Research progress on insect visual electrophysiological techniques.

Insect visual electrophysiological techniques are important to study the electrical characteristics of photoreceptor cells and visual neurons in insects, including electroretinography (ERG) and microelectrode intracellular recording (MIR). ERG records the changes of voltage or electric current in the retina of insects in response to different light stimuli, which occurs outside the cell. MIR records the changes in individual photoreceptor cells or visual neurons of an insect exposed to different lights, which occurs inside the cell. Insect visual electrophysiological techniques can explore the mechanism of electrophysiological response of insects' vision to light and reveal their sensitive light spectra and photoreceptor types. This review introduced the basic structure and the principle of ERG and MIR, and summarized their applications in insect researches in the past 20 years, which would provide references for elucidating the mechanism of light perception in insects and the use of insect phototropism to control pests.

Localization of multiple opsins in ocular and non-ocular tissues of deep-sea shrimps and the first evidence of co-localization in a rhabdomeric R8 cell (Caridea: Oplophoroidea).

Bioluminescence is a prevalent phenomenon throughout the marine realm and is often the dominant source of light in mesophotic and aphotic depth horizons. Shrimp belonging to the superfamily Oplophoroidea are mesopelagic, perform diel vertical migration, and secrete a bright burst of bioluminescent mucous when threatened. Species in the family Oplophoridae also possess cuticular light-emitting photophores presumably for camouflage via counter-illumination. Many species within the superfamily express a single visual pigment in the retina, consistent with most other large-bodied mesopelagic crustaceans studied to date. Photophore-bearing species have an expanded visual opsin repertoire and dual-sensitivity visual systems, as evidenced by transcriptomes and electroretinograms. In this study, we used immunohistochemistry to describe opsin protein localization in the retinas of four species of Oplophoroidea and non-ocular tissues of Janicella spinicauda. Our results show that Acanthephyra purpurea (Acanthephyridae) retinas possess LWS-only photoreceptors, consistent with the singular peak sensitivity previously reported. Oplophoridae retinas contain two opsin clades (LWS and MWS) consistent with dual-sensitivity. Oplophorus gracilirostris and Systellaspis debilis have LWS in the proximal rhabdom (R1-7 cells) and MWS2 localized in the distal rhabdom (R8 cell). Surprisingly, Janicella spinicauda has LWS in the proximal rhabdom (R1-7) and co-localized MWS1 and MWS2 opsin paralogs in the distal rhabdom, providing the first evidence of co-localization of opsins in a crustacean rhabdomeric R8 cell. Furthermore, opsins were found in multiple non-ocular tissues of J. spinicauda, including nerve, tendon, and photophore. These combined data demonstrate evolutionary novelty and opsin duplication within Oplophoridae, with implications for visual ecology, evolution in mesophotic environments, and a mechanistic understanding of adaptive counter-illumination using photophore bioluminescence.

Functional opsin patterning for Drosophila color vision is established through signaling pathways in adjacent object-detection neurons.

Vision is mainly based on two different tasks, object detection and color discrimination, carried out by photoreceptor (PR) cells. The Drosophila compound eye consists of ∼800 ommatidia. Every ommatidium contains eight PR cells, six outer cells (R1-R6) and two inner cells (R7 and R8), by which object detection and color vision are achieved, respectively. Expression of opsin genes in R7 and R8 is highly coordinated through the instructive signal from R7 to R8, and two major ommatidial subtypes are distributed stochastically; pale type expresses Rh3/Rh5 and yellow type expresses Rh4/Rh6 in R7/R8. The homeodomain protein Defective proventriculus (Dve) is expressed in yellow-type R7 and in six outer PRs, and it is involved in Rh3 repression to specify the yellow-type R7. dve mutant eyes exhibited atypical coupling, Rh3/Rh6 and Rh4/Rh5, indicating that Dve activity is required for proper opsin coupling. Surprisingly, Dve activity in R1 is required for the instructive signal, whereas activity in R6 and R7 blocks the signal. Our results indicate that functional coupling of two different neurons is established through signaling pathways from adjacent neurons that are functionally different.

Morphology and spectral sensitivity of long visual fibers and lamina monopolar cells in the butterfly Papilio xuthus.

Extensive analysis of the flower-visiting behavior of a butterfly, Papilio xuthus, has indicated complex interaction between chromatic, achromatic, and motion cues. Their eyes are spectrally rich with six classes of photoreceptors, respectively sensitive in the ultraviolet, violet, blue, green, red, and broad-band wavelength regions. Here, we studied the anatomy and physiology of photoreceptors and second-order neurons of P. xuthus, focusing on their spectral sensitivities and projection terminals to address where the early visual integration takes place. We thus found the ultraviolet, violet, and blue photoreceptors and all second-order neurons terminate in the distal region of the second optic ganglion, the medulla. We identified five types of second-order neurons based on the arborization in the first optic ganglion, the lamina, and the shape of the medulla terminals. Their spectral sensitivity is independent of the morphological types but reflects the combination of pre-synaptic photoreceptors. The results indicate that the distal medulla is the most plausible region for early visual integration.

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.

Physiological and behavioral evidence for multiple spectral channels in the larval stomatopod visual system.

Larval stomatopods have generally been described as having a typical larval crustacean compound eye, which lacks the visual pigment diversity and morphological specializations of the well-studied stomatopod adult eye. However, recent work has suggested that larval stomatopod eyes are more complex than previously described. In this study, we provide physiological and behavioral evidence of at least three distinct photoreceptor classes in three species of larval stomatopods: Gonodactylellus n. sp., Gonodactylaceus falcatus and Pullosquilla n. sp. First, electroretinogram recordings were used to measure the spectral sensitivity of each species. Evidence for at least three spectral classes were identified in each: an ultraviolet, peaking at 340-376 nm; a short-wavelength blue, peaking at 455-464 nm; and a long-wavelength orange, peaking at 576-602 nm. Next, the behavioral response to light was investigated. We found that each species demonstrated positive phototactic responses to monochromatic stimuli across the UV-visible spectrum. In wavelength preference trials, distinct preferences among species were identified when different colored light stimuli were presented simultaneously. All species displayed a strong response to the UV stimulus, as well as responses to blue and orange stimuli, although at different response strengths, but no response to green. The results of this study demonstrate that larval stomatopods not only have multiple physiologically active spectral classes but they also display clear and distinct responses to wavelengths across the spectrum. We propose that the spectral classes demonstrated in each are related to visually guided ecological tasks of the larvae, which may differ between species.

A thermal receptor for nonvisual sunlight detection in myriapods.

Organisms from cyanobacteria to humans have evolved a wide array of photoreceptive strategies to detect light. Sunlight avoidance behavior is common in animals without vision or known photosensory genes. While indirect light perception via photothermal conversion is a possible scenario, there is no experimental evidence for this hypothesis. Here, we show a nonvisual and extraocular sunlight detection mechanism by identifying the broad-range thermal receptor 1 (BRTNaC1, temperature range = 33 to 48 °C) in centipede antennae. BRTNaC1, a heat-activated cation-permeable ion channel, is structurally related to members of the epithelial sodium channel family. At the molecular level, heat activation of BRTNaC1 exhibits strong pH dependence controlled by two protonatable sites. Physiologically, temperature-dependent activation of BRTNaC1 upon sunlight exposure comes from a striking photothermal effect on the antennae, where a slightly acidic environment (pH 6.1) of the body fluid leads to the protonation of BRTNaC1 and switches on its high thermal sensitivity. Furthermore, testosterone potently inhibits heat activation of BRTNaC1 and the sunlight avoidance behavior of centipedes. Taken together, our study suggests a sophisticated strategy for nonvisual sunlight detection in myriapods.

A tunable reflector enabling crustaceans to see but not be seen.

Many oceanic prey animals use transparent bodies to avoid detection. However, conspicuous eye pigments, required for vision, compromise the organisms' ability to remain unseen. We report the discovery of a reflector overlying the eye pigments in larval decapod crustaceans and show how it is tuned to render the organisms inconspicuous against the background. The ultracompact reflector is constructed from a photonic glass of crystalline isoxanthopterin nanospheres. The nanospheres' size and ordering are modulated to tune the reflectance from deep blue to yellow, enabling concealment in different habitats. The reflector may also function to enhance the acuity or sensitivity of the minute eyes by acting as an optical screen between photoreceptors. This multifunctional reflector offers inspiration for constructing tunable artificial photonic materials from biocompatible organic molecules.

Opsin knockdown specifically slows phototransduction in broadband and UV-sensitive photoreceptors in Periplaneta americana.

Photoreceptors with different spectral sensitivities serve different physiological and behavioral roles. We hypothesized that such functional evolutionary optimization could also include differences in phototransduction dynamics. We recorded elementary responses to light, quantum bumps (QBs), of broadband green-sensitive and ultraviolet (UV)-sensitive photoreceptors in the cockroach, Periplaneta americana, compound eyes using intracellular recordings. In addition to control photoreceptors, we used photoreceptors from cockroaches whose green opsin 1 (GO1) or UV opsin expression was suppressed by RNA interference. In the control broadband and UV-sensitive photoreceptors average input resistances were similar, but the membrane capacitance, a proxy for membrane area, was smaller in the broadband photoreceptors. QBs recorded in the broadband photoreceptors had comparatively short latencies, high amplitudes and short durations. Absolute sensitivities of both opsin knockdown photoreceptors were significantly lower than in wild type, and, unexpectedly, their latency was significantly longer while the amplitudes were not changed. Morphologic examination of GO1 knockdown photoreceptors did not find significant differences in rhabdom size compared to wild type. Our results differ from previous findings in Drosophila melanogaster rhodopsin mutants characterized by progressive rhabdomere degeneration, where QB amplitudes were larger but phototransduction latency was not changed compared to wild type.

Opponent processing in the retinal mosaic of nymphalid butterflies.

The eyes of nymphalid butterflies, investigated with incident illumination, show colourful facet reflection patterns-the eye shine-which is uniform or heterogeneous, dependent on the species. Facet colours suggest that the ommatidia contain different sets of photoreceptors and screening pigments, but how the colours and the cell characteristics are associated has not been clearly established. Here, we analyse the retinae of two nymphalids, Apatura ilia, which has a uniform eyeshine, and Charaxes jasius, a species with a heterogeneous eye shine, using single-cell recordings, spectroscopy and optical pupillometry. Apatura has UV-, blue- and green-sensitive photoreceptors, allocated into three ommatidial types. The UV- and blue-sensitive cells are long visual fibres (LVFs), receiving opponent input from the green-sensitive short visual fibres (SVFs). Charaxes has an expanded set of photoreceptors, allocated into three additional, red-reflecting ommatidial types. All red ommatidia contain green-sensitive LVFs, receiving opponent input from red receptors. In both species, the SVFs do not receive any opponent input. The simple retina of Apatura with three ommatidial types and two colour-opponent channels can support trichromatic vision. Charaxes has six ommatidial types and three colour-opponent channels. Its expanded receptor set can support tetrachromatic vision. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Mosquito cryptochromes expressed in Drosophila confer species-specific behavioral light responses.

Cryptochrome (CRY) is a short-wavelength light-sensitive photoreceptor expressed in a subset of circadian neurons and eyes in Drosophila that regulates light-evoked circadian clock resetting. Acutely, light evokes rapid electrical excitation of the ventral lateral subset of circadian neurons and confers circadian-modulated avoidance behavioral responses to short-wavelength light. Recent work shows dramatically different avoidance versus attraction behavioral responses to short-wavelength light in day-active versus night-active mosquitoes and that these behavioral responses are attenuated by CRY protein degradation by constant light exposure in mosquitoes. To determine whether CRY1s mediate species-specific coding for behavioral and electrophysiological light responses, we used an "empty neuron" approach and transgenically expressed diurnal Aedes aegypti (AeCRY1) versus nocturnal Anopheles gambiae (AgCRY1) in a cry-null Drosophila background. AeCRY1 is much less light sensitive than either AgCRY1 or DmCRY as shown by partial behavioral rhythmicity following constant light exposure. Remarkably, expression of nocturnal AgCRY1 confers low survival to constant white light as does expression of AeCRY1 to a lesser extent. AgCRY1 mediates significantly stronger electrophysiological cell-autonomous responses to 365 nm ultraviolet (UV) light relative to AeCRY1. AgCRY1 expression mediates electrophysiological sensitivity to 635 nm red light, whereas AeCRY1 does not, consistent with species-specific mosquito red light responses. AgCRY1 and DmCRY mediate intensity-dependent avoidance behavior to UV light at different light intensity thresholds, whereas AeCRY1 does not, thus mimicking mosquito and fly behaviors. These findings highlight CRY as a key non-image-forming visual photoreceptor that mediates physiological and behavioral light responses in a species-specific fashion.

Distinct mechanisms of light adaptation of elementary responses in photoreceptors of dipteran flies and American cockroach.

Of many light adaptation mechanisms optimizing photoreceptor functioning in the compound eyes of insects, those modifying the single-photon response, the quantum bump (QB), remain least studied. Here, by recording from photoreceptors of the blow fly Protophormia terraenovae, the hover fly Volucella pellucens, and the cockroach Periplaneta americana, we investigated mechanisms of rapid light adaptation by examining how properties of QBs change after light stimulation and multiquantal impulse responses during repetitive stimulation. In P. terraenovae, light stimulation reduced latencies, characteristic durations, and amplitudes of QBs in an intensity- and duration-dependent manner. In P. americana, only QB amplitudes decreased consistently. In both species, time constants of QB parameters' recovery increased with the strength and duration of stimulation, reaching ∼30 s after bright prolonged 10-s pulses. In the blow fly, changes in QB amplitudes during recovery correlated with changes in half-widths but not latencies, suggesting at least two separate mechanisms of light adaptation: acceleration of QB onset by sensitizing transduction channels and acceleration of transduction channel inactivation causing QB shortening and decrease. In the cockroach, light adaptation reduced QB amplitude by apparently lowering the transduction channel availability. Impulse response data in the blow fly and cockroach were consistent with the inferences from the QB recovery experiments. However, in the hover fly V. pellucens, impulse response latencies and durations decreased simultaneously, whereas amplitudes decreased little, even when bright flashes were applied at high frequencies. These findings indicate the existence of dissimilar mechanisms of light adaptation in the microvilli of different species.NEW & NOTEWORTHY By studying light adaptation of elementary responses in photoreceptors of the blow fly and the cockroach we found three distinct mechanisms. In the blow fly, one mechanism speeds quantum bump onset and another accelerates quantum bump inactivation, decreasing its size. In the cockroach, quantum bump amplitude decreases without changes in kinetics, indicating decreased availability of transduction channels. The findings can be explained by expression of different transduction channels in the flies and cockroaches.

Connectome of the lamina reveals the circuit for early color processing in the visual pathway of a butterfly.

Connectomics has become a standard neuroscience methodology in a few model animals,1 with the visual system being a popular target of study.2-5 Combining connectomics with circuit and behavioral physiology, recent studies on the color vision of the fruit fly Drosophila melanogaster have focused on the mechanisms underlying early wavelength processing in the optic ganglia.6-8 However, the color vision capabilities of D. melanogaster are limited,9 compared with many flower-visiting insects.10,11 For example, a butterfly Papilio xuthus has six spectral classes of photoreceptors. Each ommatidium contains nine photoreceptors in one of three fixed combinations, making the eye an array of three spectrally distinct ommatidia types.12 Behaviorally, P. xuthus can detect 1 nm differences in light wavelength across the spectrum from ultraviolet to red, outperforming humans.13 What is the neuronal basis of such precise color vision? How does such a system evolve? Addressing these questions requires comparative studies at the circuit level. Here, we performed a connectome analysis in the first optic ganglion, the lamina, of P. xuthus. The lamina comprises cartridges, each typically containing nine photoreceptor axons from a single ommatidium and four second-order neurons. We found abundant inter-photoreceptor connections, which are absent in the lamina of D. melanogaster. We also identified connections between neighboring cartridges, particularly those receiving inputs from spectrally distinct ommatidia. The linear summation of synaptic connections well explains the spectral sensitivity of photoreceptors and second-order neurons in the lamina.

Analysis of Lipid Signaling in Drosophila Photoreceptors using Mass Spectrometry.

The activation of phospholipase Cß (PLCß) is an essential step during sensory transduction in Drosophila photoreceptors. PLCß activity results in the hydrolysis of the membrane lipid phosphatidylinositol 4,5 bisphosphate [PI(4,5)P2] leading ultimately to the activation of transient receptor potential (TRP) and TRP like (TRPL) channels. The activity of PLCß also leads subsequently to the generation of many lipid species several of which have been proposed to play a role in TRP and TRPL activation. In addition, several classes of lipids have been proposed to play key roles in organizing the cell biology of photoreceptors to optimize signaling reactions for optimal sensory transduction. Historically, these discoveries have been driven by the ability to isolate Drosophila mutants for enzymes that control the levels of specific lipids and perform analysis of photoreceptor physiology in these mutants. More recently, powerful mass spectrometry methods for isolation and quantitative analysis of lipids with high sensitivity and specificity have been developed. These are particularly suited for use in Drosophila where lipid analysis is now possible from photoreceptors without the need for radionuclide labeling. In this article, the conceptual and practical considerations in the use of lipid mass spectrometry for the robust, sensitive, and accurate quantitative assessment of various signaling lipids in Drosophila photoreceptors are covered. Along with existing methods in molecular genetics and physiological analysis such lipid is likely to enhance the power of photoreceptors as a model system for discoveries in biology.

Colour vision in stomatopod crustaceans: more questions than answers.

Stomatopod crustaceans, or mantis shrimps, are known for their extensive range of spectral sensitivity but relatively poor spectral discrimination. Instead of the colour-opponent mechanism of other colour vision systems, the 12 narrow-band colour channels they possess may underlie a different method of colour processing. We investigated one hypothesis in which the photoreceptors are proposed to act as individual wave-band detectors, interpreting colour as a parallel pattern of photoreceptor activation, rather than a ratiometric comparison of individual signals. This different form of colour detection has been used to explain previous behavioural tests in which low-saturation blue was not discriminated from grey, potentially because of similar activation patterns. Results here, however, indicate that the stomatopod Haptosquilla trispinosa was able to easily distinguish several colours, including blue of both high and low saturation, from greys. The animals did show a decrease in performance over time in an artificially lit environment, indicating plasticity in colour discrimination ability. This rapid plasticity, most likely the result of a change in opsin (visual pigment) expression, has now been noted in several animal lineages (both invertebrate and vertebrate) and is a factor we suggest needs attention and potential re-examination in any colour-based behavioural tests. As for stomatopods, it remains unclear why they achieve poor colour discrimination using the most comprehensive set of spectral sensitivities in the animal kingdom and also what form of colour processing they may utilise.

Insect visual sensitivity to long wavelengths enhances colour contrast of insects against vegetation.

The sensitivity of animal photoreceptors to different wavelengths of light strongly influence the perceived visual contrast of objects in the environment. Outside of the human visual wavelength range, ultraviolet sensitivity in many species provides important and behaviourally relevant visual contrast between objects. However, at the opposite end of the spectrum, the potential advantage of red sensitivity remains unclear. We investigated the potential benefit of long wavelength sensitivity by modelling the visual contrast of a wide range of jewel beetle colours against flowers and leaves of their host plants to hypothetical insect visual systems. We find that the presence of a long wavelength sensitive photoreceptor increases estimated colour contrast, particularly of beetles against leaves. Moreover, under our model parameters, a trichromatic visual system with ultraviolet (λmax = 355 nm), short (λmax = 445 nm) and long (λmax = 600 nm) wavelength photoreceptors performed as well as a tetrachromatic visual system, which had an additional medium wavelength photoreceptor (λmax = 530 nm). When we varied λmax for the long wavelength sensitive receptor in a tetrachromatic system, contrast values between beetles, flowers and leaves were all enhanced with increasing λmax from 580 nm to at least 640 nm. These results suggest a potential advantage of red sensitivity in visual discrimination of insect colours against vegetation and highlight the potential adaptive value of long wavelength sensitivity in insects.

Synaptic targets of photoreceptors specialized to detect color and skylight polarization in Drosophila.

Color and polarization provide complementary information about the world and are detected by specialized photoreceptors. However, the downstream neural circuits that process these distinct modalities are incompletely understood in any animal. Using electron microscopy, we have systematically reconstructed the synaptic targets of the photoreceptors specialized to detect color and skylight polarization in Drosophila, and we have used light microscopy to confirm many of our findings. We identified known and novel downstream targets that are selective for different wavelengths or polarized light, and followed their projections to other areas in the optic lobes and the central brain. Our results revealed many synapses along the photoreceptor axons between brain regions, new pathways in the optic lobes, and spatially segregated projections to central brain regions. Strikingly, photoreceptors in the polarization-sensitive dorsal rim area target fewer cell types, and lack strong connections to the lobula, a neuropil involved in color processing. Our reconstruction identifies shared wiring and modality-specific specializations for color and polarization vision, and provides a comprehensive view of the first steps of the pathways processing color and polarized light inputs.

Analysis of growth cone extension in standardized coordinates highlights self-organization rules during wiring of the Drosophila visual system.

A fascinating question in neuroscience is how ensembles of neurons, originating from different locations, extend to the proper place and by the right time to create precise circuits. Here, we investigate this question in the Drosophila visual system, where photoreceptors re-sort in the lamina to form the crystalline-like neural superposition circuit. The repeated nature of this circuit allowed us to establish a data-driven, standardized coordinate system for quantitative comparison of sparsely perturbed growth cones within and across specimens. Using this common frame of reference, we investigated the extension of the R3 and R4 photoreceptors, which is the only pair of symmetrically arranged photoreceptors with asymmetric target choices. Specifically, we found that extension speeds of the R3 and R4 growth cones are inherent to their cell identities. The ability to parameterize local regularity in tissue organization facilitated the characterization of ensemble cellular behaviors and dissection of mechanisms governing neural circuit formation.

Red-green opponency in the long visual fibre photoreceptors of brushfoot butterflies (Nymphalidae).

In many butterflies, the ancestral trichromatic insect colour vision, based on UV-, blue- and green-sensitive photoreceptors, is extended with red-sensitive cells. Physiological evidence for red receptors has been missing in nymphalid butterflies, although some species can discriminate red hues well. In eight species from genera Archaeoprepona, Argynnis, Charaxes, Danaus, Melitaea, Morpho, Heliconius and Speyeria, we found a novel class of green-sensitive photoreceptors that have hyperpolarizing responses to stimulation with red light. These green-positive, red-negative (G+R-) cells are allocated to positions R1/2, normally occupied by UV and blue-sensitive cells. Spectral sensitivity, polarization sensitivity and temporal dynamics suggest that the red opponent units (R-) are the basal photoreceptors R9, interacting with R1/2 in the same ommatidia via direct inhibitory synapses. We found the G+R- cells exclusively in butterflies with red-shining ommatidia, which contain longitudinal screening pigments. The implementation of the red colour channel with R9 is different from pierid and papilionid butterflies, where cells R5-8 are the red receptors. The nymphalid red-green opponent channel and the potential for tetrachromacy seem to have been switched on several times during evolution, balancing between the cost of neural processing and the value of extended colour information.

Characterization of cephalic and non-cephalic sensory cell types provides insight into joint photo- and mechanoreceptor evolution.

Rhabdomeric opsins (r-opsins) are light sensors in cephalic eye photoreceptors, but also function in additional sensory organs. This has prompted questions on the evolutionary relationship of these cell types, and if ancient r-opsins were non-photosensory. A molecular profiling approach in the marine bristleworm Platynereis dumerilii revealed shared and distinct features of cephalic and non-cephalic r-opsin1-expressing cells. Non-cephalic cells possess a full set of phototransduction components, but also a mechanosensory signature. Prompted by the latter, we investigated Platynereis putative mechanotransducer and found that nompc and pkd2.1 co-expressed with r-opsin1 in TRE cells by HCR RNA-FISH. To further assess the role of r-Opsin1 in these cells, we studied its signaling properties and unraveled that r-Opsin1 is a Gαq-coupled blue light receptor. Profiling of cells from r-opsin1 mutants versus wild-types, and a comparison under different light conditions reveals that in the non-cephalic cells light - mediated by r-Opsin1 - adjusts the expression level of a calcium transporter relevant for auditory mechanosensation in vertebrates. We establish a deep-learning-based quantitative behavioral analysis for animal trunk movements and identify a light- and r-Opsin-1-dependent fine-tuning of the worm's undulatory movements in headless trunks, which are known to require mechanosensory feedback. Our results provide new data on peripheral cell types of likely light sensory/mechanosensory nature. These results point towards a concept in which such a multisensory cell type evolved to allow for fine-tuning of mechanosensation by light. This implies that light-independent mechanosensory roles of r-opsins may have evolved secondarily.