acquired by an eye located on the superior tentacles, by which the slugs avoid light. Recent studies, however, suggested that the brain also plays a role as a photosensor in their negative phototaxis behavior. In the present study, we investigated how the photosensory information acquired by the eye and brain is integrated. The visual pathway in the brain was traced by incorporating tracer molecules from the cut end of an optic nerve, and commissural interactions were found in optic neuropiles located in the lateral regions of the cerebral ganglia. A cluster of neuronal cell bodies located near the dorsal surface of the cerebral ganglion had connections with the contralateral optic neuropile via gap junctions. Some of these neuronal cell bodies were Opn5A-immunoreactive, and contained numerous photic vesicle-like structures. Light-induced spikes were recorded extracellularly from the dorsal surface of these neuronal clusters, and they were synchronous with the spikes recorded from the cut end of the cerebral commissure. This study suggests that both the light information from the eye and the contralateral cerebral ganglion are integrated in the optic neuropile.

Ciliary and rhabdomeric photoreceptor-cell circuits form a spectral depth gauge in marine zooplankton

Ciliary and rhabdomeric photoreceptor cells represent two main lines of photoreceptor-cell evolution in animals. The two cell types coexist in some animals, however how these cells functionally integrate is unknown. We used connectomics to map synaptic paths between ciliary and rhabdomeric photoreceptors in the planktonic larva of the annelid Platynereis and found that ciliary photoreceptors are presynaptic to the rhabdomeric circuit. The behaviors mediated by the ciliary and rhabdomeric cells also interact hierarchically. The ciliary photoreceptors are UV-sensitive and mediate downward swimming in non-directional UV light, a behavior absent in ciliary-opsin knockout larvae. UV avoidance overrides positive phototaxis mediated by the rhabdomeric eyes such that vertical swimming direction is determined by the ratio of blue/UV light. Since this ratio increases with depth, Platynereis larvae may use it as a depth gauge during vertical migration. Our results revealed a functional integration of ciliary and rhabdomeric photoreceptor cells in a zooplankton larva.

The Last Common Ancestor of Most Bilaterian Animals Possessed at Least Nine Opsins

The opsin gene family encodes key proteins animals use to sense light and has expanded dramatically as it originated early in animal evolution. Understanding the origins of opsin diversity can offer clues to how separate lineages of animals have repurposed different opsin paralogs for different light-detecting functions. However, the more we look for opsins outside of eyes and from additional animal phyla, the more opsins we uncover, suggesting we still do not know the true extent of opsin diversity, nor the ancestry of opsin diversity in animals. To estimate the number of opsin paralogs present in both the last common ancestor of the Nephrozoa (bilaterians excluding Xenoacoelomorpha), and the ancestor of Cnidaria + Bilateria, we reconstructed a reconciled opsin phylogeny using sequences from 14 animal phyla, especially the traditionally poorly-sampled echinoderms and molluscs. Our analysis strongly supports a repertoire of at least nine opsin paralogs in the bilaterian ancestor and at least four opsin paralogs in the last common ancestor of Cnidaria + Bilateria. Thus, the kernels of extant opsin diversity arose much earlier in animal history than previously known. Further, opsins likely duplicated and were lost many times, with different lineages of animals maintaining different repertoires of opsin paralogs. This phylogenetic information can inform hypotheses about the functions of different opsin paralogs and can be used to understand how and when opsins were incorporated into complex traits like eyes and extraocular sensors.

Expression and light-dependent translocation of β-arrestin in the visual system of the terrestrial slug Limax valentianus

Vertebrates, cephalopods and arthropods are equipped with eyes that have the highest spatiotemporal resolution among the animal phyla. In parallel, only animals in these three phyla have visual arrestin specialized for the termination of visual signaling triggered by opsin, in addition to ubiquitously expressed β-arrestin that serves in terminating general G protein-coupled receptor signaling. Indeed, visual arrestin in Drosophila and rodents translocates to the opsin-rich subcellular region in response to light to reduce the overall sensitivity of photoreceptors in an illuminated environment (i.e. light adaptation). We thus hypothesized that, during evolution, visual arrestin has taken over the role of β-arrestin in those animals with eyes of high spatiotemporal resolution. If this is true, it is expected that β-arrestin plays a role similar to visual arrestin in those animals with low-resolution eyes. In the present study, we focused on the terrestrial mollusk Limax valentianus, a species related to cephalopods but that has only β-arrestin, and generated antibodies against β-arrestin. We found that β-arrestin is highly expressed in photosensory neurons, and translocates into the microvilli of the rhabdomere within 30 min in response to short wavelength light (400 nm), to which the Limax eye exhibits a robust response. These observations suggest that β-arrestin functions in the visual system of those animals that do not have visual arrestin. We also exploited anti-β-arrestin antibody to visualize the optic nerve projecting to the brain, and demonstrated its usefulness for tracing a visual ascending pathway.

Spectral Tuning of Phototaxis by a Go-Opsin in the Rhabdomeric Eyes of Platynereis

Phototaxis is characteristic of the pelagic larval stage of most bottom-dwelling marine invertebrates. Larval phototaxis is mediated by simple eyes that can express various types of light-sensitive G-protein-coupled receptors known as opsins. Since opsins diversified early during metazoan evolution in the marine environment, understanding underwater light detection could elucidate this diversification. Opsins have been classified into three major families, the r-opsins, the c-opsins, and the Go/RGR opsins, a family uniting Go-opsins, retinochromes, RGR opsins, and neuropsins. The Go-opsins form an ancient and poorly characterized group retained only in marine invertebrate genomes. Here, we characterize a Go-opsin from the marine annelid Platynereis dumerilii. We found Go-opsin1 coexpressed with two r-opsins in depolarizing rhabdomeric photoreceptor cells in the pigmented eyes of Platynereis larvae. We purified recombinant Go-opsin1 and found that it absorbs in the blue-cyan range of the light spectrum. To characterize the function of Go-opsin1, we generated a Go-opsin1 knockout Platynereis line by zinc-finger-nuclease-mediated genome engineering. Go-opsin1 knockout larvae were phototactic but showed reduced efficiency of phototaxis to wavelengths matching the in vitro Go-opsin1 spectrum. Our results highlight spectral tuning of phototaxis as a potential mechanism contributing to opsin diversity.

Three-dimensional reconstruction of the axon extending from the dermal photoreceptor cell in the extraocular photoreception system of a marine gastropod, onchidium

The marine gastropod Onchidium has a multiple photoreceptive system consisting of stalk eyes, dorsal eyes, photosensitive neurons, and extraocular dermal photoreceptor cells (DPCs). The DPCs were widespread all over the dorsal mantle and distributed singly or in groups in the dermis, but were not discernible by the naked eye. The DPC was oval in shape and large in size, and characterized by features specific to gastropod photoreceptor cells such as massive microvilli, photic vesicles, and a depolarized response. DPC-17, one of a group of 19 DPCs, was examined on serial semi-thin sections of 0.4 µm in thickness with a high-voltage transmission electron microscope (HVTEM). The axon emerged specifically from the lateral side between the distal microvillous portion and proximal cytoplasm, travelled through the connective tissue, and joined a small nerve bundle (NB). Two types of supportive cells were found along the length of the axon. The first type was a covering cell (CC) surrounding the surface of the DPC body and continuing onward to the axon sheath. DPC-17 was covered by 11 CCs, while the larger DPC-6 was only covered by four CCs. The second type was a sheath cell (ShC) wrapping the surface of the small NB where the axon of the DPC merged with undefined nerve fibers. The axon extending directly from DPC-17 was reconstructed three-dimensionally (3D) using DeltaViewer software. The 3D-reconstructed image of the sheath of the axon and the CC demonstrated the continuity between the two structures, especially when the image was rotated using DeltaViewer.

Morphological and physiological characteristics of dermal photoreceptors in Lymnaea stagnalis

Dermal photoreceptors located in the mantle of Lymnaea stagnalis were histologically and physiologically characterized. Our previous study demonstrated that the shadow response from dermal photoreceptors induces the whole-body withdrawal response. Through the interneuron, RPeD11, we detected that the light-off response indirectly originated from a dermal photoreceptor. Previous observations, based on behavioral pharmacology, revealed that cyclic guanosine monophosphate acts as a second messenger in the dermal photoreceptor. Furthermore, gastropods possess dermal photoreceptors containing rhodopsin, as a photopigment, and another photo-sensitive protein, arrestin, responsible for terminating the light response. Thus, we chose three antibodies, anti-cGMP, anti-rhodopsin, and anti-β-arrestin, to identify the dermal photoreceptor molecules in Lymnaea mantle. Extracellular recording, using a suction electrode on the mantle, revealed a light off-response from the right parietal nerve. Overlapping structures, positive against each of the antibodies, were also observed. Numerous round, granular particles of 3-47 μm in diameter with one nucleus were distributed around pneumostome and/or inside the mantle. The cells surrounding the pneumostome area, located 10 μm beneath the surface, tended to have smaller cell soma ranging from 3 to 25 μm in diameter, while cells located in other areas were distributed uniformly inside the mantle, with a larger diameter ranging from 12 to 47 μm. The histological examination using back-filing Lucifer Yellow staining of the right parietal nerve with the three dermal photoreceptor antibodies confirmed that these overlapping-stained structures were dermal photoreceptors in Lymnaea.

A novel retinol-binding protein in the retina of the swallowtail butterfly, Papilio xuthus

Retinoid-binding proteins are indispensable for visual cycles in both vertebrate and invertebrate retinas. These proteins stabilize and transport hydrophobic retinoids in the hydrophilic environment of plasma and cytoplasm, and allow regeneration of visual pigments. Here, we identified a novel retinol-binding protein in the eye of a butterfly, Papilio xuthus. The protein that we term Papilio retinol-binding protein (Papilio RBP) is a major component of retinal soluble proteins and exclusively binds 3-hydroxyretinol, and emits fluorescence peaking at 480 nm under ultraviolet (UV) illumination. The primary structure, deduced from the nucleotide sequence of the cDNA, shows no similarity to any other lipophilic ligand-binding proteins. The molecular mass and isoelectric point of the protein estimated from the amino-acid sequence are 26.4 kDa and 4.92, respectively. The absence of any signal sequence for secretion in the N-terminus suggests that the protein exists in the cytoplasmic matrix. All-trans 3-hydroxyretinol is the major ligand of the Papilio RBP in dark-adapted eyes. Light illumination of the eyes increases the 11-cis isomer of the ligand and induces redistribution of the Papilio RBP from the proximal to the distal part of the photoreceptor layer. These results suggest that the Papilio RBP is involved in visual pigment turnover.

Localization of retinal proteins in the stalk and dorsal eyes of the marine gastropod, Onchidium

Onchidium possesses stalk eye (SE) and dorsal eye (DE) which comprise part of a unique multiple photoreceptive system. The retina of SE consists of rhabdomeric-type visual cells, whereas the DE contains two types of photoreceptor cells; ciliary-type cells in the retina and rhabdomeric-type cells in the lens. High-performance liquid chromatography (HPLC) analyses revealed the presence of 11-cis-retinal as well as all-trans-retinal in both eyes. The amount of retinal of one DE (0.17 pmol) is far less than that in one SE (0.41 pmol) in the dark-adapted Onchidium. In the dark-adapted SE, the amount of all-trans-retinal was higher than that of 11-cis-retinal. This finding is consistent with the presence of photic vesicles, including retinochrome, in rhabdomeric-type visual cells. In contrast, a higher amount of 11-cis-retinal than all-trans-retinal was present in dark-adapted DE, although this was decreased in light-adapted DE. Upon UV irradiation following treatment with sodium borohydride (NaBH(4)), the fluorescence (derived from retinochrome) was observed in the somatic layer of SE. Additional fluorescence (due to rhodopsin) was observed in the villous layer upon treatment with NaBH(4) after denaturation. However, only weak, obscure fluorescence of retinyl proteins was observed in the DE, not in a specific but an indefinite area on treatment with NaBH4 with or without denaturation. With fluorescence histochemistry, the localization of rhodopsin and retinochrome was confirmed at specific regions in the retina of the SE, whereas no distinct localization of these photopigments in DE was demonstrated. The amount of retinal to detect the fluorescence may be too low in the DE, or photopigments of DE may differ in chemical nature from those of SE.