Visual fields of four batoid fishes: a comparative study

CLINICAL AND HISTOPATHOLOGIC OCULAR FINDINGS IN AQUARIUM-HOUSED COWNOSE RAYS (RHINOPTERA BONASUS)

Cownose rays (Rhinoptera bonasus) are susceptible to ocular disease with their prominent globes, but despite being popular animals housed in aquaria, there is little published information about their normal ocular anatomy and common pathologic ocular findings. A total of 63 live cownose rays (CNR) from three unrelated, separately housed groups had ocular examinations, and 5 adult rays were selected for ocular ultrasound. All examinations were performed out of the water, and most without anesthesia. Clinical findings were described, categorized, and scored by severity. Sixty-two of 63 rays (123 eyes) had clinical abnormalities, including 110 eyes with corneal pathology (mild = 76, moderate/severe = 34) and 74 eyes with intraocular pathology (mild = 44, moderate/severe = 30). Grey-to-white corneal opacities were the most common pathology (n = 58 rays/100 eyes) followed by cataracts (n = 41 rays/58 eyes), then persistent (or dysplastic) pupillary membranes (n = 14 rays/15 eyes). Most pathologic findings appeared inactive, but one aquarium had several CNR with active ocular pathology. There was a significant association between the diagnosis of moderate/severe corneal and intraocular pathology with age (P = 0.008 and P = 0.014, respectively) and weight (P = 0.001 and P = 0.039, respectively), as well as moderate/severe corneal pathology and group sampled (P = 0.03). There were no other significant variables identified. Additionally, histopathology of 14 eyes (11 rays) from two different facilities were examined, with keratitis (n = 8) and uveitis (n = 2) as the most common lesions. This study shows a high prevalence of pathologic ocular findings in cownose ray eyes with heavier adults more likely to be affected than lighter juveniles. Comprehensive ocular evaluation is important in this species and serial ocular exams and future studies should be pursued to monitor ocular disease progression and better understand possible etiologies.

Anuran interorbital distance variation: the role of ecological and behavioral factors

Eye position varies significantly among taxonomic levels, and this variation is often shaped by ecological and behavioral factors. Eye position is often positively associated with interorbital distance where species with broad visual fields possess a large distance between the left and right eye. Selective pressures underlying the evolution of the eye position are especially studied in birds and mammals. However, selective pressures underling the evolution of anuran eye position which can be indicated by interorbital distance keep unknown. Here, we investigated the effects of ecological (e.g., habitat type, light availability) and behavioral factors (e.g., activity pattern, foraging mobility, and defensive strategy) on variations in interorbital distance among 260 anuran species in China. Our results showed that variations of the interorbital distance can be significantly predicted by the activity pattern. Nocturnal species had larger interorbital distance than both nocturnal and diurnal species. We also found that foraging mobility and defensive strategy affected markedly variation of interorbital distance. Species having slower foraging mobility and possessing poison glands had larger interorbital distance than species having faster foraging mobility and possessing non-position glands. Light availability tended to be associated with variation of interorbital distance, indicating that species living weak light tending to possess larger interorbital distance. However, variations of the interorbital space were not associated with habitat type in anurans. Our findings suggest that anuran behaviors play key roles in shaping visual fields and eye position, and thus affecting the evolution of interorbital distance. This article is protected by copyright. All rights reserved.

Pupil dilation and constriction in the skate Leucoraja erinacea in a simulated natural light field

The skate Leucoraja erinacea has an elaborately shaped pupil, whose characteristics and functions have received little attention. The goal of our study was to investigate the pupil response in relation to natural ambient light intensities. First, we took a recently developed sensory–ecological approach, which gave us a tool for creating a controlled light environment for behavioural work: during a field survey, we collected a series of calibrated natural habitat images from the perspective of the skates' eyes. From these images, we derived a vertical illumination profile using custom-written software for quantification of the environmental light field (ELF). After collecting and analysing these natural light field data, we created an illumination set-up in the laboratory, which closely simulated the natural vertical light gradient that skates experience in the wild and tested the light responsiveness – in particular the extent of dilation – of the skate pupil to controlled changes in this simulated light field. Additionally, we measured pupillary dilation and constriction speeds. Our results confirm that the skate pupil changes from nearly circular under low light to a series of small triangular apertures under bright light. A linear regression analysis showed a trend towards smaller skates having a smaller dynamic range of pupil area (dilation versus constriction ratio around 4-fold), and larger skates showing larger ranges (around 10- to 20-fold). Dilation took longer than constriction (between 30 and 45 min for dilation; less than 20 min for constriction), and there was considerable individual variation in dilation/constriction time. We discuss our findings in terms of the visual ecology of L. erinacea and consider the importance of accurately simulating natural light fields in the laboratory.

The Functional Anatomy of the Cornea and Anterior Chamber in Lampreys: Insights From the Pouched Lamprey, Geotria australis (Geotriidae, Agnatha)

Extant lampreys (Petromyzontiformes) are one of two lineages of surviving jawless fishes or agnathans, and are therefore of critical importance to our understanding of vertebrate evolution. Anadromous lampreys undergo a protracted lifecycle, which includes metamorphosis from a larval ammocoete stage to an adult that moves between freshwater and saltwater with exposure to a range of lighting conditions. Previous studies have revealed that photoreception differs radically across the three extant families with the Pouched lamprey Geotria australis possessing a complex retina with the potential for pentachromacy. This study investigates the functional morphology of the cornea and anterior chamber of G. australis, which is specialised compared to its northern hemisphere counterparts. Using light microscopy, scanning and transmission electron microscopy and microcomputed tomography, the cornea is found to be split into a primary spectacle (dermal cornea) and a scleral cornea (continuous with the scleral eyecup), separated by a mucoid layer bounded on each side by a basement membrane. A number of other specialisations are described including mucin-secreting epithelial cells and microholes, four types of stromal sutures for the inhibition of stromal swelling, abundant anastomosing and branching of collagen lamellae, and a scleral endothelium bounded by basement membranes. The structure and function of the cornea including an annular and possibly a pectinate ligament and iris are discussed in the context of the evolution of the eye in vertebrates.

Why animals swirl and how they group

We report a possible solution for the long-standing problem of the biological function of swirling motion, when a group of animals orbits a common center of the group. We exploit the hypothesis that learning processes in the nervous system of animals may be modelled by reinforcement learning (RL) and apply it to explain the phenomenon. In contrast to hardly justified models of physical interactions between animals, we propose a small set of rules to be learned by the agents, which results in swirling. The rules are extremely simple and thus applicable to animals with very limited level of information processing. We demonstrate that swirling may be understood in terms of the escort behavior, when an individual animal tries to reside within a certain distance from the swarm center. Moreover, we reveal the biological function of swirling motion: a trained for swirling swarm is by orders of magnitude more resistant to external perturbations, than an untrained one. Using our approach we analyze another class of a coordinated motion of animals-a group locomotion in viscous fluid. On a model example we demonstrate that RL provides an optimal disposition of coherently moving animals with a minimal dissipation of energy.

Transfer entropy dependent on distance among agents in quantifying leader-follower relationships

Synchronized movement of (both unicellular and multicellular) systems can be observed almost everywhere. Understanding of how organisms are regulated to synchronized behavior is one of the challenging issues in the field of collective motion. It is hypothesized that one or a few agents in a group regulate(s) the dynamics of the whole collective, known as leader(s). The identification of the leader (influential) agent(s) is very crucial. This article reviews different mathematical models that represent different types of leadership. We focus on the improvement of the leader-follower classification problem. It was found using a simulation model that the use of interaction domain information significantly improves the leader-follower classification ability using both linear schemes and information-theoretic schemes for quantifying influence. This article also reviews different schemes that can be used to identify the interaction domain using the motion data of agents.

Learning to flock through reinforcement

Flocks of birds, schools of fish, and insect swarms are examples of the coordinated motion of a group that arises spontaneously from the action of many individuals. Here, we study flocking behavior from the viewpoint of multiagent reinforcement learning. In this setting, a learning agent tries to keep contact with the group using as sensory input the velocity of its neighbors. This goal is pursued by each learning individual by exerting a limited control on its own direction of motion. By means of standard reinforcement learning algorithms we show that (i) a learning agent exposed to a group of teachers, i.e., hard-wired flocking agents, learns to follow them, and (ii) in the absence of teachers, a group of independently learning agents evolves towards a state where each agent knows how to flock. In both scenarios, the emergent policy (or navigation strategy) corresponds to the polar velocity alignment mechanism of the well-known Vicsek model. These results (a) show that such a velocity alignment may have naturally evolved as an adaptive behavior that aims at minimizing the rate of neighbor loss, and (b) prove that this alignment does not only favor (local) polar order, but it corresponds to the best policy or strategy to keep group cohesion when the sensory input is limited to the velocity of neighboring agents. In short, to stay together, steer together.

Vision in sharks and rays: Opsin diversity and colour vision

The visual sense of elasmobranch fishes is poorly studied compared to their bony cousins, the teleosts. Nevertheless, the elasmobranch eye features numerous specialisations that have no doubt facilitated the diversification and evolutionary success of this fascinating taxon. In this review, I highlight recent discoveries on the nature and phylogenetic distribution of visual pigments in sharks and rays. Whereas most rays appear to be cone dichromats, all sharks studied to date are cone monochromats and, as a group, have likely abandoned colour vision on multiple occasions. This situation in sharks mirrors that seen in other large marine predators, the pinnipeds and cetaceans, which leads us to reassess the costs and benefits of multiple cone pigments and wavelength discrimination in the marine environment.

Collective motion patterns of self-propelled agents with both velocity alignment and aggregation interactions

We combine the velocity alignment and aggregation mechanisms to study the collective motion of active agents in noisy circumstances. The agents are located on a two-dimensional square plane, and the proportion of velocity alignment and aggregation interactions are, respectively, set to be k and 1-k. In the case of k=1 our model is similar to the classical Vicsek model, while it degenerates to the view angle model for k=0. By tuning the intensity of the external noise η and the proportional coefficient k, and carrying out extensive numerical simulations, we find that the system can exhibit diverse dynamic patterns widely observed in real biological systems. By means of finite-size scaling analysis, we confirm that the presence of the aggregation interaction affects not only the position of the critical noise η_{c} (beyond which the agents display disordered motion) but also the type of the phase transition of the collective motion. In particular, under a weak external noise environment, the transition from disordered to ordered state by increasing k (i.e., by decreasing the proportion of aggregation interaction) is found to be of first order. Besides, for moderate external noise, we also find the existence of the optimal proportion of the aggregation interaction for the system to achieve the highest degree of order. Our results highlights the important role of the aggregation interaction in the collective motion and may have promising potential applications in natural self-propelled particles and artificial multiagent systems.

Retinal specializations and visual ecology in an animal with an extremely elaborate pupil shape: the little skate Leucoraja (Raja) erinacea Mitchell, 1825

Investigating retinal specializations offers insights into eye functionality. Using retinal wholemount techniques, we investigated the distribution of retinal ganglion cells in the Little skate Leucoraja erinacea by (a) dye-backfilling into the optic nerve prior to retinal wholemounting; (b) Nissl-staining of retinal wholemounts. Retinas were examined for regional specializations (higher numbers) of ganglion cells that would indicate higher visual acuity in those areas. Total ganglion cell number were low compared to other elasmobranchs (backfilled: average 49,713 total ganglion cells, average peak cell density 1,315 ganglion cells mm^-2 ; Nissl-stained: average 47,791 total ganglion cells, average peak cell density 1,319 ganglion cells mm^-2 ). Ganglion cells fit into three size categories: small (5-20 µm); medium (20-30 µm); large: (≥ 30 µm), and they were not homogeneously distributed across the retina. There was a dorsally located horizontal visual streak with increased ganglion cell density; additionally, there were approximately three local maxima in ganglion cell distribution (potential areae centrales) within this streak in which densities were highest. Using computerized tomography (CT) and micro-CT, geometrical dimensions of the eye were obtained. Combined with ganglion cell distributions, spatial resolving power was determined to be between 1.21 and 1.37 cycles per degree. Additionally, photoreceptor sizes across different retinal areas varied; photoreceptors were longest within the horizontal visual streak. Variations in the locations of retinal specializations appear to be related to the animal's anatomy: shape of the head and eyes, position of eyes, location of tapetum, and shape of pupil, as well as the visual demands associated with lifestyle and habitat type.

Enucleation in a Cownose Ray (Rhinoptera bonasus)

Trauma is a common problem in Cownose Ray during mating season in both wild and captive rays. Enucleation is indicated when there is an ocular trauma. A 5-year-old female Cownose Ray (Rhinoptera bonasus) from Aquaria of Kuala Lumpur Convention Centre (KLCC) was presented to University Veterinary Hospital (UVH), Universiti Putra Malaysia, with a complaint of protruding left eye, which resulted from crushing into artificial coral during mating season. There were a hyphema in the traumatic left eye, periorbital tissue tear, exposed left eye socket, and multiple abrasions on both pectoral fins. The Cownose was anaesthetized and maintained with isoeugenol and on-field emergency enucleation of the left eye was performed. It was managed medically with postoperative enrofloxacin, tobramycin ointment, and povidone iodine. No suture breakdown and secondary infection were observed at day 7 after enucleation during revisit. At day 24 after enucleation, the Cownose responded well to treatment with excellent healing progression and no surgical complication was observed.

Validating two-dimensional leadership models on three-dimensionally structured fish schools

Identifying leader-follower interactions is crucial for understanding how a group decides where or when to move, and how this information is transferred between members. Although many animal groups have a three-dimensional structure, previous studies investigating leader-follower interactions have often ignored vertical information. This raises the question of whether commonly used two-dimensional leader-follower analyses can be used justifiably on groups that interact in three dimensions. To address this, we quantified the individual movements of banded tetra fish (Astyanax mexicanus) within shoals by computing the three-dimensional trajectories of all individuals using a stereo-camera technique. We used these data firstly to identify and compare leader-follower interactions in two and three dimensions, and secondly to analyse leadership with respect to an individual's spatial position in three dimensions. We show that for 95% of all pairwise interactions leadership identified through two-dimensional analysis matches that identified through three-dimensional analysis, and we reveal that fish attend to the same shoalmates for vertical information as they do for horizontal information. Our results therefore highlight that three-dimensional analyses are not always required to identify leader-follower relationships in species that move freely in three dimensions. We discuss our results in terms of the importance of taking species' sensory capacities into account when studying interaction networks within groups.

Large-Scale Patterns in a Minimal Cognitive Flocking Model: Incidental Leaders, Nematic Patterns, and Aggregates

We study a minimal cognitive flocking model, which assumes that the moving entities navigate using the available instantaneous visual information exclusively. The model consists of active particles, with no memory, that interact by a short-ranged, position-based, attractive force, which acts inside a vision cone (VC), and lack velocity-velocity alignment. We show that this active system can exhibit-due to the VC that breaks Newton's third law-various complex, large-scale, self-organized patterns. Depending on parameter values, we observe the emergence of aggregates or millinglike patterns, the formation of moving-locally polar-files with particles at the front of these structures acting as effective leaders, and the self-organization of particles into macroscopic nematic structures leading to long-ranged nematic order. Combining simulations and nonlinear field equations, we show that position-based active models, as the one analyzed here, represent a new class of active systems fundamentally different from other active systems, including velocity-alignment-based flocking systems. The reported results are of prime importance in the study, interpretation, and modeling of collective motion patterns in living and nonliving active systems.

First-order phase transition in a model of self-propelled particles with variable angular range of interaction

We have carried out a Monte Carlo simulation of a modified version of Vicsek model for the motion of self-propelled particles in two dimensions. In this model the neighborhood of interaction of a particle is a sector of the circle with the particle at the center (rather than the whole circle as in the original Vicsek model). The sector is centered along the direction of the velocity of the particle, and the half-opening angle of this sector is called the "view angle." We vary the view angle over its entire range and study the change in the nature of the collective motion of the particles. We find that ordered collective motion persists down to remarkably small view angles. And at a certain transition view angle the collective motion of the system undergoes a first-order phase transition to a disordered state. We also find that the reduction in the view angle can in fact increase the order in the system significantly. We show that the directionality of the interaction, and not only the radial range of the interaction, plays an important role in the determination of the nature of the above phase transition.

Comparative anatomy of the extraocular muscles in four Myliobatoidei rays (Batoidea, Myliobatiformes)

Extraocular muscles are classically grouped as four rectus and two oblique muscles. However, their description and potential associations with species behavior are limited. The objective was to characterize extraocular muscles in four Myliobatoidei rays from diverse habitats with divergent behaviors. Heads (10 per species) of Dasyatis hypostigma, Gymnura altavela, Mobula thurstoni and Pteroplatytrygon violacea were decalcified and dissected to characterize and describe extraocular muscles. Principal component analysis (PCA) was used to evaluate relationships between muscle length and species; for P. violacea, D. hypostigma and G. altavela, these were qualitatively and quantitatively consistent with the general pattern of extraocular muscles in vertebrates. In contrast, for M. thurstoni, the two oblique muscles were completely fused and there was a seventh extraocular muscle, named m. lateral rectus β (both were apparently novel findings in this species). There were also significant differences in eye disposition in the chondrocranium. The PCA axis 1 (rectus muscles) and PCA axis 2 (oblique muscles) accounted for 98.47% of data variability. Extraocular muscles had significant differences in length and important anatomical differences among sampled species that facilitated grouping species according to their life history. In conclusion, extraocular muscles are not uniform in all vertebrate species, thereby providing another basis for comparative studies.

Vision in two cyprinid fish: implications for collective behavior

Many species of fish rely on their visual systems to interact with conspecifics and these interactions can lead to collective behavior. Individual-based models have been used to predict collective interactions; however, these models generally make simplistic assumptions about the sensory systems that are applied without proper empirical testing to different species. This could limit our ability to predict (and test empirically) collective behavior in species with very different sensory requirements. In this study, we characterized components of the visual system in two species of cyprinid fish known to engage in visually dependent collective interactions (zebrafish Danio rerio and golden shiner Notemigonus crysoleucas) and derived quantitative predictions about the positioning of individuals within schools. We found that both species had relatively narrow binocular and blind fields and wide visual coverage. However, golden shiners had more visual coverage in the vertical plane (binocular field extending behind the head) and higher visual acuity than zebrafish. The centers of acute vision (areae) of both species projected in the fronto-dorsal region of the visual field, but those of the zebrafish projected more dorsally than those of the golden shiner. Based on this visual sensory information, we predicted that: (a) predator detection time could be increased by >1,000% in zebrafish and >100% in golden shiners with an increase in nearest neighbor distance, (b) zebrafish schools would have a higher roughness value (surface area/volume ratio) than those of golden shiners, (c) and that nearest neighbor distance would vary from 8 to 20 cm to visually resolve conspecific striping patterns in both species. Overall, considering between-species differences in the sensory system of species exhibiting collective behavior could change the predictions about the positioning of individuals in the group as well as the shape of the school, which can have implications for group cohesion. We suggest that more effort should be invested in assessing the role of the sensory system in shaping local interactions driving collective behavior.

Visual Specializations in Five Sympatric Species of Stingrays from the Family Dasyatidae

The eyes of five ray species (Taeniura lymma, Neotrygon kuhlii, Pastinachus atrus, Himantura uarnak and Urogymnus asperrimus) from the same taxonomic family (Dasyatidae) and the same geographic region (Ningaloo Reef, Western Australia) were studied to identify differences in retinal specializations that may reflect niche specialization. The topographic distributions of photoreceptors (rods and all cones) and ganglion cells were assessed and used to identify localized peaks in cell densities that indicate specializations for acute vision. These data were also used to calculate summation ratios of photoreceptors to ganglion cells in each species and estimate the anatomical spatial resolving power of the eye. Subtle differences in the distribution of retinal neurons appear to be related to the ecology of these closely related species of stingrays. The main specialization in the retinal cell density distribution is the dorsal streak that allows these animals to scan the substrate for potential prey. The blue-spotted fantail ray, T. lymma, showed the highest peak density of rods (86,700 rods mm-2) suggesting a specialization for scotopic vision. The highest peak density of cones (9,970 cones mm-2) was found in H. uarnak, and the highest peak density of ganglion cells (4,500 cells mm-2) was found in P. atrus. The proportion of rods to cones in the dorsal streak was higher in the two smaller species (12.5-14:1 in T. lymma and N. kuhlii) than the larger stingrays (6-8:1 in P. atrus, H. uarnak and U. asperrimus). Visual specializations in different sympatric species are subtle but may reflect specializations to specific ecological niches.

Prey capture kinematics in batoids using different prey types: investigating the role of the cephalic lobes

Cephalic lobes are novel structures found in some myliobatid stingrays. While undulatory batoids utilize the pectoral fins for prey capture and locomotion, lobed species partition locomotion to the pectoral fins, utilizing the lobes exclusively for prey capture. We investigated the use of the anterior pectoral fins and cephalic lobes in prey capture in five batoid species. The purpose of this study was to investigate the: (1) prey capture kinematics and use of the cephalic lobes in lobed and lobeless batoids; (2) role of the cephalic lobes in modulating capture behavior based on prey type. It was hypothesized that lobed species would display unique capture behaviors resulting in faster and more successful capture of prey, and display greater modulation in capture behavior. Findings showed that lobed species used only the head region for capture, were faster at pouncing and tenting, but slower at mouth opening. The cephalic lobes were more movable than the anterior pectoral fins of lobeless species. Modulation occurred in all species. Elusive prey increased tent duration for the lobeless species, increased mouth opening duration in the lobed Aetobatus narinari, and were farther away from the mouth than non-elusive prey during biting for all species. All species had few prey escapes. Overall, species with cephalic lobes captured prey faster but did not display increased modulatory ability or feeding success. The cephalic lobes help localize prey capture to the head region, speeding up the prey capture event and maintaining an efficient capture rate despite having less flexible pectoral fins.

Does nocturnality drive binocular vision? Octodontine rodents as a case study

Binocular vision is a visual property that allows fine discrimination of in-depth distance (stereopsis), as well as enhanced light and contrast sensitivity. In mammals enhanced binocular vision is structurally associated with a large degree of frontal binocular overlap, the presence of a corresponding retinal specialization containing a fovea or an area centralis, and well-developed ipsilateral retinal projections to the lateral thalamus (GLd). We compared these visual traits in two visually active species of the genus Octodon that exhibit contrasting visual habits: the diurnal Octodon degus, and the nocturnal Octodon lunatus. The O. lunatus visual field has a prominent 100° frontal binocular overlap, much larger than the 50° of overlap found in O. degus. Cells in the retinal ganglion cell layer were 40% fewer in O. lunatus (180,000) than in O. degus (300,000). O. lunatus has a poorly developed visual streak, but a well developed area centralis, located centrally near the optic disk (peak density of 4,352 cells/mm²). O. degus has a highly developed visual streak, and an area centralis located more temporally (peak density of 6,384 cells/mm²). The volumes of the contralateral GLd and superior colliculus (SC) are 15% larger in O. degus compared to O. lunatus. However, the ipsilateral projections to GLd and SC are 500% larger in O. lunatus than in O. degus. Other retinorecipient structures related to ocular movements and circadian activity showed no statistical differences between species. Our findings strongly suggest that nocturnal visual behavior leads to an enhancement of the structures associated with binocular vision, at least in the case of these rodents. Expansion of the binocular visual field in nocturnal species may have a beneficial effect in light and contrast sensitivity, but not necessarily in stereopsis. We discuss whether these conclusions can be extended to other mammalian and non-mammalian amniotes.

Linking sensory biology and fisheries bycatch reduction in elasmobranch fishes: a review with new directions for research

Incidental capture, or bycatch, in fisheries represents a substantial threat to the sustainability of elasmobranch populations worldwide. Consequently, researchers are increasingly investigating elasmobranch bycatch reduction methods, including some focused on these species' sensory capabilities, particularly their electrosensory systems. To guide this research, we review current knowledge of elasmobranch sensory biology and feeding ecology with respect to fishing gear interactions and include examples of bycatch reduction methods used for elasmobranchs as well as other taxonomic groups. We discuss potential elasmobranch bycatch reduction strategies for various fishing gear types based on the morphological, physiological, and behavioural characteristics of species within this diverse group. In select examples, we indicate how an understanding of the physiology and sensory biology of vulnerable, bycatch-prone, non-target elasmobranch species can help in the identification of promising options for bycatch reduction. We encourage collaboration among researchers studying bycatch reduction across taxa to provide better understanding of the broad effects of bycatch reduction methods.

Vision in elasmobranchs and their relatives: 21st century advances

This review identifies a number of exciting new developments in the understanding of vision in cartilaginous fishes that have been made since the turn of the century. These include the results of studies on various aspects of the visual system including eye size, visual fields, eye design and the optical system, retinal topography and spatial resolving power, visual pigments, spectral sensitivity and the potential for colour vision. A number of these studies have covered a broad range of species, thereby providing valuable information on how the visual systems of these fishes are adapted to different environmental conditions. For example, oceanic and deep-sea sharks have the largest eyes amongst elasmobranchs and presumably rely more heavily on vision than coastal and benthic species, while interspecific variation in the ratio of rod and cone photoreceptors, the topographic distribution of the photoreceptors and retinal ganglion cells in the retina and the spatial resolving power of the eye all appear to be closely related to differences in habitat and lifestyle. Multiple, spectrally distinct cone photoreceptor visual pigments have been found in some batoid species, raising the possibility that at least some elasmobranchs are capable of seeing colour, and there is some evidence that multiple cone visual pigments may also be present in holocephalans. In contrast, sharks appear to have only one cone visual pigment. There is evidence that ontogenetic changes in the visual system, such as changes in the spectral transmission properties of the lens, lens shape, focal ratio, visual pigments and spatial resolving power, allow elasmobranchs to adapt to environmental changes imposed by habitat shifts and niche expansion. There are, however, many aspects of vision in these fishes that are not well understood, particularly in the holocephalans. Therefore, this review also serves to highlight and stimulate new research in areas that still require significant attention.

Body plan convergence in the evolution of skates and rays (Chondrichthyes: Batoidea)

Skates, rays and allies (Batoidea) comprise more than half of the species diversity and much of the morphological disparity among chondrichthyan fishes, the sister group to all other jawed vertebrates. While batoids are morphologically well characterized and have an excellent fossil record, there is currently no consensus on the interrelationships of family-level taxa. Here we construct a resolved, robust and time-calibrated batoid phylogeny using mitochondrial genomes, nuclear genes, and fossils, sampling densely across taxa. Data partitioning schemes, biases in the sequence data, and the relative informativeness of each fossil are explored. The molecular phylogeny is largely congruent with morphology crownward in the tree, but the branching orders of major batoid groups are mostly novel. Body plan convergence appears to be widespread in batoids. A depressed, rounded pectoral disk supported to the snout tip by fin radials, common to skates and stingrays, is indicated to have been derived independently by each group, while the long, spiny rostrum of sawfishes similarly appears to be convergent with that of sawsharks, which are not batoids. The major extant batoid lineages are inferred to have arisen relatively rapidly from the Late Triassic into the Jurassic, with long stems followed by subsequent radiations in each group around the Cretaceous/Tertiary boundary. The fossil record indicates that batoids were affected with disproportionate severity by the end-Cretaceous extinction event.

Enhanced visual fields in hammerhead sharks

Several factors that influence the evolution of the unusual head morphology of hammerhead sharks (family Sphyrnidae) are proposed but few are empirically tested. In this study we tested the 'enhanced binocular field' hypothesis (that proposes enhanced frontal binocularity) by comparison of the visual fields of three hammerhead species: the bonnethead shark, Sphyrna tiburo, the scalloped hammerhead shark, Sphyrna lewini, and the winghead shark, Eusphyra blochii, with that of two carcharhinid species: the lemon shark, Negaprion brevirostris, and the blacknose shark, Carcharhinus acronotus. Additionally, eye rotation and head yaw were quantified to determine if species compensate for large blind areas anterior to the head. The winghead shark possessed the largest anterior binocular overlap (48 deg.) and was nearly four times larger than that of the lemon (10 deg.) and blacknose (11 deg.) sharks. The binocular overlap in the scalloped hammerhead sharks (34 deg.) was greater than the bonnethead sharks (13 deg.) and carcharhinid species; however, the bonnethead shark did not differ from the carcharhinids. These results indicate that binocular overlap has increased with lateral head expansion in hammerhead sharks. The hammerhead species did not demonstrate greater eye rotation in the anterior or posterior direction. However, both the scalloped hammerhead and bonnethead sharks exhibited greater head yaw during swimming (16.9 deg. and 15.6 deg., respectively) than the lemon (15.1 deg.) and blacknose (15.0 deg.) sharks, indicating a behavioral compensation for the anterior blind area. This study illustrates the larger binocular overlap in hammerhead species relative to their carcharhinid sister taxa and is consistent with the 'enhanced binocular field' hypothesis.

Functional consequences of structural differences in stingray sensory systems. Part I: mechanosensory lateral line canals

Short range hydrodynamic and electrosensory signals are important during final stages of prey capture in elasmobranchs (sharks, skates and rays), and may be particularly useful for dorso-ventrally flattened batoids with mouths hidden from their eyes. In stingrays, both the lateral line canal and electrosensory systems are highly modified and complex with significant differences on ventral surfaces that relate to feeding ecology. This study tests functional hypotheses based on quantified differences in sensory system morphology of three stingray species, Urobatis halleri, Myliobatis californica and Pteroplatytrygon violacea. Part I investigates the mechanosensory lateral line canal system whereas part II focuses on the electrosensory system. Stingray lateral line canals include both pored and non-pored sections and differ in branching complexity and distribution. A greater proportion of pored canals and high pore numbers were predicted to correspond to increased response to water flow. Behavioral experiments were performed to compare responses of stingrays to weak water jets mimicking signals produced by potential prey at velocities of 10–20 cm s–1. Bat rays, M. californica, have the most complex and broadly distributed pored canal network and demonstrated both the highest response rate and greater response intensity to water jet signals. Results suggest that U. halleri and P. violacea may rely on additional sensory input, including tactile and visual cues, respectively, to initiate stronger feeding responses. These results suggest that stingray lateral line canal morphology can indicate detection capabilities through responsiveness to weak water jets.