Whole-body photoreceptor networks are independent of 'lenses' in brittle stars

'Distributed' vision and the architecture of animal visual systems

More than a century of research, of which JEB has published a substantial selection, has highlighted the rich diversity of animal eyes. From these studies have emerged numerous examples of visual systems that depart from our own familiar blueprint, a single pair of lateral cephalic eyes. It is now clear that such departures are common, widespread and highly diverse, reflecting a variety of different eye types, visual abilities and architectures. Many of these examples have been described as 'distributed' visual systems, but this includes several fundamentally different systems. Here, I re-examine this term, suggest a new framework within which to evaluate visual system distribution in both spatial and functional senses, and propose a roadmap for future work. The various architectures covered by this term reflect three broad strategies that offer different opportunities and require different approaches for study: the duplication of functionally identical eyes, the expression of multiple, functionally distinct eye types in parallel and the use of dispersed photoreceptors to mediate visual behaviour without eyes. Within this context, I explore some of the possible implications of visual system architecture for how visual information is collected and integrated, which has remained conceptually challenging in systems with a large degree of spatial and/or functional distribution. I highlight two areas that should be prioritised in future investigations: the whole-organism approach to behaviour and signal integration, and the evolution of visual system architecture across Metazoa. Recent advances have been made in both areas, through well-designed ethological experiments and the deployment of molecular tools.

Photobehaviours guided by simple photoreceptor systems

Light provides a widely abundant energy source and valuable sensory cue in nature. Most animals exposed to light have photoreceptor cells and in addition to eyes, there are many extraocular strategies for light sensing. Here, we review how these simpler forms of detecting light can mediate rapid behavioural responses in animals. Examples of these behaviours include photophobic (light avoidance) or scotophobic (shadow) responses, photokinesis, phototaxis and wavelength discrimination. We review the cells and response mechanisms in these forms of elementary light detection, focusing on aquatic invertebrates with some protist and terrestrial examples to illustrate the general principles. Light cues can be used very efficiently by these simple photosensitive systems to effectively guide animal behaviours without investment in complex and energetically expensive visual structures.

Polarization sensitivity and decentralized visual processing in an animal with a distributed visual system

The marine mollusc Acanthopleura granulata (Mollusca; Polyplacophora) has a distributed visual array composed of hundreds of small image-forming eyes embedded within its eight dorsal shell plates. As in other animals with distributed visual systems, we still have a poor understanding of the visual capabilities of A. granulata and we have yet to learn where and how it processes visual information. Using behavioral trials involving isoluminant looming visual stimuli, we found that A. granulata demonstrates spatial vision with an angular resolution of 6 deg. We also found that A. granulata responds to looming stimuli defined by contrasting angles of linear polarization. To learn where and how A. granulata processes visual information, we traced optic nerves using fluorescent lipophilic dyes. We found that the optic nerves innervate the underlying lateral neuropil, a neural tissue layer that circumnavigates the body. Adjacent optic nerves innervate the lateral neuropil with highly overlapping arborizations, suggesting it is the site of an integrated visuotopic map. Using immunohistochemistry, we found that the lateral neuropil of A. granulata is subdivided into two separate layers. In comparison, we found that a chiton with eyespots (Chiton tuberculatus) and two eyeless chitons (Ischnochiton papillosus and Chaetopleura apiculata) have lateral neuropil that is a singular circular layer without subdivision, findings consistent with previous work on chiton neuroanatomy. Overall, our results suggest that A. granulata effectuates its visually mediated behaviors using a unique processing scheme: it extracts spatial and polarization information using a distributed visual system, and then integrates and processes that information using decentralized neural circuits.

Algal Ocelloids and Plant Ocelli

Vision is essential for most organisms, and it is highly variable across kingdoms and domains of life. The most known and understood form is animal and human vision based on eyes. Besides the wide diversity of animal eyes, some animals such as cuttlefish and cephalopods enjoy so-called dermal or skin vision. The most simple and ancient organ of vision is the cell itself and this rudimentary vision evolved in cyanobacteria. More complex are so-called ocelloids of dinoflagellates which are composed of endocellular organelles, acting as lens- and cornea/retina-like components. Although plants have almost never been included into the recent discussions on organismal vision, their plant-specific ocelli had already been proposed by Gottlieb Haberlandt already in 1905. Here, we discuss plant ocelli and their roles in plant-specific vision, both in the shoots and roots of plants. In contrast to leaf epidermis ocelli, which are distributed throughout leaf surface, the root apex ocelli are located at the root apex transition zone and serve the light-guided root navigation. We propose that the plant ocelli evolved from the algal ocelloids, are part of complex plant sensory systems and guide cognition-based plant behavior.

Panoramic spatial vision in the bay scallop Argopecten irradians

We have a growing understanding of the light-sensing organs and light-influenced behaviours of animals with distributed visual systems, but we have yet to learn how these animals convert visual input into behavioural output. It has been suggested they consolidate visual information early in their sensory-motor pathways, resulting in them being able to detect visual cues (spatial resolution) without being able to locate them (spatial vision). To explore how an animal with dozens of eyes processes visual information, we analysed the responses of the bay scallop Argopecten irradians to both static and rotating visual stimuli. We found A. irradians distinguish between static visual stimuli in different locations by directing their sensory tentacles towards them and were more likely to point their extended tentacles towards larger visual stimuli. We also found that scallops track rotating stimuli with individual tentacles and with rotating waves of tentacle extension. Our results show, to our knowledge for the first time that scallops have both spatial resolution and spatial vision, indicating their sensory-motor circuits include neural representations of their visual surroundings. Exploring a wide range of animals with distributed visual systems will help us learn the different ways non-cephalized animals convert sensory input into behavioural output.

The Diversity of Eyes and Vision

Every aspect of vision, from the opsin proteins to the eyes and the ways that they serve animal behavior, is incredibly diverse. It is only with an evolutionary perspective that this diversity can be understood and fully appreciated. In this review, I describe and explain the diversity at each level and try to convey an understanding of how the origin of the first opsin some 800 million years ago could initiate the avalanche that produced the astonishing diversity of eyes and vision that we see today. Despite the diversity, many types of photoreceptors, eyes, and visual roles have evolved multiple times independently in different animals, revealing a pattern of eye evolution strictly guided by functional constraints and driven by the evolution of gradually more demanding behaviors. I conclude the review by introducing a novel distinction between active and passive vision that points to uncharted territories in vision research. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Run and hide: visual performance in a brittle star

Spatial vision was recently reported in a brittle star, Ophiomastix wendtii, which lacks discrete eyes, but little is known about its visual ecology. Our aim was to better characterize the vision and visual ecology of this unusual visual system. We tested animal orientation relative to vertical bar stimuli at a range of angular widths and contrasts, to identify limits of angular and contrast detection. We also presented dynamic shadow stimuli, either looming towards or passing the animal overhead, to test for potential defensive responses. Finally, we presented animals lacking a single arm with a vertical bar stimulus known to elicit a response in intact animals. We found that O. wendtii orients to large (≥50 deg), high-contrast vertical bar stimuli, consistent with a shelter-seeking role and with photoreceptor acceptance angles estimated from morphology. We calculate poor optical sensitivity for individual photoreceptors, and predict dramatic oversampling for photoreceptor arrays. We also report responses to dark stimuli moving against a bright background - this is the first report of responses to moving stimuli in brittle stars and suggests additional defensive uses for vision in echinoderms. Finally, we found that animals missing a single arm orient less well to static stimuli, which requires further investigation.

Neural anatomy of echinoid early juveniles and comparison of nervous system organization in echinoderms

The echinoderms are a phylum of marine deuterostomes characterized by the pentaradial (five fold) symmetry of their adult bodies. Due to this unusual body plan, adult echinoderms have long been excluded from comparative analyses aimed at understanding the origin and evolution of deuterostome nervous systems. Here, we investigated the neural anatomy of early juveniles of representatives of three of the five echinoderm classes: the echinoid Paracentrotus lividus, the asteroid Patiria miniata, and the holothuroid Parastichopus parvimensis. Using whole mount immunohistochemistry and confocal microscopy, we found that the nervous system of echinoid early juveniles is composed of three main structures: a basiepidermal nerve plexus, five radial nerve cords connected by a circumoral nerve ring, and peripheral nerves innervating the appendages. Our whole mount preparations further allowed us to obtain thorough descriptions of these structures and of several innervation patterns, in particular at the level of the appendages. Detailed comparisons of the echinoid juvenile nervous system with those of asteroid and holothuroid juveniles moreover supported a general conservation of the main neural structures in all three species, including at the level of the appendages. Our results support the previously proposed hypotheses for the existence of two neural units in echinoderms: one consisting of the basiepidermal nerve plexus to process sensory stimuli locally and one composed of the radial nerve cords and the peripheral nerves constituting a centralized control system. This study provides the basis for more in-depth comparisons of the echinoderm adult nervous system with those of other animals, in particular hemichordates and chordates, to address the long-standing controversies about deuterostome nervous system evolution.

Troglomorphism in the brittle star Ophionereis commutabilis Bribiesca-Contreras et al., 2019 (Echinodermata, Ophiuroidea, Ophionereididae)

Due to their peculiar and sometimes bizarre morphology, cave fauna (across invertebrates and vertebrates from both aquatic and terrestrial cave habitats) have fascinated researchers throughout history. Despite their success in colonizing most marine ecosystems, the adaptations of cave brittle stars (Ophiuroidea) to a stygobiotic lifestyle have been scarcely examined. Employing comparative methods on a data set of two species belonging to the genus Ophionereis, this study addresses whether a cave-dwelling species from Cozumel exhibited similar troglomorphic traits as those of other taxa inhabiting caves. Our work demonstrated that some characters representing potential morphological cave adaptations in O. commutabilis were: bigger sizes, elongation of arms and tube feet and the presence of traits potentially paedomorphic. In addition, an element of ophiuroid’s photoreceptor system, as well as pigmentation, was observed to be peculiar in this stygobiotic species, plausibly as a result of inhabiting a low light-energy environment. Finally, we add evidence to the statement that O. commutabilis is a cave endemic species, already supported by demography, distribution and origin of this species, and now by a typical array of troglomorphisms.

Visual Ecology: Now You See, Now You Don't

During the day, the brittle star Ophiocoma wendtii demonstrates spatial vision due to a distributed network of extraocular photoreceptors whose fields of view are restricted by chromatophores. At night, these chromatophores contract and O. wendtii loses spatial vision.

The crowns have eyes: multiple opsins found in the eyes of the crown-of-thorns starfish Acanthaster planci

Background

Opsins are G protein-coupled receptors used for both visual and non-visual photoreception, and these proteins evolutionarily date back to the base of the bilaterians. In the current sequencing age, phylogenomic analysis has proven to be a powerful tool, facilitating the increase in knowledge about diversity within the opsin subclasses and, so far, at least nine types of opsins have been identified. Within echinoderms, opsins have been studied in Echinoidea and Ophiuroidea, which do not possess proper image forming eyes, but rather widely dispersed dermal photoreceptors. However, most species of Asteroidea, the starfish, possess true eyes and studying them will shed light on the diversity of opsin usage within echinoderms and help resolve the evolutionary history of opsins.

Results

Using high-throughput RNA sequencing, we have sequenced and analyzed the transcriptomes of different Acanthaster planci tissue samples: eyes, radial nerve, tube feet and a mixture of tissues from other organs. At least ten opsins were identified, and eight of them were found significantly differentially expressed in both eyes and radial nerve, with R-opsin being the most highly expressed in the eye.

Conclusion

This study provides new important insight into the involvement of opsins in visual and nonvisual photoreception. Of relevance, we found the first indication of an r-opsin photopigment expressed in a well-developed visual eye in a deuterostome animal. Additionally, we provided tissue specific A. planci transcriptomes that will aid in future Evo Devo studies.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1276-0) contains supplementary material, which is available to authorized users.

Phylogenomics, life history and morphological evolution of ophiocomid brittlestars

Brittlestars in the family Ophiocomidae are large and colourful inhabitants of tropical shallow water habitats across the globe. Here we use targeted capture and next-generation sequencing to generate robust phylogenomic trees for 39 of the 43 species in order to test the monophyly of existing genera. The large genus Ophiocoma, as currently constituted, is paraphyletic on our trees and required revision. Four genera are recognised herein: an expanded Ophiomastix (now including Ophiocoma wendtii, O. occidentalis, O. endeani, O. macroplaca, and Ophiarthrum spp), Ophiocomella (now including the non-fissiparous Ophiocoma pumila, aethiops and valenciae) and Breviturma (now including Ophiocoma pica, O. pusilla, O. paucigranulata and O. longispina) and a restricted Ophiocoma. The resulting junior homonym Ophiomastix elegans is renamed O. brocki. The genus Ophiomastix exhibits relatively high rates of morphological disparity compared to other lineages. Ophiomastix flaccida and O. (formerly Ophiarthrum) pictum have divergent mitochondrial genomes, characterised by gene-order rearrangements, strand recoding, enriched GT base composition, and a corresponding divergence of nuclear mitochondrial protein genes. The new phylogeny indicates that larval and developmental transitions occurred rarely. Larval culture trials show that species with abbreviated lecithotrophic larval development occur only within Ophiomastix, although the possible monophyly of these species is obscured by the rapid early radiation within this genus. Asexual reproduction by fission is limited to one species-complex within Ophiocomella, also characterised by elevated levels of allelic heterozygosity, and which has achieved a relatively rapid global distribution. The crown ages of the new genera considerably predate the closure of the Tethyan seaway and all four are distributed in both the Atlantic and Indo-Pacific Oceans. Two species pairs appear to reflect the closure of the Panama Seaway, although their fossil-calibrated node ages (12-14 ± 6 my), derived from both concatenated sequence and multispecies coalescent analyses, considerably predate the terminal closure event. Ophiocoma erinaceus has crossed the East Pacific barrier and is recorded from Clipperton Island, SW of Mexico.

Evidence for spatial vision in Chiton tuberculatus, a chiton with eyespots

To better understand relationships between the structures and functions of the distributed visual systems of chitons, we compare how morphological differences between the light-sensing structures of these animals relate to their visually guided behaviors. All chitons have sensory organs - termed aesthetes - embedded within their protective shell plates. In some species, the aesthetes are interspersed with small, image-forming eyes. In other species, the aesthetes are paired with pigmented eyespots. Previously, we compared the visually influenced behaviors of chitons with aesthetes to those of chitons with both aesthetes and eyes. Here, we characterize the visually influenced behaviors of chitons with aesthetes and eyespots. We find that chitons with eyespots engage in behaviors consistent with spatial vision, but appear to use spatial vision for different tasks than chitons with eyes. Unlike chitons with eyes, Chiton tuberculatus and C. marmoratus fail to distinguish between sudden appearances of overhead objects and equivalent, uniform changes in light levels. We also find that C. tuberculatus orients to static objects with angular sizes as small as 10 deg. Thus, C. tuberculatus demonstrates spatial resolution that is at least as fine as that demonstrated by chitons with eyes. The eyespots of Chiton are smaller and more numerous than the eyes found in other chitons and they are separated by angles of <0.5 deg, suggesting that the light-influenced behaviors of Chiton may be more accurately predicted by the network properties of their distributed visual system than by the structural properties of their individual light-detecting organs.

Whole-body photoreceptor networks are independent of 'lenses' in brittle stars

Photoreception and vision are fundamental aspects of animal sensory biology and ecology, but important gaps remain in our understanding of these processes in many species. The colour-changing brittle star Ophiocoma wendtii is iconic in vision research, speculatively possessing a unique whole-body visual system that incorporates information from nerve bundles underlying thousands of crystalline 'microlenses'. The hypothesis that these might form a sophisticated compound eye-like system regulated by chromatophores has been extensively reiterated, with investigations into biomimetic optics and similar supposedly 'visual' structures in living and fossil taxa. However, no photoreceptors or visual behaviours have ever been identified. We present the first evidence of photoreceptor networks in three Ophiocoma species, both with and without microlenses and colour-changing behaviour. High-resolution microscopy, immunohistochemistry and synchrotron tomography demonstrate that putative photoreceptors cover the animals' oral, lateral and aboral surfaces, but are absent at the hypothesized focal points of the microlenses. The structural optics of these crystal 'lenses' are an exaptation and do not fulfil any apparent visual role. This contradicts previous studies, yet the photoreceptor network in Ophiocoma appears even more widespread than previously anticipated, both taxonomically and anatomically.