Enhanced visual fields in hammerhead sharks

Embryonic development in the bonnethead (Sphyrna tiburo), a viviparous hammerhead shark

BACKGROUND

The hammerhead sharks (family Sphyrnidae) are an immediately recognizable group of sharks due to their unique head shape. Though there has long been an interest in hammerhead development, there are currently no explicit staging tables published for any members of the group. The bonnethead Sphyrna tiburo is the smallest member of Sphyrnidae and is abundant in estuarine and nearshore waters in the Gulf of Mexico and Western North Atlantic Ocean. Due to their relative abundance, close proximity to shore, and brief gestation period, it has been possible to collect and document multiple embryonic specimens at progressive stages of development.

RESULTS

We present the first comprehensive embryonic staging series for the Bonnethead, a viviparous hammerhead shark. Our stage series covers a period of development from stages that match the vertebrate phylotypic period, from Stage 23, through stages of morphological divergence to complete development at birth-Stage 35). Notably, we use a variety of techniques to document crucial stages that lead to their extreme craniofacial diversity, resulting in the formation of one of the most distinctive characters of any shark species, the cephalofoil or hammer-like head.

CONCLUSION

Documenting the development of hard-to-access vertebrates, like this viviparous shark species, offers important information about how new and diverse morphologies arise that otherwise may remain poorly studied. This work will serve as a platform for future comparative developmental research both within sharks and across the phylogeny of vertebrates, underpinning the extreme potential of craniofacial development and morphological diversity in vertebrate animals.

Temporal niche partitioning as a novel mechanism promoting co-existence of sympatric predators in marine systems

Niche partitioning of time, space or resources is considered the key to allowing the coexistence of competitor species, and particularly guilds of predators. However, the extent to which these processes occur in marine systems is poorly understood due to the difficulty in studying fine-scale movements and activity patterns in mobile underwater species. Here, we used acceleration data-loggers to investigate temporal partitioning in a guild of marine predators. Six species of co-occurring large coastal sharks demonstrated distinct diel patterns of activity, providing evidence of strong temporal partitioning of foraging times. This is the first instance of diel temporal niche partitioning described in a marine predator guild, and is probably driven by a combination of physiological constraints in diel timing of activity (e.g. sensory adaptations) and interference competition (hierarchical predation within the guild), which may force less dominant predators to suboptimal foraging times to avoid agonistic interactions. Temporal partitioning is often thought to be rare compared to other partitioning mechanisms, but the occurrence of temporal partitioning here and similar characteristics in many other marine ecosystems (multiple predators simultaneously present in the same space with dietary overlap) introduces the question of whether this is a common mechanism of resource division in marine systems.

A first observation of spermatogenesis in mature male scalloped hammerheads (Sphyrna lewini) from Zinkwazi, KwaZulu-Natal, South Africa

Sharks are not only threatened, but also have a low fecundity as they are being overfished. The shark family, Sphyrnidae, consists of nine species of which three are found in South African oceans. One of the three Sphyrnidae species, the scalloped hammerhead (Sphyrna lewini) are the most common, but their biology and mode of reproduction are not extensively studied in terms of their reproductive biology. The aim of this study was to describe the germ cell development in the testes of sexually mature male scalloped hammerheads. Three individual male S. lewini were caught at Zinkwazi, KwaZulu-Natal, South Africa. The sharks and their reproductive organs were weighed and measured to collect the biometric data for the condition factor and the gonado-somatic index. Following standard necropsy, the testes were fixed in Bouin's solution and processed for histological assessment. The histological assessment revealed that the testes of S. lewini consist of seminiferous tubules which form part of a larger lobular structure with germ cells in different stages of development, from spermatogonia to mature spermatozoa. Seven stages of development were identified during the process of spermatogenesis, similar to what has been described for elasmobranchs. In conclusion, this study provides evidence that the testes of S. lewini are diametrical and polyspermatocystic and conforms to the testes structure of elasmobranch males.

Volitional Swimming Kinematics of Blacktip Sharks, Carcharhinus limbatus, in the Wild

Recent work showed that two species of hammerhead sharks operated as a double oscillating system, where frequency and amplitude differed in the anterior and posterior parts of the body. We hypothesized that a double oscillating system would be present in a large, volitionally swimming, conventionally shaped carcharhinid shark. Swimming kinematics analyses provide quantification to mechanistically examine swimming within and among species. Here, we quantify blacktip shark (Carcharhinus limbatus) volitional swimming kinematics under natural conditions to assess variation between anterior and posterior body regions and demonstrate the presence of a double oscillating system. We captured footage of 80 individual blacktips swimming in the wild using a DJI Phantom 4 Pro aerial drone. The widespread accessibility of aerial drone technology has allowed for greater observation of wild marine megafauna. We used Loggerpro motion tracking software to track five anatomical landmarks frame by frame to calculate tailbeat frequency, tailbeat amplitude, speed, and anterior/posterior variables: amplitude and frequency of the head and tail, and the body curvature measured as anterior and posterior flexion. We found significant increases in tailbeat frequency and amplitude with increasing swimming speed. Tailbeat frequency decreased and tailbeat amplitude increased as posterior flexion amplitude increased. We found significant differences between anterior and posterior amplitudes and frequencies, suggesting a double oscillating modality of wave propagation. These data support previous work that hypothesized the importance of a double oscillating system for increased sensory perception. These methods demonstrate the utility of quantifying swimming kinematics of wild animals through direct observation, with the potential to apply a biomechanical perspective to movement ecology paradigms.

Oil gland and oil pores in billfishes: in search of a function

Billfishes are well known for their distinctive elongated rostra, i.e. bills. The functional significance of billfish rostra has been frequently discussed and the recent discovery of an oil gland (glandula oleofera) at the base of the rostrum in swordfish, Xiphias gladius, has added an interesting facet to this discussion regarding the potential co-evolution of gland and rostra. Here, we investigated the oil gland and oil pores (through which the oil is brought to the skin surface) of four billfish species - swordfish, Atlantic blue marlin (Makaira nigricans), Indo-Pacific sailfish (Istiophorus platypterus) and striped marlin (Kajikia audax) - and provide detailed evidence for the presence of an oil gland in the last three. All four species had a high density of oil pores on the forehead which is consistent with the hypothesis of hydrodynamic benefits of the oil. The extension of the pores onto the front half of the rostrum in sailfish and striped marlin, but not in swordfish or blue marlin, suggests that the oil may have additional functions. One such function could be linked to the antibacterial and anti-inflammatory properties of the oil. However, the available evidence on predatory rostrum use (and hence the likelihood of tissue damage) is only partly consistent with the extension of pores on rostra across species. We conclude that the oil gland probably serves multiple, non-mutually exclusive functions. More detailed information on rostrum use in blue marlin and swordfish is needed to better link behavioural and morphological data with the aim of accomplishing a full comparative analysis.

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.

Visual system development of the spotted unicornfish, Naso brevirostris (Acanthuridae)

Ontogenetic changes of the visual system are often correlated to shifts in habitat and feeding behaviour of animals. Coral reef fishes begin their lives in the pelagic zone and then migrate to the reef. This habitat transition frequently involves a change in diet and light environment as well as major morphological modifications. The spotted unicornfish, Naso brevirostris, is known to shift diet from zooplankton to algae and back to mainly zooplankton when transitioning from larval to juvenile and then to adult stages. Concurrently, N. brevirostris also moves from an open pelagic to a coral-associated habitat before migrating up in the water column when reaching adulthood. Using retinal mapping techniques, we discovered that the distribution and density of ganglion and photoreceptor cells in N. brevirostris mostly changes during the transition from the larval to the juvenile stage, with only minor modifications thereafter. Similarly, visual gene (opsin) expression based on RNA sequencing, although qualitatively similar between stages (all fishes mainly expressed the same three cone opsins; SWS2B, RH2B, RH2A), also showed the biggest quantitative difference when transitioning from larvae to juveniles. The juvenile stage in particular seems mismatched with its reef-associated ecology, which may be due to this stage only lasting a fraction of the lifespan of these fishes. Hence, the visual ontogeny found in N. brevirostris is very different from the progressive changes found in other reef fishes calling for a thorough analysis of visual system development of the reef fish community.

An Analysis of Extraocular Muscle Forces in the Piked Dogfish (Squalus acanthias)

Vertebrates utilize six extraocular muscles that attach to a tough, protective sclera to rotate the eye. The goal of the study was to describe the maximum tetanic forces, as well as the torques produced by the six extraocular muscles of the piked dogfish Squalus acanthias to understand the forces exerted on the eye. The lateral rectus extraocular muscle of Squalus acanthias was determined to be parallel fibered with the muscle fibers bundled into discrete fascicles. The extraocular muscles attach to the sclera by muscular insertions. The total tensile forces generated by the extraocular muscles ranged from 1.18 N to 2.21 N. The torques of the extraocular muscles ranged from 0.39 N to 2.34 N. The torques were greatest in the principal direction of movement for each specific muscle. The lateral rectus produced the greatest total tensile force, as well as the greatest torque force component, while the medial rectus produced the second greatest. This is likely due to the constant rotational movement of the eye anteriorly and posteriorly to stabilize the visual image, as well as increase the effective visual field during swimming. Rotational forces in dimensions other than the primary direction of movement may contribute to motion in directions other than the principal direction during multi-muscle contraction that occurs in the vertebrate eye. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:837-844, 2019. © 2018 Wiley Periodicals, Inc.

Scene through the eyes of an apex predator: a comparative analysis of the shark visual system

The eyes of apex predators, such as the shark, have fascinated comparative visual neuroscientists for hundreds of years with respect to how they perceive the dark depths of their ocean realm or the visual scene in search of prey. As the earliest representatives of the first stage in the evolution of jawed vertebrates, sharks have an important role to play in our understanding of the evolution of the vertebrate eye, including that of humans. This comprehensive review covers the structure and function of all the major ocular components in sharks and how they are adapted to a range of underwater light environments. A comparative approach is used to identify: species-specific diversity in the perception of clear optical images; photoreception for various visual behaviours; the trade-off between image resolution and sensitivity; and visual processing under a range of levels of illumination. The application of this knowledge is also discussed with respect to the conservation of this important group of cartilaginous fishes.

The pupillary light responses of animals; a review of their distribution, dynamics, mechanisms and functions

The timecourse and extent of changes in pupil area in response to light are reviewed in all classes of vertebrate and cephalopods. Although the speed and extent of these responses vary, most species, except the majority of teleost fish, show extensive changes in pupil area related to light exposure. The neuromuscular pathways underlying light-evoked pupil constriction are described and found to be relatively conserved, although the precise autonomic mechanisms differ somewhat between species. In mammals, illumination of only one eye is known to cause constriction in the unilluminated pupil. Such consensual responses occur widely in other animals too, and their function and relation to decussation of the visual pathway is considered. Intrinsic photosensitivity of the iris muscles has long been known in amphibia, but is in fact widespread in other animals. The functions of changes in pupil area are considered. In the majority of species, changes in pupil area serve to balance the conflicting demands of high spatial acuity and increased sensitivity in different light levels. In the few teleosts in which pupil movements occur they do not serve a visual function but play a role in camouflaging the eye of bottom-dwelling species. The occurrence and functions of the light-independent changes in pupil size displayed by many animals are also considered. Finally, the significance of the variations in pupil shape, ranging from circular to various orientations of slits, ovals, and other shapes, is discussed.

Regional variation in undulatory kinematics of two hammerhead species: the bonnethead (Sphyrna tiburo) and the scalloped hammerhead (Sphyrna lewini)

Hammerhead sharks (Sphyrnidae) exhibit a large amount of morphological variation within the family, making them the focus of many studies. The size of the laterally expanded head, or cephalofoil, is inversely correlated with pectoral fin area. The inverse relationship between cephalofoil and pectoral fin size in this family suggests that they might serve a complementary role in lift generation. The cephalofoil is also hypothesized to increase olfaction, electroreception and vision; however, little is known about how morphological variation impacts post-cranial swimming kinematics. Previous studies demonstrate that the bonnethead and scalloped hammerhead have significantly different yaw amplitude, and we hypothesized that these species utilize varied frequency and amplitude of undulation along the body. We analyzed video of free-swimming sharks to examine kinematics and 2D morphological variables of the bonnethead and scalloped hammerhead. We also examined the second moment of area along the length of the body and over a size range of animals to determine whether there were shape differences along the body of these species and whether those changed over ontogeny. We found that both species swim with the same standardized velocity and Strouhal number, but there was no correlation between two-dimensional morphology and swimming kinematics. However, the bonnethead has a dorso-ventrally compressed anterior trunk and undulates with greater amplitude, whereas the scalloped hammerhead has a laterally compressed anterior trunk and undulates with lower amplitude. We propose that differences in cross-sectional trunk morphology account for interspecific differences in undulatory amplitude. We also found that for both species, undulatory frequency is significantly greater in the anterior body compared with all other body regions. We hypothesize that the bonnethead and scalloped hammerhead swim with a double oscillation system.

The effects of Biorock-associated electric fields on the Caribbean reef shark (Carcharhinus perezi) and the bull shark (Carcharhinus leucas)

Healthy coral reefs are biologically diverse and provide vital ecosystem services. However, decreasing water quality and global warming are key contributors to coral reef decline, which poses substantial environmental threats. In response to this degradation, an innovative coral reef restoration technology, called Biorock, utilizes weak direct current electric fields to cause limestone deposition on conductive materials, inevitably inducing prolific coral reef growth. Although expediting coral growth, research on how the associated electric fields may impact the behavioural patterns of teleosts and/or organisms (i.e. elasmobranchs) possessing electroreception capabilities is lacking. Therefore, we studied the behavioural responses of two shark species, the bull shark (Carcharhinus leucas) and the Caribbean reef shark (Carcharhinus perezi) and multiple teleost species towards weak direct current electric fields in Bimini, Bahamas. Generalized linear mixed model analyses based on 90 trials illustrate that both the feeding and avoidance behaviors of C. leucas and C. perezi were significantly associated with treatment type, with the weak experimental electrode treatments resulting in the greatest quantity of avoidances and fewest feedings for both species. However, data analyses illustrate that teleost feeding behavior was not observably impacted by experimental treatments. Although the Biorock technology exhibits promise in coral reef restoration, the findings from this study illustrate a need for future large-scale studies assessing shark behavioral patterns around these devices, since the deterrence of apex predators may impact ecosystem balance.

Visual resolution and contrast sensitivity in two benthic sharks

Sharks have long been described as having 'poor' vision. They are cone monochromats and anatomical estimates suggest they have low spatial resolution. However, there are no direct behavioural measurements of spatial resolution or contrast sensitivity. This study estimates contrast sensitivity and spatial resolution of two species of benthic sharks, the Port Jackson shark, Heterodontus portusjacksoni, and the brown-banded bamboo shark, Chiloscyllium punctatum, by recording eye movements in response to optokinetic stimuli. Both species tracked moving low spatial frequency gratings with weak but consistent eye movements. Eye movements ceased at 0.38 cycles per degree, even for high contrasts, suggesting low spatial resolution. However, at lower spatial frequencies, eye movements were elicited by low contrast gratings, 1.3% and 2.9% contrast in H portusjacksoni and C. punctatum, respectively. Contrast sensitivity was higher than in other vertebrates with a similar spatial resolving power, which may reflect an adaptation to the relatively low contrast encountered in aquatic environments. Optokinetic gain was consistently low and neither species stabilised the gratings on their retina. To check whether restraining the animals affected their optokinetic responses, we also analysed eye movements in free-swimming C. punctatum We found no eye movements that could compensate for body rotations, suggesting that vision may pass through phases of stabilisation and blur during swimming. As C. punctatum is a sedentary benthic species, gaze stabilisation during swimming may not be essential. Our results suggest that vision in sharks is not 'poor' as previously suggested, but optimised for contrast detection rather than spatial resolution.

Molecular and pathological characterization of Fusarium solani species complex infection in the head and lateral line system of Sphyrna lewini

A severe fungal infection affecting the head and lateral line system was diagnosed in 7 captive scalloped hammerhead sharks Sphyrna lewini in an aquarium in Thailand. Extensive and severe necrotizing cellulitis was consistently observed microscopically along the cephalic and lateral line canals in conjunction with positive fungal cultures for Fusarium sp. Molecular phylogenetic analysis was performed from 3 isolates based on the nucleotide sequences containing internally transcribed spacer (ITS) and a portion of 5.8S and 28S rDNA. The fungus was highly homologous (100%) and closely related to F. solani species complex 2 (FSSC 2), which belongs to Clade 3 of the FSSC. Our results illustrate the histopathological findings and expand upon our knowledge of the prevalence of invasive fusariosis in the head and lateral line system of hammerhead sharks.

Laterality strength is linked to stress reactivity in Port Jackson sharks (Heterodontus portusjacksoni)

Cerebral lateralization is an evolutionarily deep-rooted trait, ubiquitous among the vertebrates and present even in some invertebrates. Despite the advantages of cerebral lateralization in enhancing cognition and facilitating greater social cohesion, large within population laterality variation exists in many animal species. It is proposed that this variation is maintained due links with inter-individual personality trait differences. Here we explored for lateralization in Port Jackson sharks (Heterodontus portusjacksoni) using T-maze turn and rotational swimming tasks. Additionally, we explored for a link between personality traits, boldness and stress reactivity, and cerebral lateralization. Sharks demonstrated large individual and sex biased laterality variation, with females demonstrating greater lateralization than males overall. Stress reactivity, but not boldness, was found to significantly correlate with lateralization strength. Stronger lateralized individuals were more reactive to stress. Demonstrating laterality in elasmobranchs for the first time indicates ancient evolutionary roots of vertebrate lateralization approximately 240 million years old. Greater lateralization in female elasmobranchs may be related enhancing females' ability to process multiple stimuli during mating, which could increase survivability and facilitate insemination. Despite contrasting evidence in teleost fishes, the results of this study suggest that stress reactivity, and other personality traits, may be linked to variation in lateralization.

Eye retraction in the giant guitarfish, Rhynchobatus djiddensis (Elasmobranchii: Batoidea): a novel mechanism for eye protection in batoid fishes

Eye retraction behavior has evolved independently in some vertebrate linages such as mudskippers (fish), frogs and salamanders (amphibians), and cetaceans (mammals). In this paper, we report the eye retraction behavior of the giant guitarfish (Rhynchobatus djiddensis) for the first time, and discuss its mechanism and function. The eye retraction distance was nearly the same as the diameter of the eyeball itself, indicating that eye retraction in the giant guitarfish is probably one of the largest among vertebrates. Eye retraction is achieved by unique arrangement of the eye muscle: one of the anterior eye muscles (the obliquus inferior) is directed ventrally from the eyeball and attaches to the ventral surface of the neurocranium. Due to such muscle arrangement, the obliquus inferior can pull the eyeball ventrally. This mechanism was also confirmed by electrical stimulation of the obliquus inferior. The eye retraction ability of the giant guitarfish likely represents a novel eye protection behavior of elasmobranch fishes.

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.

Not So Fast: Swimming Behavior of Sailfish during Predator-Prey Interactions using High-Speed Video and Accelerometry

Billfishes are considered among the fastest swimmers in the oceans. Despite early estimates of extremely high speeds, more recent work showed that these predators (e.g., blue marlin) spend most of their time swimming slowly, rarely exceeding 2 m s(-1). Predator-prey interactions provide a context within which one may expect maximal speeds both by predators and prey. Beyond speed, however, an important component determining the outcome of predator-prey encounters is unsteady swimming (i.e., turning and accelerating). Although large predators are faster than their small prey, the latter show higher performance in unsteady swimming. To contrast the evading behaviors of their highly maneuverable prey, sailfish and other large aquatic predators possess morphological adaptations, such as elongated bills, which can be moved more rapidly than the whole body itself, facilitating capture of the prey. Therefore, it is an open question whether such supposedly very fast swimmers do use high-speed bursts when feeding on evasive prey, in addition to using their bill for slashing prey. Here, we measured the swimming behavior of sailfish by using high-frequency accelerometry and high-speed video observations during predator-prey interactions. These measurements allowed analyses of tail beat frequencies to estimate swimming speeds. Our results suggest that sailfish burst at speeds of about 7 m s(-1) and do not exceed swimming speeds of 10 m s(-1) during predator-prey interactions. These speeds are much lower than previous estimates. In addition, the oscillations of the bill during swimming with, and without, extension of the dorsal fin (i.e., the sail) were measured. We suggest that extension of the dorsal fin may allow sailfish to improve the control of the bill and minimize its yaw, hence preventing disturbance of the prey. Therefore, sailfish, like other large predators, may rely mainly on accuracy of movement and the use of the extensions of their bodies, rather than resorting to top speeds when hunting evasive prey.

Constructional morphology within the head of hammerhead sharks (sphyrnidae)

The study of functional trade-offs is important if a structure, such as the cranium, serves multiple biological roles, and is, therefore, shaped by multiple selective pressures. The sphyrnid cephalofoil presents an excellent model for investigating potential trade-offs among sensory, neural, and feeding structures. In this study, hammerhead shark species were chosen to represent differences in head form through phylogeny. A combination of surface-based geometric morphometrics, computed tomography (CT) volumetric analysis, and phylogenetic analyses were utilized to investigate potential trade-offs within the head. Hammerhead sharks display a diversity of cranial morphologies where the position of the eyes and nares vary among species, with only minor changes in shape, position, and volume of the feeding apparatus through phylogeny. The basal winghead shark, Eusphyra blochii, has small anteriorly positioned eyes. Through phylogeny, the relative size and position of the eyes change, such that derived species have larger, more medially positioned eyes. The lateral position of the external nares is highly variable, showing no phylogenetic trend. Mouth size and position are conserved, remaining relatively unchanged. Volumetric CT analyses reveal no trade-offs between the feeding apparatus and the remaining cranial structures. The few trade-offs were isolated to the nasal capsule volume's inverse correlation with braincase, chondrocranial, and total cephalofoil volume. Eye volume also decreased as cephalofoil width increased. These data indicate that despite considerable changes in head shape, much of the head is morphologically conserved through sphyrnid phylogeny, particularly the jaw cartilages and their associated feeding muscles, with shape change and morphological trade-offs being primarily confined to the lateral wings of the cephalofoil and their associated sensory structures.

Multisensory integration and behavioral plasticity in sharks from different ecological niches

The underwater sensory world and the sensory systems of aquatic animals have become better understood in recent decades, but typically have been studied one sense at a time. A comprehensive analysis of multisensory interactions during complex behavioral tasks has remained a subject of discussion without experimental evidence. We set out to generate a general model of multisensory information extraction by aquatic animals. For our model we chose to analyze the hierarchical, integrative, and sometimes alternate use of various sensory systems during the feeding sequence in three species of sharks that differ in sensory anatomy and behavioral ecology. By blocking senses in different combinations, we show that when some of their normal sensory cues were unavailable, sharks were often still capable of successfully detecting, tracking and capturing prey by switching to alternate sensory modalities. While there were significant species differences, odor was generally the first signal detected, leading to upstream swimming and wake tracking. Closer to the prey, as more sensory cues became available, the preferred sensory modalities varied among species, with vision, hydrodynamic imaging, electroreception, and touch being important for orienting to, striking at, and capturing the prey. Experimental deprivation of senses showed how sharks exploit the many signals that comprise their sensory world, each sense coming into play as they provide more accurate information during the behavioral sequence of hunting. The results may be applicable to aquatic hunting in general and, with appropriate modification, to other types of animal behavior.

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.

Electroreception in elasmobranchs: sawfish as a case study

The ampullae of Lorenzini are the electroreceptors of elasmobranchs. Ampullary pores located in the elasmobranch skin are each connected to a gel-filled canal that ends in an ampullary bulb, in which the sensory epithelium is located. Each ampulla functions as an independent receptor that measures the potential difference between the ampullary pore opening and the body interior. In the elasmobranch head, the ampullary bulbs of different ampullae are aggregated in 3-6 bilaterally symmetric clusters, which can be surrounded by a connective tissue capsule. Each cluster is innervated by one branch of the anterior lateral line nerve (ALLN). Only the dorsal root of the ALLN carries electrosensory fibers, which terminate in the dorsal octavo-lateral nucleus (DON) of the medulla. Each ampullary cluster projects into a distinctive area in the central zone of the DON, where projection areas are somatotopically arranged. Sharks and rays can possess thousands of ampullae. Amongst other functions, the use of electroreception during prey localization is well documented. The distribution of ampullary pores in the skin of elasmobranchs is influenced by both the phylogeny and ecology of a species. Pores are grouped in distinct pore fields, which remain recognizable amongst related taxa. However, the density of pores within a pore field, which determines the electroreceptive resolution, is influenced by the ecology of a species. Here, I compare the pore counts per pore field between rhinobatids (shovelnose rays) and pristids (sawfish). In both groups, the number of ampullary pores on the ventral side of the rostrum is similar, even though the pristid rostrum can comprise about 20% of the total length. Ampullary pore numbers in pristids are increased on the upper side of the rostrum, which can be related to a feeding strategy that targets free-swimming prey in the water column. Shovelnose rays pin their prey onto the substrate with their disk, while repositioning their mouth for ingestion and thus possess large numbers of pores ventrally around the mouth and in the area between the gills.

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.

Temporal resolution and spectral sensitivity of the visual system of three coastal shark species from different light environments

Visual temporal resolution and scotopic spectral sensitivity of three coastal shark species (bonnethead Sphyrna tiburo, scalloped hammerhead Sphyrna lewini, and blacknose shark Carcharhinus acronotus) were investigated by electroretinogram. Temporal resolution was quantified under photopic and scotopic conditions using response waveform dynamics and maximum critical flicker-fusion frequency (CFF). Photopic CFF(max) was significantly higher than scotopic CFF(max) in all species. The bonnethead had the shortest photoreceptor response latency time (23.5 ms) and the highest CFF(max) (31 Hz), suggesting that its eyes are adapted for a bright photic environment. In contrast, the blacknose had the longest response latency time (34.8 ms) and lowest CFF(max) (16 Hz), indicating its eyes are adapted for a dimmer environment or nocturnal lifestyle. Scotopic spectral sensitivity revealed maximum peaks (480 nm) in the bonnethead and blacknose sharks that correlated with environmental spectra measured during twilight, which is a biologically relevant period of heightened predation.