Repeated eye reduction events reveal multiple pathways to degeneration in a family of marine snails

Evolutionary loss of shell pigmentation, pattern, and eye structure in deep-sea snails in the dysphotic zone

Adaptations to habitats lacking light, such as the reduction or loss of eyes and pigmentation, have fascinated biologists for centuries, yet have rarely been studied in the deep sea, the earth's oldest and largest light-limited habitat. Here, we investigate the evolutionary loss of shell pigmentation, pattern, and eye structure across a family of deep-sea gastropods (Solariellidae). We show that within our phylogenetic framework, loss of these traits evolves without reversal, at different rates (faster for shell traits than eye structure), and over different depth ranges. Using a Bayesian approach, we find support for correlated evolution of trait loss with increasing depth within the dysphotic region. A transition to trait loss occurs for pattern and eye structure at 400-500 m and for pigmentation at 600-700 m. We also show that one of the sighted, shallow-water species, Ilanga navakaensis, which may represent the "best-case" scenario for vision for the family, likely has poor spatial acuity and contrast sensitivity. We therefore propose that pigmentation and pattern are not used for intraspecific communication but are important for camouflage from visual predators, and that the low-resolution vision of solariellids is likely to require high light intensity for basic visual tasks, such as detecting predators.

Evolution and development of complex eyes: a celebration of diversity

For centuries, the eye has fascinated scientists and philosophers alike, and as a result the visual system has always been at the forefront of integrating cutting-edge technology in research. We are again at a turning point at which technical advances have expanded the range of organisms we can study developmentally and deepened what we can learn. In this new era, we are finally able to understand eye development in animals across the phylogenetic tree. In this Review, we highlight six areas in comparative visual system development that address questions that are important for understanding the developmental basis of evolutionary change. We focus on the opportunities now available to biologists to study the developmental genetics, cell biology and morphogenesis that underlie the incredible variation of visual organs found across the Metazoa. Although decades of important work focused on gene expression has suggested homologies and potential evolutionary relationships between the eyes of diverse animals, it is time for developmental biologists to move away from this reductive approach. We now have the opportunity to celebrate the differences and diversity in visual organs found across animal development, and to learn what it can teach us about the fundamental principles of biological systems and how they are built.

Hard to get, easy to lose: Evolution of mantle photoreceptor organs in bivalves (Bivalvia, Pteriomorphia)

Morphologically diverse eyes have evolved numerous times, yet little is known about how eye gain and loss is related to photic environment. The pteriomorphian bivalves (e.g., oysters, scallops, and ark clams), with a remarkable range of photoreceptor organs and ecologies, are a suitable system to investigate the association between eye evolution and ecological shifts. The present phylogenetic framework was based on amino acid sequences from transcriptome datasets and nucleotide sequences of five additional genes. In total, 197 species comprising 22 families from all five pteriomorphian orders were examined, representing the greatest taxonomic sampling to date. Morphological data were acquired for 162 species and lifestyles were compiled from the literature for all 197 species. Photoreceptor organs occur in 11 families and have arisen exclusively in epifaunal lineages, that is, living above the substrate, at least five times independently. Models for trait evolution consistently recovered higher rates of loss over gain. Transitions to crevice-dwelling habit appear associated with convergent gains of eyespots in epifaunal lineages. Once photoreceptor organs have arisen, multiple losses occurred in lineages that shift to burrowing lifestyles and deep-sea habitats. The observed patterns suggest that eye evolution in pteriomorphians might have evolved in association with light-guided behaviors, such as phototaxis, body posture, and alarm responses.

The utility of micro-computed tomography for the non-destructive study of eye microstructure in snails

Molluscan eyes exhibit an enormous range of morphological variation, ranging from tiny pigment-cup eyes in limpets, compound eyes in ark clams and pinhole eyes in Nautilus, through to concave mirror eyes in scallops and the large camera-type eyes of the more derived cephalopods. Here we assess the potential of non-destructive micro-computed tomography (µ-CT) for investigating the anatomy of molluscan eyes in three species of the family Solariellidae, a group of small, deep-sea gastropods. We compare our results directly with those from traditional histological methods applied to the same specimens, and show not only that eye microstructure can be visualised in sufficient detail for meaningful comparison even in very small animals, but also that μ-CT can provide additional insight into gross neuroanatomy without damaging rare and precious specimens. Data from μ-CT scans also show that neurological innervation of eyes is reduced in dark-adapted snails when compared with the innervation of cephalic tentacles, which are involved in mechanoreception and possibly chemoreception. Molecular tests also show that the use of µ-CT and phosphotungstic acid stain do not prevent successful downstream DNA extraction, PCR amplification or sequencing. The use of µ-CT methods is therefore highly recommended for the investigation of difficult-to-collect or unique specimens.

The Kingdom of the Blind: Disentangling Fundamental Drivers in the Evolution of Eye Loss

Light is a fundamentally important biological cue used by almost every animal on earth, to maintain daily rhythms, navigate, forage, find mates, or avoid predators. But an enormous number of species live in darkness: in subterranean caves, deep oceans, underground burrows, and within parasitic host bodies, and the loss of eyes appears consistently across these ecosystems. However, the evolutionary mechanisms that lead to the reduction of the visual system remain the subject of great interest and debate more than 150 years after Darwin tackled the issue. Studies of model taxa have discovered significant roles for natural selection, neutral evolution, and pleiotropy, but the interplay between them remains unclear. To nail down unifying concepts surrounding the evolution of eye loss, we must embrace the enormous range of affected animals and habitats. The fine developmental details of model systems such as the Mexican cave tetra Astyanax mexicanus have transformed and enriched the field, but these should be complemented by wider studies to identify truly overarching patterns that apply throughout animals. Here, the major evolutionary drivers are placed within a conceptual cost-benefit framework that incorporates the fundamental constraints and forces that influence evolution in the dark. Major physiological, ecological, and environmental factors are considered within the context of this framework, which appears faithful to observed patterns in deep-sea and cavernicolous animals. To test evolutionary hypotheses, a comparative phylogenetic approach is recommended, with the goal of studying large groups exhibiting repeated reduction, and then comparing these across habitats, taxa, and lifestyles. Currently, developmental and physiological methods cannot feasibly be used on such large scales, but penetrative imaging techniques could provide detailed morphological data non-invasively and economically for large numbers of species. Comprehensive structural datasets can then be contextualized phylogenetically to examine recurrent trends and associations, and to reconstruct character histories through multiple independent transitions into darkness. By assessing these evolutionary trajectories within an energetic cost-benefit framework, the relationships between fundamental influences can be inferred and compared across different biological and physical parameters. However, substantial numbers of biological and environmental factors affect the evolutionary trajectory of loss, and it is critical that researchers make fair and reasonable comparisons between objectively similar groups.

Regressed but Not Gone: Patterns of Vision Gene Loss and Retention in Subterranean Mammals

Regressive evolution involves the degradation of formerly useful traits as organisms invade novel ecological niches. In animals, committing to a strict subterranean habit can lead to regression of the eyes, likely due to a limited exposure to light. Several lineages of subterranean mammals show evidence of such degeneration, which can include decreased organization of the retina, malformation of the lens, and subcutaneous positioning of the eye. Advances in DNA sequencing have revealed that this regression co-occurs with a degradation of genomic loci encoding visual functions, including protein-coding genes. Other dim light-adapted vertebrates with normal ocular anatomy, such as nocturnal and aquatic species, also demonstrate evidence of visual gene loss, but the absence of comparative studies has led to the untested assumption that subterranean mammals are special in the degree of this genomic regression. Additionally, previous studies have shown that not all vision genes have been lost in subterranean mammals, but it is unclear whether they are under relaxed selection and will ultimately be lost, are maintained due to pleiotropy or if natural selection is favoring the retention of the eye and certain critical underlying loci. Here I report that vision gene loss in subterranean mammals tends to be more extensive in quantity and differs in distribution from other dim light-adapted mammals, although some committed subterranean mammals demonstrate significant overlap with nocturnal microphthalmic species. In addition, blind subterranean mammals retain functional orthologs of non-pleiotropic visual genes that are evolving at rates consistent with purifying selection. Together, these results suggest that although living underground tends to lead to major losses of visual functions, natural selection is maintaining genes that support the eye, perhaps as an organ for circadian and/or circannual entrainment.