True Eyes Research Articles

Phylogenomic analyses shed light on the relationships of chiton superfamilies and shell-eye evolution.

Mollusca is the second-largest animal phylum with over 100,000 extant species representing eight classes. Across 1000 extant species in the class Polyplacophora, chitons have a relatively constrained morphology but with some notable deviations. Several genera possess "shell eyes", i.e., true eyes with a lens and retina that are embedded within the dorsal shells. The phylogeny of the major chiton clades is mostly well established, in a set of superfamily-level and higher level taxa supported by various approaches, including morphological studies, multiple gene markers, mitogenome-phylogeny, and phylotranscriptomic approaches. However, one critical lineage has remained unclear, namely Schizochiton which was controversially suggested as being the potential independent origin of chiton shell eyes. Here, with the draft genome sequencing of Schizochiton incisus (superfamily Schizochitonoidea) plus assemblies of transcriptome data from other polyplacophorans, we present phylogenetic reconstructions using both mitochondrial genomes and phylogenomic approaches with multiple methods. We found that phylogenetic trees from mitogenomic data are inconsistent, reflecting larger scale confounding factors in molluscan mitogenomes. However, a consistent and robust topology was generated with protein-coding genes using different models and methods. Our results support Schizochitonoidea as the sister group to other Chitonoidea in Chitonina, in agreement with the established classification. Combined with evidence from fossils, our phylogenetic results suggest that the earliest origin of shell eyes is in Schizochitonoidea, and that these structures were also gained secondarily in other genera in Chitonoidea. Our results have generated a holistic review of the internal relationship within Polyplacophora, and a better understanding of the evolution of Polyplacophora.

Habitat degradation and predators have independent trait-mediated effects on prey

Coral reefs are degrading globally leading to a catastrophic loss of biodiversity. While shifts in the species composition of communities have been well documented associated with habitat change, the mechanisms that underlie change are often poorly understood. Our study experimentally examines the effects of coral degradation on the trait-mediated effects of predators on the morphology, behaviour and performance of a juvenile coral reef fish. Juvenile damselfish were exposed to predators or controls (omnivore or nothing) in seawater that had flowed over either live or dead-degraded coral over a 45d period. No interaction between water source and predator exposure was found. However, fish exposed to degraded water had larger false eyespots relative to the size of their true eyes, and were more active, both of which may lead to a survival advantage. Non-consumptive effects of predators on prey occurred regardless of water source and included longer and deeper bodies, large false eyespots that may distract predator strikes away from the vulnerable head region, and shorter latencies in their response to a simulated predator strike. Research underscores that phenotypic plasticity may assist fishes in coping with habitat degradation and promote greater resilience to habitat change than may otherwise be predicted.

A poorly preserved fish-like animal from the Chengjiang Lagerstätte (Cambrian Series 2, Stage 3)

Specimens of a poorly preserved animal, including individuals and body parts, have been discovered from the Chengjiang Lagerstätte. These specimens are interpreted to be a fish-like animal and are described as an unnamed myllokunmingiid. The morphologies and preservation of individuals and body parts are described, and the relative decay patterns are briefly discussed. These specimens are from the event mudstone beds of the Maotianshan Shale Member, but exhibit the classic preservation of organisms in the background mudstone beds: slow accumulation, and this might indicate that different preservation modes of organisms co-exist in the event mudstone beds. Based on the facts that lenses cannot be discerned on all the specimens of myllokunmingiids, their visual organs may have been more like eyespots than true eyes.

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

BackgroundOpsins 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.ResultsUsing 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.ConclusionThis 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.

Bionic function surface inspired by the preventing moisture property mechanism of <italic>Ephemera pictiventris</italic> mcLachlan compound eyes

The fogging phenomenon affects light transmittance on the surface of a transparent substrate, and what’s more, the substrate surface will therefore become wet, which leads to some secondary problems such as corrosion and deterioration etc. There are two ways to solve the antifogging problem. One is hydrophilic and the other is hydrophobic. However, hydrophilic antifogging method cannot change the “wet” nature. The anti-moisture function on Ephemera pictiventris mcLachlan compound eyes surface was studied in this paper, and the compound eyes structure was characterized via Scanning Electron Microscope (SEM) and Laser Scanning Confocal Microscope (LSCM). Then anti-moisture mechanism was explained by Cassie model based on above structure data. Next artificial Ephemera pictiventris mcLachlan compound eyes surfaces were prepared by direct three-beam Laser Interference Lithography (LIL) and Colloidal Lithography (CL) respectively, and further wettability was also discussed. Results show that the compound eyes surface of Ephemera pictiventris mcLachlan exhibits striking hydrophobic anti-moisture property. The compound eyes are composed of hundreds of micron-scale ommatidia closely arranged on ellipsoidal surface. The morphology of each ommatidium is almost uniform and made up of two parts. The upper part is spherical and the lower is frustum of a cone with the upper spherical diameter of about 20 μ m and the overall height of about 11 μ m. There is no more tiny nanostructure on the ommatidia surface. The morphology and dimension of bionic convex array surface that is artificial compound eyes surface are close to the parameters of true compound eyes surface. The water contact angle (WCA) value of this surface can reach 123°, showing a strong hydrophobic property, after etched 2 min by HF with the concentration of 5%. The mechanism analysis results show that the calculated apparent WCA value is 119.2°, in agreement with the experimental value. Also the apparent WCA value increases with the increase of intrinsic WCA value. The smaller the ratio of ommatidia radius, the greater the apparent WCA value. When the ratio is less than or equal to 0.4, the apparent WCA value is up to 150° or more. The static WCA value and water sliding angle (WSA) value of artificial optimized Ephemera pictiventris mcLachlan compound eyes surface with the feature size of 350 nm are 160°±5° and 2°, respectively, showing excellent superhydrophobic and self-cleaning functions. The result indicates that Ephemera pictiventris mcLachlan compound eyes is dry-style antifogging function structure surface. The synthesis of proper size, compact arrangement of ommatidia and arc shape of the compound eyes is one of the origins of hydrophobic antifogging function. Artificial optimized compound eyes surface shows excellent superhydrophobic and self-cleaning functions.

Predator mimicry, not conspicuousness, explains the efficacy of butterfly eyespots.

Large conspicuous eyespots on butterfly wings have been shown to deter predators. This has been traditionally explained by mimicry of vertebrate eyes, but recently the classic eye-mimicry hypothesis has been challenged. It is proposed that the conspicuousness of the eyespot, not mimicry, is what causes aversion due to sensory biases, neophobia or sensory overloads. We conducted an experiment to directly test whether the eye-mimicry or the conspicuousness hypothesis better explain eyespot efficacy. We used great tits (Parus major) as model predator, and tested their reaction towards animated images on a computer display. Birds were tested against images of butterflies without eyespots, with natural-looking eyespots, and manipulated spots with the same contrast but reduced resemblance to an eye, as well as images of predators (owls) with and without eyes. We found that mimetic eyespots were as effective as true eyes of owls and more efficient in eliciting an aversive response than modified, less mimetic but equally contrasting eyespots. We conclude that the eye-mimicry hypothesis explains our results better than the conspicuousness hypothesis and is thus likely to be an important mechanism behind the evolution of butterfly eyespots.

Infrared Organs of Snakes: An Integral Part of Vision

Abstract The infrared organs of boas, pythons, and pit vipers are true eyes that function not by a photochemical reaction but on the basis of heat generated in the receptors (called terminal nerve masses, TNMs), by electromagnetic radiation. In the pythons and pit vipers, the pit opening acts as the aperture of a pinhole camera, a virtual lens that permits the receptors to encode the movements of an infrared source sufficiently for the brain to form an image. Many boid snakes possess TNMs identical to those of the pythons but lack an opening that could serve as a lens. All TNMs are irrigated by a dense capillary network that serves as a heat regulator, mimicking the role of the photochemical cycle in the lateral eyes. Thus, the pits are an integral part of the snakes' visual system, which makes use of the longer waves of the electromagnetic spectrum for which there are no appropriate photoreceptive pigments in nature; they do everything the eyes do. They are definitely not, as they have often been treated...

Ultrastructure of pigmented adult eyes in errant polychaetes (Annelida): implications for annelid evolution

The evolution of photoreceptor cells and eyes in Metazoa is far from being resolved, although recent developmental and structural studies have provided strong evidence for a common origin of photoreceptor cells and existence of sister cell types already in early metazoans. These sister cell types are ciliary and rhabdomeric photoreceptor cells, depending on which part of each cell is involved in photoreception proper. However, a crucial point in eye evolution is how the enormous structural diversity of photoreceptor cells and visual systems developed, given the general molecular conservation of the photoreceptor cells. One example of this diversity can be observed in Annelida. Within the polychaetes the errant forms, taxon Aciculata, constitute the only group possessing true multicellular eyes in the adult stage. Thus far these organs have been investigated only in taxa of Phyllodocida, a subgroup of Aciculata. Data on Eunicida and Amphinomida as well as certain phyllodocidan taxa had been lacking. The ultrastructure of these adult eyes was investigated in various species of errant polychaetes, belonging to Amphinomidae, Eunicidae and Hesionidae, to elucidate whether they provide any phylogenetic clues regarding either the evolution of visual systems in Annelida or lophotrochozoan phylogeny in general. These eyes are composed of numerous supportive pigment cells and rhabdomeric photoreceptor cells and sometimes additional cell types. As a rule the pigment and rhabdomeric cell types form a continuous epithelium in which the two types intermingle. Presence of granules with shading pigment in sensory cells is a common feature but is apparently restricted to a taxon comprising Phyllodocida and Eunicida s. str. Very likely a lens-like structure does not belong to the ground pattern of annelid eyes, despite its presence in Phyllodocida. These lens-like structures are formed by secretions or cellular processes of the pigment cells. In many species the eye cup communicates with the exterior via a small cuticularized canal. This canal is interpreted as a rudiment due to the mode of formation in the epidermis. With respect to current phylogenetic hypotheses, these multicellular eyes have either been developed in the stem species of a taxon Aciculata nested within the polychaetes or have been evolved in the stem lineage of Annelida. Similarities to gastropod eyes are interpreted as convergent and not as indication of common origin. Except for the photoreceptor cells proper, the structure of the adult eyes in polychaetes most likely does not help to resolve lophotrochozoan phylogeny.

Arthropodization in the Hirudinea: evidence for a phylogenetic link with insects and other Uniramia?

The Hirudinea have a number of arthropod-like characters, including a true haemocoel and compound eyes. This ‘arthropodization’ may have bearing on the origin of the arthropod phylum Uniramia. Certain traits shared by Clitellata and Uniramia are interpreted as primitive and compatible with the view that a monophyletic link exists between these two groups: ‘internal’ fertilization, egg protected by cocoon/chorion, egg yolky with direct development not invoking a planktotrophic larval stage, formation and fate of presumptive areas in early embryology, uniramous lobopodial-like structures and the presence of uniramous mouthparts. Still other traits shared by Hirudinea and Hexapoda are interpreted as advanced and evidence for a paraphyletic relationship between leeches and pterygote insects: oogenesis with nurse cells, cephalization and segmental constancy. It is proposed that a major theme in uniramian evolution is the invasion of land from freshwater by a pre-lobopodial clitellate-like ancestor, the lobopodium being an adaptation to terrestrial locomotion. These views give new significance to the poorly studied fossil segmented worms from freshwater and terrestrial palaeohabitats.

THE FUNCTION OF THE EYESPOT IN CHLAM YDOMONAS

The function of the eyespot in flagellates in general, and in Chlamydomonas in particular, has been the subject of much discussion, but a definite conclusion which is generally acceptable has yet to be reached. This indecision has been mainly due to the inability to make any truly direct approach to the problem. Until methods in microtechnique become sufficiently refined to allow the removal of eyespots from individual cells, it appears inevitable that discussion of the subject must continue to be based on indirect evidence only. By its very name which has long been in common usage, the eyespot is assumed to be photosensitive, and its similarity in colour to the true eyes of certain lower animals established this assumption long before there was any experimental evidence. Strasburger (I878) was among the first to bring a more critical attitude towards the subject by pointing out that many flagellates without eyespots were nevertheless reactive to light. He postulated that sensitivity to light was a property of the entire protoplasm in those organisms which showed it. Soon afterwards Engelmann (i882) published the results of experiments with Euglena. In this organism the eyespot is situated at the anterior end of the cell. He passed a sharply defined shadow forward along the length of a cell swimming towards a source of light, and observed that this had no effect until the shadow covered the region of the eyespot. At this point the cell shortened itself and turned away from the original direction of motion. Mast (I927) claimed to have repeated and confirmed this experiment. Luntz (I93I), however, was unable to produce a shadow with an edge so distinct that part of a cell could be shaded and the rest in the light, and he questioned the validity of Engelmann's results. Such criticism may be unjustified, but the value of discussing it in this paper is doubtful since there is good reason to believe that the eyespots of Euglenophyta on the one hand, and of Chlorophyta on the other, are structurally and functionally dissimilar. In Euglenophyta the eyespot consists of an irregular mass of pigment granules, whereas in Chlorophyta the eyespot has a definite outline and appears to be more homogeneous. A complex structure, including a cup-shaped pigment layer and a lens which concentrates the light, has been postulated for certain members of the Volvocales (Mast, 1927), In addition to these structural differences, the position of the eyespot is significant. In Euglenophyta the eyespot is always located at the anterior end of the cell close to the base of the flagellum. Wager (I900) observed in Euglena an enlargement of the base of the flagellum adjacent to the eyespot. By this arrangement the eyespot casts a shadow on the enlargement when the light comes from a particular direction, and possibly affects