Deep-sea starfish from the Arctic have well-developed eyes in the dark

Eyes, Vision, and Bioluminescence in Deep-Sea Brisingid Sea Stars

AbstractSea stars are a major component of the megabenthos in most marine habitats, including those within the deep sea. Being radially symmetric, sea stars have sensory structures that are evenly distributed along the arms, with a compound eye located on each arm tip of most examined species. Surprisingly, eyes with a spatial resolution that rivals the highest acuity known among sea stars so far were recently found in Novodinia americana, a member of the deep-sea sea star order Brisingida. Here, we examined 21 species across 11 brisingid genera for the presence of eyes; where eyes were present, we used morphological characteristics to evaluate spatial resolution and sensitivity. This study found that eyes were present within 43% of the examined species. These brisingid eyes were relatively large compared to those of other deep-sea sea stars, with a high number of densely packed ommatidia. One of the examined species, Brisingaster robillardi, had more than 600 ommatidia per eye, which is the highest number of ommatidia found in any sea star eye so far. Combined, the results indicate that brisingid eyes are adapted for spatial resolution over sensitivity. Together with results showing that many brisingids are bioluminescent, this relatively high spatial resolution suggests that the group may use their eyes to support visually guided intraspecific communication based on bioluminescent signals. Phylogenetic analysis indicated that the common ancestor of brisingids had eyes (P = 0.72) and that eyes were lost once within the clade.

Variation in eye abundance among scallops reveals ontogenetic and evolutionary convergence associated with life habits

Eyes are remarkable systems to investigate the complex interaction between ecological drivers and phenotypic outcomes. Some animals, such as scallops, have many eyes for visual perception, but to date, the evolution of multiple-eye systems remains obscure. For instance, it is unclear whether eye number changes over a lifetime or varies among species. Scallops are a suitable model group to investigate these questions considering the interspecific variation of adult size and ecological diversity. We tested whether eye abundance scales with body size among individuals and species and whether it varies with life habits. We performed comparative analyses, including a phylogenetic ANCOVA and evolutionary model comparisons, based on eye count and shell height (as a proxy of body size) across 31 scallop species. Our analyses reveal that patterns of increasing relationship with body size are not concordant among taxa and suggest ontogenetic convergence caused by similar ecologies. Accordingly, selective optima in eye numbers are associated with shifts in life habits. For instance, species with increased mobility have significantly more eyes than less mobile species. The convergent evolution of greater eye abundance in more mobile scallops likely indicates a visual improvement based on increased levels of oversampling of the surrounding environment.

On the genus Crossaster (Echinodermata: Asteroidea) and its distribution

Several starfish (Echinodermata, Asteroidea) are keystone species of marine ecosystems, but some of the species are difficult to identify using morphological criteria only. The common sunstar, Crossaster papposus (Linnaeus, 1767), is a conspicuous species with a wide circumboreal distribution. In 1900, a closely similar species, C. squamatus (Döderlein, 1900) was described from the NE Atlantic Ocean, but subsequent authors have differed in their views on whether this is a valid taxon or rather an ecotype associated with temperature variations. We assessed the differentiating morphological characters of specimens from Norwegian and Greenland waters identified as C. papposus and C. squamatus and compared their distributions in the NE Atlantic as inferred from research cruises. The field data show that C. papposus is found mainly in temperate and shallow waters, whereas C. squamatus resides on the shelf-break in colder, mixed water masses. Intraspecific diversity and interspecific genetic differentiation of the two putative species, and their phylogenetic relationships to several Crossaster congeners worldwide, were explored using mitochondrial and nuclear DNA sequences. The molecular evidence suggests that C. papposus is the more diverse and geographically structured taxon, in line with its wide distribution. C. papposus and C. squamatus are closely related, yet clearly distinct taxa, while C. papposus and C. multispinus H.L. Clark, 1916, the latter from the South Pacific Ocean, are closely related, possibly sister taxa.

Distribution and quantification of bioluminescence as an ecological trait in the deep sea benthos

Bioluminescence is a prominent functional trait used for visual communication. A recent quantification showed that in pelagic ecosystems more than 75% of individual macro-planktonic organisms are categorized as able to emit light. In benthic ecosystems, only a few censuses have been done, and were based on a limited number of observations. In this study, our dataset is based on observations from remotely operated vehicle (ROV) dives conducted from 1991-2016, spanning 0-3,972 m depth. Data were collected in the greater Monterey Bay area in central California, USA and include 369,326 pelagic and 154,275 epibenthic observations at Davidson Seamount, Guide Seamount, Sur Ridge and Monterey Bay. Because direct observation of in situ bioluminescence remains a technical challenge, taxa from ROV observations were categorized based on knowledge gained from the literature to assess bioluminescence status. We found that between 30-41% of the individual observed benthic organisms were categorized as capable of emitting light, with a strong difference between benthic and pelagic ecosystems. We conclude that overall variability in the distribution of bioluminescent organisms is related to the major differences between benthic and pelagic habitats in the deep ocean. This study may serve as the basis of future investigations linking the optical properties of various habitats and the variability of bioluminescent organism distributions.

Eyes and negative phototaxis in juvenile crown-of-thorns starfish, Acanthaster species complex

As a corallivore, the crown-of-thorns starfish (COTS; Acanthaster species complex), has significant impacts on coral mortality and community structure on tropical reefs throughout its Indo-Pacific range. COTS form aggregations which systematically move through and across reefs causing significant loss in hard coral cover. Previous work has shown that their behaviours on the reef are influenced by rheotaxis, olfaction and vision, with vision guiding adult animals to their coral habitat at short distances. As the compound eye of starfish grows throughout life the visual capacity of juvenile eyes is putatively less than for adult animals. Here we show this to be the case. Juvenile eyes have approximately the same visual field as adult eyes but significantly lower spatial resolution. They display negative phototaxis, as observed in adults, but we found no direct proof for the use of spatial resolution in this behaviour. Our results show that juveniles are able to use their eyes to locate their habitat: the coral reef. However, their putatively lower spatial resolution would make this visual task more difficult than for the adults.

This article has an associated First Person interview with the first author of the paper.

Summary: The juvenile crown-of-thorns starfish has much smaller eyes than adults do, which results in worse spatial resolution. Still, they use the eyes for negative phototaxis seeking shelter.

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.