A unique advantage for giant eyes in giant squid

Will I stay or will I go? Eye morphology predicts individual migratory propensity in a partial migrant

Billions of animals undertake migratory journeys every year, with powerful consequences for ecosystem dynamics. Key behaviours that enable successful migration are often guided by the visual system. The amount and quality of information that animals can extract from visual scenes are directly related to structural eye size-larger eyes can house larger pupils, enhancing light-gathering capacity and vision by improving visual acuity and contrast sensitivity. Migration should exert strong demands on individual visual performance, for example via foraging, antipredator benefits or navigational requirements. Yet, it remains elusive whether variations in eye morphology and corresponding visual capabilities are associated with migratory propensity. Here, we capitalize upon intra-population variation in migratory propensity (also known as partial migration) in roach, a common freshwater fish, to directly test for migration-associated variation in image-forming eyes within a species. In a multi-year field study tracking the migration decisions of over 2000 individuals in two different lake systems, we found that relative pupil size was positively associated with individual migration propensity. Computational simulations of the visual ecology associated with the observed differences in pupil size show that migrants have an extended visual detection range and that the performance gain is most pronounced for viewing small targets (e.g. planktonic prey) under low-light conditions. These results suggest that the larger pupils of migrants represent an adaptation for increased foraging efficiency to aid in the accumulation of critical pre-migratory energy reserves. Together, our anatomical and functional findings provide new perspectives on visual system design in relation to individual-level migratory decision-making.

Evolution of Large Eyes in Stromboidea (Gastropoda): Impact of Photic Environment and Life History Traits

Eyes within the marine gastropod superfamily Stromboidea range widely in size, from 0.2 to 2.3 mm-the largest eyes known in any gastropod. Despite this interesting variation, the underlying evolutionary pressures remain unknown. Here, we use the wealth of material available in museum collections to explore the evolution of stromboid eye size and structure. Our results suggest that depth is a key light-limiting factor in stromboid eye evolution; here, increasing water depth is correlated with increasing aperture width relative to lens diameter, and therefore an increasing investment in sensitivity in dim light environments. In the major clade containing all large-eyed stromboid families, species observed active during the day and the night had wider eye apertures relative to lens sizes than species observed active during the day only, thereby prioritizing sensitivity over resolution. Species with no consistent diel activity pattern also had smaller body sizes than exclusively day-active species, which may suggest that smaller animals are more vulnerable to shell-crushing predators, and avoid the higher predation pressure experienced by animals active during the day. Within the same major clade, ancestral state reconstruction suggests that absolute eye size increased above 1 mm twice. The unresolved position of Varicospira, however, weakens this hypothesis and further work with additional markers is needed to confirm this result.

Multiple Pathways of Visual Adaptations for Water Column Usage in an Antarctic Adaptive Radiation

Evolutionary transitions in water column usage have played a major role in shaping ray-finned fish diversity. However, the extent to which vision-associated trait complexity and water column usage is coupled remains unclear. Here we investigated the relationship between depth niche, eye size, and the molecular basis of light detection across the Antarctic notothenioid adaptive radiation. Integrating a phylogenetic comparative framework with data on eye size and depth occupancy, we provide support for an acceleration in the rate of eye size diversification nearly 20 million years after the initial radiation. Our results further reveal that levels of eye size divergence are often highest between closely related taxa. We further analyzed opsin tuning site sequences and found changes representing repeated instances of independent tuning site changes across the notothenioid phylogeny that are generally not associated with habitat depth or species eye size. Collectively, our results strongly support that multiple evolutionary pathways underlie the diversification of visual adaptations in this iconic adaptive radiation.

Open questions in marine mammal sensory research

Although much research has focused on marine mammal sensory systems over the last several decades, we still lack basic knowledge for many of the species within this diverse group of animals. Our conference workshop allowed all participants to present recent developments in the field and culminated in discussions on current knowledge gaps. This report summarizes open questions regarding marine mammal sensory ecology and will hopefully serve as a platform for future research.

A new computational model illuminates the extraordinary eyes of Phronima

Vision in the midwater of the open ocean requires animals to perform visual tasks quite unlike those of any other environment. These tasks consist of detecting small, low contrast objects and point sources against a relatively dim and uniform background. Deep-sea animals have evolved many extraordinary visual adaptations to perform these tasks. Linking eye anatomy to specific selective pressures, however, is challenging, not least because of the many difficulties of studying deep-sea animals. Computational modelling of vision, based on detailed morphological reconstructions of animal eyes, along with underwater optics, offers a chance to understand the specific visual capabilities of individual visual systems. Prior to the work presented here, comprehensive models for apposition compound eyes in the mesopelagic, the dominant eye form of crustaceans, were lacking. We adapted a model developed for single-lens eyes and used it to examine how different parameters affect the model’s ability to detect point sources and extended objects. This new model also allowed us to examine spatial summation as a means to improve visual performance. Our results identify a trade-off between increased depth range over which eyes function effectively and increased distance at which extended objects can be detected. This trade-off is driven by the size of the ommatidial acceptance angle. We also show that if neighbouring ommatidia have overlapping receptive fields, spatial summation helps with all detection tasks, including the detection of bioluminescent point sources. By applying our model to the apposition compound eyes of Phronima, a mesopelagic hyperiid amphipod, we show that the specialisations of the large medial eyes of Phronima improve both the detection of point sources and of extended objects. The medial eyes outperformed the lateral eyes at every modelled detection task. We suggest that the small visual field size of Phronima’s medial eyes and the strong asymmetry between the medial and lateral eyes reflect Phronima’s need for effective vision across a large depth range and its habit of living inside a barrel. The barrel’s narrow aperture limits the usefulness of a large visual field and has allowed a strong asymmetry between the medial and lateral eyes. The model provides a useful tool for future investigations into the visual abilities of apposition compound eyes in the deep sea.

How do animals see the world? This is particularly an interesting question when the animal’s eyes look very different from our own, or if they inhabit an environment that is visually very different from ours. Biologists approach this question by seeking to determine not only how animal eyes function but also what selective pressures led to the evolution of their eyes. Understanding the eyes of deep-sea animals is particularly intriguing and more challenging than usual because their visual world is so dramatically different from our own and they are inaccessible and therefore hard to study. Understanding their visual capabilities by behavioural or physiological experiments is at best extremely challenging and often impossible. However, modelling of their visual abilities, by combining knowledge about ocular anatomy with information about the way light propagates in the deep sea, is comparatively tractable. Here we present a computational model that predicts the ability of apposition compound eyes (eyes that are widely found in many arthropod invertebrates) to detect salient visual targets in the deep sea between 200 and 700 m below the surface. We use this model specifically to examine the extraordinary ‘double eyes’ of the midwater hyperiid amphipod Phronima that have perplexed scientists for decades. This allowed us to put forward a new hypothesis about the selective pressures that have led to Phronima’s unusual eyes. The predictive model we present here also provides a framework for future assessments of visual performance of apposition compound eyes in other deep-sea animals.

Genomes of Two Flying Squid Species Provide Novel Sights into Adaptations of Cephalopods to Pelagic Life

Pelagic cephalopods have evolved a series of fascinating traits, such as excellent visual acuity, high-speed agility, and photophores for adaptation to open pelagic oceans. However, the genetic mechanisms underpinning these traits are not well understood. Thus, in this study, we obtained high-quality genomes of two purpleback flying squid species (Sthenoteuthis oualaniensis and Sthenoteuthis sp.), with sizes of 5450 Mb and 5651 Mb, respectively. Comparative genomic analyses revealed that the S-crystallin subfamily SL20-1 associated with visual acuity in the purpleback flying squid lineage was significantly expanded, and the evolution of high-speed agility for the species was accompanied by significant positive selection pressure on genes related to energy metabolism. These molecular signals might have contributed to the evolution of their adaptative predatory and anti-predatory traits. In addition, the transcriptomic analysis provided clear indications of the evolution of the photophores of purpleback flying squids, especially the recruitment of new genes and energy metabolism-related genes which may have played key functional roles in the process.

Ecological signal in the size and shape of marine amniote teeth

Amniotes have been a major component of marine trophic chains from the beginning of the Triassic to present day, with hundreds of species. However, inferences of their (palaeo)ecology have mostly been qualitative, making it difficult to track how dietary niches have changed through time and across clades. Here, we tackle this issue by applying a novel geometric morphometric protocol to three-dimensional models of tooth crowns across a wide range of raptorial marine amniotes. Our results highlight the phenomenon of dental simplification and widespread convergence in marine amniotes, limiting the range of tooth crown morphologies. Importantly, we quantitatively demonstrate that tooth crown shape and size are strongly associated with diet, whereas crown surface complexity is not. The maximal range of tooth shapes in both mammals and reptiles is seen in medium-sized taxa; large crowns are simple and restricted to a fraction of the morphospace. We recognize four principal raptorial guilds within toothed marine amniotes (durophages, generalists, flesh cutters and flesh piercers). Moreover, even though all these feeding guilds have been convergently colonized over the last 200 Myr, a series of dental morphologies are unique to the Mesozoic period, probably reflecting a distinct ecosystem structure.

Ecological Predictors and Functional Implications of Eye Size in Deep-Sea Shrimps

Constraints on energy resources and available light in the deep sea should place strong selection pressure on eye size, a fundamental determinant of visual ability. By examining eye size among 16 species (454 individuals) of deep-sea sergestid shrimps, we show significant differences in intraspecific eye growth rates and species eye-size averages that are correlated to different aspects of ecology and result in variable sighting distance thresholds of bioluminescence, one measure of visual performance. We used linear regressions modeling the lowest and highest bounds of phylogenetic signal to test for ecological correlates of relative and absolute eye size, which indicate the allocation of energetic resources toward eyes and an optical basis of visual capability, respectively. Of the ecological variables tested [mean depth, diel vertical migration (DVM) distance, habitat type, and light organ type], light organ type was the only significant correlate of both relative and absolute eye size, suggesting that bioluminescence plays a particularly important role in the evolution of sergestid vision and that these animals may be reliant on bioluminescent signaling. Our findings also suggest that the DVM imposes visual demands distinct from the average depths occupied by a species. While DVM distance correlated with relative eye size, mean depth correlated with absolute eye size, revealing that eye size increases with depth before 1,000 m, then decreases in bathypelagic (aphotic) zone. By applying measured eye sizes to models of visual performance, we estimated that sergestids can detect a bioluminescent point source from ≤3.77 m away, and that these sighting distance thresholds vary between species by a factor of three. In relative terms, however, all sergestids under the test conditions had a common detection threshold at ∼63.5 body lengths, suggesting that bioluminescence sighting distance is proportional among species and may be related to shared behaviors of swarming and copulation. By considering the effects of evolutionary history, light and nutrient availability, and the constraints of body size, our study reveals new patterns of deep-sea eye size evolution and provides new insights into the visual ecology of this diverse and important deep-sea group.

Net illumination reduces fisheries bycatch, maintains catch value, and increases operational efficiency

Small-scale fisheries are vital for food security, nutrition, and livelihoods in coastal areas throughout the world's oceans.^1-9 As intricately linked social-ecological systems, small-scale fisheries require management approaches that help ensure both ecological and socioeconomic sustainability.^7,10-14 Given their ease of use and lucrative nature, coastal gillnet fisheries are globally ubiquitous.^10,15 However, these fisheries often result in high discarded capture of non-target organisms (bycatch) that can lead to significant cascading effects throughout trophic chains^16-18 and costly fisheries restrictions that result in important revenue losses in coastal communities with scarce economic alternatives.^19,20 Despite these challenges, few solutions have been developed and broadly adopted to decrease bycatch in coastal gillnet fisheries, particularly in developing nations.^5,21 Here we used controlled experiments along Mexico's Baja California peninsula to show that illuminating gillnets with green LED lights-an emerging technology originally developed to mitigate sea turtle bycatch-significantly reduced mean rates of total discarded bycatch biomass by 63%, which included significant decreases in elasmobranch (95%), Humboldt squid (81%), and unwanted finfish (48%). Moreover, illuminated nets significantly reduced the mean time required to retrieve and disentangle nets by 57%. In contrast, there were no significant differences in target fish catch or value. These findings advance our understanding of how artificial illumination affects operational efficiency and changes in catch rates in coastal gillnet fisheries, while illustrating the value of assessing broad-scale ecological and socioeconomic effects of species-specific conservation strategies.

Comparative brain structure and visual processing in octopus from different habitats

Octopods are masters of camouflage and solve complex tasks, and their cognitive ability is said to approach that of some small mammals. Despite intense interest and some research progress, much of our knowledge of octopus neuroanatomy and its links to behavior and ecology comes from one coastal species, the European common octopus, Octopus vulgaris. Octopod species are found in habitats including complex coral reefs and the relatively featureless mid-water. There they encounter different selection pressures, may be nocturnal or diurnal, and are mostly solitary or partially social. How these different ecologies and behavioral differences influence the octopus central nervous system (CNS) remains largely unknown. Here we present a phylogenetically informed comparison between diurnal and nocturnal coastal and a deep-sea species using brain imaging techniques. This study shows that characteristic neuroanatomical changes are linked to their habits and habitats. Enlargement and division of the optic lobe as well as structural foldings and complexity in the underlying CNS are linked to behavioral adaptation (diurnal versus nocturnal; social versus solitary) and ecological niche (reef versus deep sea), but phylogeny may play a part also. The difference between solitary and social life is mirrored within the brain including the formation of multiple compartments (gyri) in the vertical lobe, which is likened to the vertebrate cortex. These findings continue the case for convergence between cephalopod and vertebrate brain structure and function. Notably, within the current push toward comparisons of cognitive abilities, often with unashamed anthropomorphism at their root, these findings provide a firm grounding from which to work.

Domestication-induced reduction in eye size revealed in multiple common garden experiments: The case of Atlantic salmon (Salmo salar L.)

Domestication leads to changes in traits that are under directional selection in breeding programmes, though unintentional changes in nonproduction traits can also arise. In offspring of escaping fish and any hybrid progeny, such unintentionally altered traits may reduce fitness in the wild. Atlantic salmon breeding programmes were established in the early 1970s, resulting in genetic changes in multiple traits. However, the impact of domestication on eye size has not been studied. We measured body size corrected eye size in 4000 salmon from six common garden experiments conducted under artificial and natural conditions, in freshwater and saltwater environments, in two countries. Within these common gardens, offspring of domesticated and wild parents were crossed to produce 11 strains, with varying genetic backgrounds (wild, domesticated, F1 hybrids, F2 hybrids and backcrosses). Size-adjusted eye size was influenced by both genetic and environmental factors. Domesticated fish reared under artificial conditions had smaller adjusted eye size when compared to wild fish reared under identical conditions, in both the freshwater and marine environments, and in both Irish and Norwegian experiments. However, in parr that had been introduced into a river environment shortly after hatching and sampled at the end of their first summer, differences in adjusted eye size observed among genetic groups were of a reduced magnitude and were nonsignificant in 2-year-old sea migrating smolts sampled in the river immediately prior to sea entry. Collectively, our findings could suggest that where natural selection is present, individuals with reduced eye size are maladapted and consequently have reduced fitness, building on our understanding of the mechanisms that underlie a well-documented reduction in the fitness of the progeny of domesticated salmon, including hybrid progeny, in the wild.

Causal and non-causal explanations in code biology

In the philosophy of science, we can consider debates about the nature of non-causal explanations in general (e.g. Reutlinger, Saatsi 2018; Lange 2017) and then especially those in the life sciences (e.g. Huneman, 2018; Kostić 2020). These debates are accompanied by the development of a new mechanism that is becoming the major response to the nature of scientific explanation in the life sciences (e.g. Craver, Darden 2013; Craver 2006); and also by the development of a design explanation (e.g. Eck, Mennes 2016) that represents a modern variant of a functional explanation. In this paper, we will methodically: 1. evaluate the plurality of explanatory strategies in contemporary science (chapter 2). 2. describe the mechanical philosophy and mechanistic explanation (Glennan 2016; Craver, Darden 2013, etc.) (chapter 3). 3. explicate the role of mechanisms in code biology (Barbieri 2015, 2002, etc.) and its relation to the new mechanism (chapter 4). 4. fulfill the main goal of the paper - to apply mechanistic explanations in code biology (Barbieri 2019, etc.) and to apply their suitability for this scientific domain (chapter 5).

Vertical escape tactics and movement potential of orthoconic cephalopods

Measuring locomotion tactics available to ancient sea animals can link functional morphology with evolution and ecology over geologic timescales. Externally-shelled cephalopods are particularly important for their central roles in marine trophic exchanges, but most fossil taxa lack sufficient modern analogues for comparison. In particular, phylogenetically diverse cephalopods produced orthoconic conchs (straight shells) repeatedly through time. Persistent re-evolution of this morphotype suggests that it possesses adaptive value. Practical lateral propulsion is ruled out as an adaptive driver among orthoconic cephalopods due to the stable, vertical orientations of taxa lacking sufficient counterweights. However, this constraint grants the possibility of rapid (or at least efficient) vertical propulsion. We experiment with this form of movement using 3D-printed models of Baculites compressus, weighted to mimic hydrostatic properties inferred by virtual models. Furthermore, model buoyancy was manipulated to impart simulated thrust within four independent scenarios (Nautilus-like cruising thrust; a similar thrust scaled by the mantle cavity of Sepia; sustained peak Nautilus-like thrust; and passive, slightly negative buoyancy). Each model was monitored underwater with two submerged cameras as they rose/fell over ~2 m, and their kinematics were computed with 3D motion tracking. Our results demonstrate that orthocones require very low input thrust for high output in movement and velocity. With Nautilus-like peak thrust, the model reaches velocities of 1.2 m/s (2.1 body lengths per second) within one second starting from a static initial condition. While cephalopods with orthoconic conchs likely assumed a variety of life habits, these experiments illuminate some first-order constraints. Low hydrodynamic drag inferred by vertical displacement suggests that vertical migration would incur very low metabolic cost. While these cephalopods likely assumed low energy lifestyles day-to-day, they may have had a fighting chance to escape from larger, faster predators by performing quick, upward dodges. The current experiments suggest that orthocones sacrifice horizontal mobility and maneuverability in exchange for highly streamlined, vertically-stable, upwardly-motile conchs.

Coordinated evolution of brain size, structure, and eye size in Trinidadian killifish

Brain size, brain architecture, and eye size vary extensively in vertebrates. However, the extent to which the evolution of these components is intricately connected remains unclear. Trinidadian killifish, Anablepsoides hartii, are found in sites that differ in the presence and absence of large predatory fish. Decreased rates of predation are associated with evolutionary shifts in brain size; males from sites without predators have evolved a relatively larger brain and eye size than males from sites with predators. Here, we evaluated the extent to which the evolution of brain size, brain structure, and eye size covary in male killifish. We utilized wild-caught and common garden-reared specimens to determine whether specific components of the brain have evolved in response to differences in predation and to determine if there is covariation between the evolution of brain size, brain structure, and eye size. We observed consistent shifts in brain architecture in second generation common garden reared, but not wild caught preserved fish. Male killifish from sites that lack predators exhibited a significantly larger telencephalon, optic tectum, cerebellum, and dorsal medulla when compared with fish from sites with predators. We also found positive connections between the evolution of brain structure and eye size but not between overall brain size and eye size. These results provide evidence for evolutionary covariation between the components of the brain and eye size. Such results suggest that selection, directly or indirectly, acts upon specific regions of the brain, rather than overall brain size, to enhance visual capabilities.

More than meets the eye: Predator-induced pupil size plasticity in a teleost fish

Most animals are visually oriented, and their eyes provide their 'window to the world'. Eye size correlates positively with visual performance, because larger eyes can house larger pupils that increase photon catch and contrast discrimination, particularly under dim light, which have positive effects on behaviours that enhance fitness, including predator avoidance and foraging. Recent studies have linked predation risk to selection for larger eyes and pupils, and such changes should be of importance for the majority of teleost fishes as they have a pupil that is fixed in size (eyes lack a pupillary sphincter muscle) and, hence, do not respond to changes in light conditions. Here, we quantify eye and pupil size of individual crucian carp, a common freshwater fish, following controlled manipulations of perceived predation risk (presence/absence). We also tested if crucian carp responded to increased predation risk by shifts in diel activity patterns. We found that crucian carp show phenotypic plasticity with regards to pupil size, but not eye size, as pupil size increased when exposed to predators (pike). Predator-exposed crucian carp also shifted from diurnal to nocturnal activity. Using a modelling exercise, we moreover show that the plastically enlarged pupils significantly increase visual range, especially for small objects under dim light conditions. Overall, our results provide compelling evidence for predator-induced pupil enlargement resulting in enhanced visual capabilities in a teleost fish. Pupil size plasticity in combination with the observed shift towards nocturnal activity may allow for efficient foraging also under dark conditions when predation risk from diurnal and visually oriented predators is reduced. The data highlight the powerful role of predation risk for eye development and evolution.

Sensorial Hierarchy in Octopus vulgaris's Food Choice: Chemical vs. Visual

Simple Summary

Coleoids are cephalopods endowed with a highly sophisticated nervous system with keen sense organs and an exceptionally large brain that includes more than 30 differentiated lobes. Within this group, Octopus vulgaris, well known as an intelligent soft-bodied animal, has a significant number of lobes in the nervous system dedicated to decoding and integrating visual, tactile, and chemosensory perceptions. In this study, we aimed to understand the key role of chemical and visual cues during food selection in O. vulgaris. We first defined the preferred food, and subsequently, we set up five different problem-solving tasks, in which the animal’s choice is guided by visual and chemosensory signals, either alone or together, to evaluate whether individual O. vulgaris uses a sensorial hierarchy. Our behavioural experiments show that this species does integrate different sensory information from chemical and visual cues during food selection; however, our results indicate that chemical perception provides accurate and faster information leading to food choice. This research opens new perspectives on O. vulgaris’ predation strategies.

Abstract

Octopus vulgaris possesses highly sophisticated sense organs, processed by the nervous system to generate appropriate behaviours such as finding food, avoiding predators, identifying conspecifics, and locating suitable habitat. Octopus uses multiple sensory modalities during the searching and selection of food, in particular, the chemosensory and visual cues. Here, we examined food choice in O. vulgaris in two ways: (1) We tested octopus’s food preference among three different kinds of food, and established anchovy as the preferred choice (66.67%, Friedman test p < 0.05); (2) We exposed octopus to a set of five behavioural experiments in order to establish the sensorial hierarchy in food choice, and to evaluate the performance based on the visual and chemical cues, alone or together. Our data show that O. vulgaris integrates sensory information from chemical and visual cues during food choice. Nevertheless, food choice resulted in being more dependent on chemical cues than visual ones (88.9%, Friedman test p < 0.05), with a consistent decrease of the time spent identifying the preferred food. These results define the role played by the senses with a sensorial hierarchy in food choice, opening new perspectives on the O. vulgaris’ predation strategies in the wild, which until today were considered to rely mainly on visual cues.

The Eye of the Common Octopus (Octopus vulgaris)

Octopus vulgaris, well-known from temperate waters of the Mediterranean Sea and a well-cited model species among the cephalopods, has large eyes with which it scans its environment actively and which allow the organism to discriminate objects easily. On cursory examination, the single-chambered eyes of octopus with their spherical lenses resemble vertebrate eyes. However there are also apparent differences. For example, the retina of the octopus is everted instead of inverted, and it is equipped with primary rhabdomeric photoreceptors rather than secondary ciliary variety found in the retina of the vertebrate eye. The eyes of octopus are well adapted to the habitat and lifestyle of the species; the pupil closes quickly as a response to sudden light stimuli mimicking a situation in which the octopus leaves its den in shallow water during daytime. Although the many general anatomical and physiological features of octopus vision have been described elsewhere, our review reveals that a lot of information is still missing. Investigations that remain to be undertaken include a detailed examination of the dioptric apparatus or the visual functions such as brightness discrimination as well as a conclusive test for a faculty analogous to, or in lieu of, color vision. For a better understanding of the octopus eye and the functions mediated by it, we suggest that future studies focus on knowledge gaps that we outline in the present review.

Evolution: An Irresistibly Clear View of Land

Visually guided predation is range-limited in water, but works over long distances on land. This may have driven our last aquatic ancestors to evolve large eyes on the top of their head for spotting crunchy meals on ancient riverbanks.

Vision using multiple distinct rod opsins in deep-sea fishes

Vertebrate vision is accomplished through light-sensitive photopigments consisting of an opsin protein bound to a chromophore. In dim light, vertebrates generally rely on a single rod opsin [rhodopsin 1 (RH1)] for obtaining visual information. By inspecting 101 fish genomes, we found that three deep-sea teleost lineages have independently expanded their RH1 gene repertoires. Among these, the silver spinyfin (Diretmus argenteus) stands out as having the highest number of visual opsins in vertebrates (two cone opsins and 38 rod opsins). Spinyfins express up to 14 RH1s (including the most blueshifted rod photopigments known), which cover the range of the residual daylight as well as the bioluminescence spectrum present in the deep sea. Our findings present molecular and functional evidence for the recurrent evolution of multiple rod opsin-based vision in vertebrates.

The Current State of Cephalopod Science and Perspectives on the Most Critical Challenges Ahead From Three Early-Career Researchers

Here, three researchers who have recently embarked on careers in cephalopod biology discuss the current state of the field and offer their hopes for the future. Seven major topics are explored: genetics, aquaculture, climate change, welfare, behavior, cognition, and neurobiology. Recent developments in each of these fields are reviewed and the potential of emerging technologies to address specific gaps in knowledge about cephalopods are discussed. Throughout, the authors highlight specific challenges that merit particular focus in the near-term. This review and prospectus is also intended to suggest some concrete near-term goals to cephalopod researchers and inspire those working outside the field to consider the revelatory potential of these remarkable creatures.

The effect of aggregation on visibility in open water

Aggregation is a common life-history trait in open-water taxa. Qualitative understanding of how aggregation by prey influences their encounter rates with predators is critical for understanding pelagic predator-prey interactions and trophic webs. We extend a recently developed theory on underwater visibility to predict the consequences of grouping in open-water species in terms of increased visual detection of groups by predators. Our model suggests that enhanced visibility will be relatively modest, with maximum detection distance typically only doubling for a 100-fold increase in the number of prey in a group. This result suggests that although larger groups are more easily detected, this cost to aggregation will in many cases be dominated by benefits, especially through risk dilution in situations where predators cannot consume all members of a discovered group. This, in turn, helps to explain the ubiquity of grouping across a great variety of open-water taxa.

Cephalopods as Predators: A Short Journey among Behavioral Flexibilities, Adaptions, and Feeding Habits

The diversity of cephalopod species and the differences in morphology and the habitats in which they live, illustrates the ability of this class of molluscs to adapt to all marine environments, demonstrating a wide spectrum of patterns to search, detect, select, capture, handle, and kill prey. Photo-, mechano-, and chemoreceptors provide tools for the acquisition of information about their potential preys. The use of vision to detect prey and high attack speed seem to be a predominant pattern in cephalopod species distributed in the photic zone, whereas in the deep-sea, the development of mechanoreceptor structures and the presence of long and filamentous arms are more abundant. Ambushing, luring, stalking and pursuit, speculative hunting and hunting in disguise, among others are known modes of hunting in cephalopods. Cannibalism and scavenger behavior is also known for some species and the development of current culture techniques offer evidence of their ability to feed on inert and artificial foods. Feeding requirements and prey choice change throughout development and in some species, strong ontogenetic changes in body form seem associated with changes in their diet and feeding strategies, although this is poorly understood in planktonic and larval stages. Feeding behavior is altered during senescence and particularly in brooding octopus females. Cephalopods are able to feed from a variety of food sources, from detritus to birds. Their particular requirements of lipids and copper may help to explain why marine crustaceans, rich in these components, are common prey in all cephalopod diets. The expected variation in climate change and ocean acidification and their effects on chemoreception and prey detection capacities in cephalopods are unknown and needs future research.

Mismatch between the eye and the optic lobe in the giant squid

Giant squids (Architeuthis) are a legendary species among the cephalopods. They live in the deep sea and are well known for their enormous body and giant eyes. It has been suggested that their giant eyes are not adapted for the detection of either mates or prey at distance, but rather are best suited for monitoring very large predators, such as sperm whales, at distances exceeding 120 m and at a depth below 600 m (Nilsson et al. 2012 Curr. Biol.22, 683-688. (doi:10.1016/j.cub.2012.02.031)). However, it is not clear how the brain of giant squids processes visual information. In this study, the optic lobe of a giant squid (Architeuthis dux, male, mantle length 89 cm), which was caught by local fishermen off the northeastern coast of Taiwan, was scanned using high-resolution magnetic resonance imaging in order to examine its internal structure. It was evident that the volume ratio of the optic lobe to the eye in the giant squid is much smaller than that in the oval squid (Sepioteuthis lessoniana) and the cuttlefish (Sepia pharaonis). Furthermore, the cell density in the cortex of the optic lobe is significantly higher in the giant squid than in oval squids and cuttlefish, with the relative thickness of the cortex being much larger in Architeuthis optic lobe than in cuttlefish. This indicates that the relative size of the medulla of the optic lobe in the giant squid is disproportionally smaller compared with these two cephalopod species. This morphological study of the giant squid brain, though limited only to the optic lobe, provides the first evidence to support that the optic lobe cortex, the visual information processing area in cephalopods, is well developed in the giant squid. In comparison, the optic lobe medulla, the visuomotor integration centre in cephalopods, is much less developed in the giant squid than other species. This finding suggests that, despite the giant eye and a full-fledged cortex within the optic lobe, the brain of giant squids has not evolved proportionally in terms of performing complex tasks compared with shallow-water cephalopod species.

Massive increase in visual range preceded the origin of terrestrial vertebrates

The evolution of terrestrial vertebrates, starting around 385 million years ago, is an iconic moment in evolution that brings to mind images of fish transforming into four-legged animals. Here, we show that this radical change in body shape was preceded by an equally dramatic change in sensory abilities akin to transitioning from seeing over short distances in a dense fog to seeing over long distances on a clear day. Measurements of eye sockets and simulations of their evolution show that eyes nearly tripled in size just before vertebrates began living on land. Computational simulations of these animal's visual ecology show that for viewing objects through water, the increase in eye size provided a negligible increase in performance. However, when viewing objects through air, the increase in eye size provided a large increase in performance. The jump in eye size was, therefore, unlikely to have arisen for seeing through water and instead points to an unexpected hybrid of seeing through air while still primarily inhabiting water. Our results and several anatomical innovations arising at the same time suggest lifestyle similarity to crocodiles. The consequent combination of the increase in eye size and vision through air would have conferred a 1 million-fold increase in the amount of space within which objects could be seen. The "buena vista" hypothesis that our data suggest is that seeing opportunities from afar played a role in the subsequent evolution of fully terrestrial limbs as well as the emergence of elaborated action sequences through planning circuits in the nervous system.

Complex Visual Adaptations in Squid for Specific Tasks in Different Environments

In common with their major competitors, the fish, squid are fast moving visual predators that live over a great range of depths in the ocean. Both squid and fish show a variety of adaptations with respect to optical properties, receptors and their underlying neural circuits, and these adaptations are often linked to the light conditions of their specific niche. In contrast to the extensive investigations of adaptive strategies in fish, vision in response to the varying quantity and quality of available light, our knowledge of visual adaptations in squid remains sparse. This study therefore undertook a comparative study of visual adaptations and capabilities in a number of squid species collected between 0 and 1,200 m. Histology, magnetic resonance imagery (MRI), and depth distributions were used to compare brains, eyes, and visual capabilities, revealing that the squid eye designs reflect the lifestyle and the versatility of neural architecture in its visual system. Tubular eyes and two types of regional retinal deformation were identified and these eye modifications are strongly associated with specific directional visual tasks. In addition, a combination of conventional and immuno-histology demonstrated a new form of a complex retina possessing two inner segment layers in two mid-water squid species which they rhythmically move across a broad range of depths (50–1,000 m). In contrast to their relatives with the regular single-layered inner segment retina live in the upper mesopelagic layer (50–400 m), the new form of retinal interneuronal layers suggests that the visual sensitivity of these two long distance vertical migrants may increase in response to dimmer environments.

Bioluminescence as an ecological factor during high Arctic polar night

Bioluminescence commonly influences pelagic trophic interactions at mesopelagic depths. Here we characterize a vertical gradient in structure of a generally low species diversity bioluminescent community at shallower epipelagic depths during the polar night period in a high Arctic fjord with in situ bathyphotometric sampling. Bioluminescence potential of the community increased with depth to a peak at 80 m. Community composition changed over this range, with an ecotone at 20–40 m where a dinoflagellate-dominated community transitioned to dominance by the copepod Metridia longa. Coincident at this depth was bioluminescence exceeding atmospheric light in the ambient pelagic photon budget, which we term the bioluminescence compensation depth. Collectively, we show a winter bioluminescent community in the high Arctic with vertical structure linked to attenuation of atmospheric light, which has the potential to influence pelagic ecology during the light-limited polar night.

Spectral tuning by opsin coexpression in retinal regions that view different parts of the visual field

Vision frequently mediates critical behaviours, and photoreceptors must respond to the light available to accomplish these tasks. Most photoreceptors are thought to contain a single visual pigment, an opsin protein bound to a chromophore, which together determine spectral sensitivity. Mechanisms of spectral tuning include altering the opsin, changing the chromophore and incorporating pre-receptor filtering. A few exceptions to the use of a single visual pigment have been documented in which a single mature photoreceptor coexpresses opsins that form spectrally distinct visual pigments, and in these exceptions the functional significance of coexpression is unclear. Here we document for the first time photoreceptors coexpressing spectrally distinct opsin genes in a manner that tunes sensitivity to the light environment. Photoreceptors of the cichlid fish, Metriaclima zebra, mix different pairs of opsins in retinal regions that view distinct backgrounds. The mixing of visual pigments increases absorbance of the corresponding background, potentially aiding the detection of dark objects. Thus, opsin coexpression may be a novel mechanism of spectral tuning that could be useful for detecting prey, predators and mates. However, our calculations show that coexpression of some opsins can hinder colour discrimination, creating a trade-off between visual functions.

The presence of lateral photophores correlates with increased speciation in deep-sea bioluminescent sharks

The vast majority of species within the lanternshark genus Etmopterus harbour complex luminescent markings on their flanks, whose functional significance has long remained obscure. Recent studies, however, suggest these enigmatic photophore aggregations to play a role in intraspecific communication. Using visual modelling based on in vivo luminescence measurements from a common lanternshark species, we show that etmopterid flank markings can potentially work as a medium range signal for intraspecific detection/recognition. In addition, using molecular phylogenetic analyses, we demonstrate that the Etmopterus clade exhibits a greater than expected species richness given its age. This is not the case for other bioluminescent shark clades with no (or only few) species with flank markings. Our results therefore suggest that etmopterid flank markings may provide a way for reproductive isolation and hence may have facilitated speciation in the deep-sea.

Retinal Development and Ommin Pigment in the Cranchiid Squid Teuthowenia pellucida (Cephalopoda: Oegopsida)

The cranchiid Teuthowenia pellucida, like many deep-sea squid species, possesses large eyes that maximise light sensitivity in a nearly aphotic environment. To assess ontogenetic changes in the visual system, we conducted morphometric and histological analyses of the eyes using specimens from New Zealand collections. While the ratio between eye diameter and mantle length maintained a linear relationship throughout development, histological sections of the retina revealed that the outer photoreceptor layer became proportionally longer as the animal aged, coincident with a habitat shift into deeper, darker ocean strata. Other retinal layers maintained the same absolute thickness as was observed in paralarvae. Granules of the pigment ommin, normally located in the screening layer positioned at the base of the photoreceptors, were also observed at the outer end of the photoreceptor segments throughout the retina in young and mid-sized specimens. Early developmental stages of this species, dwelling in shallow waters, may therefore rely on migratory ommin to help shield photoreceptors from excess light and prevent over-stimulation. The oldest, deeper-dwelling specimens of T. pellucida examined had longer photoreceptors, and little or no migrated ommin was observed; we suggest therefore that short-term adaptive mechanisms for bright light conditions may be used primarily during epipelagic, early life stages in this species.

Role of olfaction in Octopus vulgaris reproduction

The olfactory system in any animal is the primary sensory system that responds to chemical stimuli emanating from a distant source. In aquatic animals "Odours" are molecules in solution that guide them to locate food, partners, nesting sites, and dangers to avoid. Fish, crustaceans and aquatic molluscs possess sensory systems that have anatomical similarities to the olfactory systems of land-based animals. Molluscs are a large group of aquatic and terrestrial animals that rely heavily on chemical communication with a generally dispersed sense of touch and chemical sensitivity. Cephalopods, the smallest class among extant marine molluscs, are predators with high visual capability and well developed vestibular, auditory, and tactile systems. Nevertheless they possess a well developed olfactory organ, but to date almost nothing is known about the mechanisms, functions and modulation of this chemosensory structure in octopods. Cephalopod brains are the largest of all invertebrate brains and across molluscs show the highest degree of centralization. The reproductive behaviour of Octopus vulgaris is under the control of a complex set of signal molecules such as neuropeptides, neurotransmitters and sex steroids that guide the behaviour from the level of individuals in evaluating mates, to stimulating or deterring copulation, to sperm-egg chemical signalling that promotes fertilization. These signals are intercepted by the olfactory organs and integrated in the olfactory lobes in the central nervous system. In this context we propose a model in which the olfactory organ and the olfactory lobe of O. vulgaris could represent the on-off switch between food intake and reproduction.

Deep-sea low-light radiometer system

Two single-waveband low-light radiometers were developed to characterize properties of the underwater light field relevant to biological camouflage at mesopelagic ocean depths. Phenomena of interest were vertical changes in downward irradiance of ambient light at wavelengths near 470 nm and 560 nm, and flashes from bioluminescent organisms. Depth profiles were acquired at multiple deep stations in different geographic regions. Results indicate significant irradiance magnitudes at 560 nm, providing direct evidence of energy transfer as described by Raman scattering. Analysis of a night profile yielded multiple examples of bioluminescent flashes. The selection of high-sensitivity, high-speed silicon photomultipliers as detectors enabled measurement of spectrally-resolved irradiance to greater than 400 m depth.

Iso-luminance counterillumination drove bioluminescent shark radiation

Counterilluminating animals use ventral photogenic organs (photophores) to mimic the residual downwelling light and cloak their silhouette from upward-looking predators. To cope with variable conditions of pelagic light environments they typically adjust their luminescence intensity. Here, we found evidence that bioluminescent sharks instead emit a constant light output and move up and down in the water column to remain cryptic at iso-luminance depth. We observed, across 21 globally distributed shark species, a correlation between capture depth and the proportion of a ventral area occupied by photophores. This information further allowed us, using visual modelling, to provide an adaptive explanation for shark photophore pattern diversity: in species facing moderate predation risk from below, counterilluminating photophores were partially co-opted for bioluminescent signalling, leading to complex patterns. In addition to increase our understanding of pelagic ecosystems our study emphasizes the importance of bioluminescence as a speciation driver.

The visual ecology of a deep-sea fish, the escolar Lepidocybium flavobrunneum (Smith, 1843)

Escolar (Lepidocybium flavobrunneum, family Gempylidae) are large and darkly coloured deep-sea predatory fish found in the cold depths (more than 200 m) during the day and in warm surface waters at night. They have large eyes and an overall low density of retinal ganglion cells that endow them with a very high optical sensitivity. Escolar have banked retinae comprising six to eight layers of rods to increase the optical path length for maximal absorption of the incoming light. Their retinae possess two main areae of higher ganglion cell density, one in the ventral retina viewing the dorsal world above (with a moderate acuity of 4.6 cycles deg(-1)), and the second in the temporal retina viewing the frontal world ahead. Electrophysiological recordings of the flicker fusion frequency (FFF) in isolated retinas indicate that escolar have slow vision, with maximal FFF at the highest light levels and temperatures (around 9 Hz at 23°C) which fall to 1-2 Hz in dim light or cooler temperatures. Our results suggest that escolar are slowly moving sit-and-wait predators. In dim, warm surface waters at night, their slow vision, moderate dorsal resolution and highly sensitive eyes may allow them to surprise prey from below that are silhouetted in the downwelling light.

Computational visual ecology in the pelagic realm

Visual performance and visual interactions in pelagic animals are notoriously hard to investigate because of our restricted access to the habitat. The pelagic visual world is also dramatically different from benthic or terrestrial habitats, and our intuition is less helpful in understanding vision in unfamiliar environments. Here, we develop a computational approach to investigate visual ecology in the pelagic realm. Using information on eye size, key retinal properties, optical properties of the water and radiance, we develop expressions for calculating the visual range for detection of important types of pelagic targets. We also briefly apply the computations to a number of central questions in pelagic visual ecology, such as the relationship between eye size and visual performance, the maximum depth at which daylight is useful for vision, visual range relations between prey and predators, counter-illumination and the importance of various aspects of retinal physiology. We also argue that our present addition to computational visual ecology can be developed further, and that a computational approach offers plenty of unused potential for investigations of visual ecology in both aquatic and terrestrial habitats.

Absence of suction feeding ichthyosaurs and its implications for triassic mesopelagic paleoecology

Mesozoic marine reptiles and modern marine mammals are often considered ecological analogs, but the extent of their similarity is largely unknown. Particularly important is the presence/absence of deep-diving suction feeders among Mesozoic marine reptiles because this would indicate the establishment of mesopelagic cephalopod and fish communities in the Mesozoic. A recent study suggested that diverse suction feeders, resembling the extant beaked whales, evolved among ichthyosaurs in the Triassic. However, this hypothesis has not been tested quantitatively. We examined four osteological features of jawed vertebrates that are closely linked to the mechanism of suction feeding, namely hyoid corpus ossification/calcification, hyobranchial apparatus robustness, mandibular bluntness, and mandibular pressure concentration index. Measurements were taken from 18 species of Triassic and Early Jurassic ichthyosaurs, including the presumed suction feeders. Statistical comparisons with extant sharks and marine mammals of known diets suggest that ichthyosaurian hyobranchial bones are significantly more slender than in suction-feeding sharks or cetaceans but similar to those of ram-feeding sharks. Most importantly, an ossified hyoid corpus to which hyoid retractor muscles attach is unknown in all but one ichthyosaur, whereas a strong integration of the ossified corpus and cornua of the hyobranchial apparatus has been identified in the literature as an important feature of suction feeders. Also, ichthyosaurian mandibles do not narrow rapidly to allow high suction pressure concentration within the oral cavity, unlike in beaked whales or sperm whales. In conclusion, it is most likely that Triassic and Early Jurassic ichthyosaurs were 'ram-feeders', without any beaked-whale-like suction feeder among them. When combined with the inferred inability for dim-light vision in relevant Triassic ichthyosaurs, the fossil record of ichthyosaurs does not suggest the establishment of modern-style mesopelagic animal communities in the Triassic. This new interpretation matches the fossil record of coleoids, which indicates the absence of soft-bodied deepwater species in the Triassic.