A content analysis of 32 years of Shark Week documentaries.

Despite evidence of their importance to marine ecosystems, at least 32% of all chondrichthyan species are estimated or assessed as threatened with extinction. In addition to the logistical difficulties of effectively conserving wide-ranging marine species, shark conservation is believed to have been hindered in the past by public perceptions of sharks as dangerous to humans. Shark Week is a high-profile, international programming event that has potentially enormous influence on public perceptions of sharks, shark research, shark researchers, and shark conservation. However, Shark Week has received regular criticism for poor factual accuracy, fearmongering, bias, and inaccurate representations of science and scientists. This research analyzes the content and titles of Shark Week episodes across its entire 32 years of programming to determine if there are trends in species covered, research techniques featured, expert identity, conservation messaging, type of programming, and portrayal of sharks. We analyzed titles from 272 episodes (100%) of Shark Week programming and the content of all available (201; 73.9%) episodes. Our data demonstrate that the majority of episodes are not focused on shark bites, although such shows are common and many Shark Week programs frame sharks around fear, risk, and adrenaline. While criticisms of disproportionate attention to particular charismatic species (e.g. great whites, bull sharks, and tiger sharks) are accurate and supported by data, 79 shark species have been featured briefly at least once. Shark Week's depictions of research and of experts are biased towards a small set of (typically visual and expensive) research methodologies and (mostly white, mostly male) experts, including presentation of many white male non-scientists as scientific experts. While sharks are more often portrayed negatively than positively, limited conservation messaging does appear in 53% of episodes analyzed. Results suggest that as a whole, while Shark Week is likely contributing to the collective public perception of sharks as bad, even relatively small alterations to programming decisions could substantially improve the presentation of sharks and shark science and conservation issues.

Swimming kinematics of the Caribbean reef shark, Carcharhinus perezi.

The Caribbean reef shark, Carcharhinus perezi, is known to rest on the substrate, a behaviour not documented in any of its congeners. We quantified the swimming kinematics of C. perezi in the wild and found that the head yawed at a frequency 15% greater than the tail beat, but that the amplitude of the tail exceeded the head yaw by approximately 80% across the range of velocities measured. We found that C. perezi velocity, head yaw frequency, and tailbeat frequency were all less than its obligate ram ventilating congener C. limbatus.

The use of an unoccupied aerial vehicle to survey shark species over sand and rocky-reef habitats in a marine protected area.

Cabo Pulmo National Park was established in 1995 and has since seen a large increase in fish biomass. An unoccupied aerial vehicle (UAV) was used to survey shallow coastal habitat in which lemon sharks (Negaprion brevirostris), bull sharks (Carcharhinus leucas) and Pacific nurse sharks (Ginglymostoma unami) were recorded. Sharks were more common in the afternoon, potentially using warmer shallow areas to behaviourally thermoregulate. This study highlights UAV surveying to be a viable tool for species identification, a limitation of previous terrestrial surveys conducted in the area.

Caudal Spine Morphology and Puncture Performance of Two Coastal Stingrays.

A diagnostic characteristic of stingrays in the family Dasyatidae is the presence of a defensive, partially serrated spine located on the tail. We assessed the contribution of caudal spine morphology on puncture and withdrawal performance from two congeneric, co-occurring stingrays, the Atlantic stingray, Hypanus sabinus, and the bluntnose stingray, Hypanus say. Spines exhibited a high degree of morphological variability. Stingray spines were serrated along 50.8% (H. sabinus) or 62.3% (H. say) of their length. Hypanus say had a greater number of serrations along each side of the spine (30.4) compared with H. sabinus (20.7) but the pitch did not differ between species. We quantified spine puncture and withdrawal forces using porcine skin as a model for human skin. Puncture and withdrawal forces did not differ significantly between species, or within H. say, but withdrawal force was greater than puncture force for H. sabinus. We incorporated micro-computed tomography scanning to quantify tissue mineral density and found that for both species, the shaft of the spine was more heavily mineralized than the base, and midway (50%) along the length of the spine was more heavily mineralized than the tip. The mineralization variability along the spine shaft may create a stiff structure that can fracture once embedded within the target tissue and act as an effective predator deterrent.

Adult blacktip sharks (Carcharhinus limbatus) use shallow water as a refuge from great hammerheads (Sphyrna mokarran).

A refuge can be any space that keeps an organism safe from danger. Prey usually seek protection in the closest refuge available to minimize cost while maximizing survival. Aerial drone footage of blacktip sharks, Carcharhinus limbatus, along the coast of southeast Florida, USA, shows adult blacktips fleeing to the shallow water adjacent to the beach when confronted with or chased by a predatory great hammerhead shark, Sphyrna mokarran. To authors' knowledge, this is the first evidence of adult C. limbatus using shallow waters as a refuge.

Electroreception in marine fishes: chondrichthyans.

Electroreception in marine fishes occurs across a variety of taxa and is best understood in the chondrichthyans (sharks, skates, rays, and chimaeras). Here, we present an up-to-date review of what is known about the biology of passive electroreception and we consider how electroreceptive fishes might respond to electric and magnetic stimuli in a changing marine environment. We briefly describe the history and discovery of electroreception in marine Chondrichthyes, the current understanding of the passive mode, the morphological adaptations of receptors across phylogeny and habitat, the physiological function of the peripheral and central nervous system components, and the behaviours mediated by electroreception. Additionally, whole genome sequencing, genetic screening and molecular studies promise to yield new insights into the evolution, distribution, and function of electroreceptors across different environments. This review complements that of electroreception in freshwater fishes in this special issue, which provides a comprehensive state of knowledge regarding the evolution of electroreception. We conclude that despite our improved understanding of passive electroreception, several outstanding gaps remain which limits our full comprehension of this sensory modality. Of particular concern is how electroreceptive fishes will respond and adapt to a marine environment that is being increasingly altered by anthropogenic electric and magnetic fields.

Effect of Deepwater Horizon Crude Oil Water Accommodated Fraction on Olfactory Function in the Atlantic Stingray, Hypanus sabinus.

The Deepwater Horizon oil spill was the largest accidental marine oil spill in history, releasing nearly 5 million barrels of crude oil. Crude oil causes both lethal and sublethal effects on marine organisms, and sensory systems have the potential to be strongly affected. Marine fishes rely upon the effective functioning of their sensory systems for detection of prey, mates, and predators. However, despite the obvious importance of sensory systems, the impact of crude oil exposure upon sensory function remains largely unexplored. Here we show that olfactory organ responses to amino acids are significantly depressed in oil exposed stingrays. We found that the response magnitude of the electro-olfactogram (EOG) to 1 mM amino acids decreased by an average of 45.8% after 48 h of exposure to an oil concentration replicating that measured in coastal areas. Additionally, in oil exposed individuals, the EOG response onset was significantly slower, and the clearing time was protracted. This study is the first to employ an electrophysiological assay to demonstrate crude oil impairment of the olfactory system in a marine fish. We show that stingrays inhabiting an area impacted by an oil spill experience reduced olfactory function, which would detrimentally impact fitness, could lead to premature death, and could cause additional cascading effects through lower trophic levels.

Etmopterus lailae sp. nov., a new lanternshark (Squaliformes: Etmopteridae) from the Northwestern Hawaiian Islands.

A new species of lanternshark, Etmopterus lailae (Squaliformes: Etmopteridae), is described from the Northwestern Hawaiian Islands, in the central North Pacific Ocean. The new species resembles other members of the "Etmopterus lucifer" clade in having linear rows of dermal denticles, and most closely resembles E. lucifer from Japan. The new species occurs along insular slopes around seamounts at depths between 314-384 m. It can be distinguished from other members of the E. lucifer clade by a combination of characteristics, including a longer anterior flank marking branch, arrangement of dermal denticles on the ventral snout surface and body, flank and caudal markings, and meristic counts including number of spiral valve turns, and precaudal vertebrate. A key to species of the Etmopterus lucifer-clade is included.

Magnetic field discrimination, learning, and memory in the yellow stingray (Urobatis jamaicensis).

Elasmobranch fishes (sharks, skates, and rays) have been hypothesized to use the geomagnetic field as a cue for orienting and navigating across a wide range of spatial scales. Magnetoreception has been demonstrated in many invertebrate and vertebrate taxa, including elasmobranchs, but this sensory modality and the cognitive abilities of cartilaginous fishes are poorly studied. Wild caught yellow stingrays, Urobatis jamaicensis (N = 8), underwent conditioning to associate a magnetic stimulus with a food reward in order to elicit foraging behaviors. Behavioral conditioning consisted of burying magnets and non-magnetic controls at random locations within a test arena and feeding stingrays as they passed over the hidden magnets. The location of the magnets and controls was changed for each trial, and all confounding sensory cues were eliminated. The stingrays learned to discriminate the magnetic stimuli within a mean of 12.6 ± 0.7 SE training sessions of four trials per session. Memory probes were conducted at intervals between 90 and 180 days post-learning criterion, and six of eight stingrays completed the probes with a ≥75% success rate and minimum latency to complete the task. These results show the fastest rate of learning and longest memory window for any batoid (skate or ray) to date. This study demonstrates that yellow stingrays, and possibly other elasmobranchs, can use a magnetic stimulus as a geographic marker for the location of resources and is an important step toward understanding whether these fishes use geomagnetic cues during spatial navigation tasks in the natural environment.

Quantification of Massive Seasonal Aggregations of Blacktip Sharks (Carcharhinus limbatus) in Southeast Florida.

Southeast Florida witnesses an enormous seasonal influx of upper trophic level marine predators each year as massive aggregations of migrating blacktip sharks (Carcharhinus limbatus) overwinter in nearshore waters. The narrow shelf and close proximity of the Gulf Stream current to the Palm Beach County shoreline drive tens of thousands of sharks to the shallow, coastal environment. This natural bottleneck provides a unique opportunity to estimate relative abundance. Over a four year period from 2011-2014, an aerial survey was flown approximately biweekly along the length of Palm Beach County. A high definition video camera and digital still camera mounted out of the airplane window provided a continuous record of the belt transect which extended 200 m seaward from the shoreline between Boca Raton Inlet and Jupiter Inlet. The number of sharks within the survey transect was directly counted from the video. Shark abundance peaked in the winter (January-March) with a maximum in 2011 of 12,128 individuals counted within the 75.6 km(-2) belt transect. This resulted in a maximum density of 803.2 sharks km(-2). By the late spring (April-May), shark abundance had sharply declined to 1.1% of its peak, where it remained until spiking again in January of the following year. Shark abundance was inversely correlated with water temperature and large numbers of sharks were found only when water temperatures were less than 25 °C. Shark abundance was also correlated with day of the year but not with barometric pressure. Although shark abundance was not correlated with photoperiod, the departure of the sharks from southeast Florida occurred around the vernal equinox. The shark migration along the United States eastern seaboard corresponds spatially and temporally with the spawning aggregations of various baitfish species. These baseline abundance data can be compared to future studies to determine if shark population size is changing and if sharks are restricting their southward migration as global water temperatures increase.

Freezing behaviour facilitates bioelectric crypsis in cuttlefish faced with predation risk.

Cephalopods, and in particular the cuttlefish Sepia officinalis, are common models for studies of camouflage and predator avoidance behaviour. Preventing detection by predators is especially important to this group of animals, most of which are soft-bodied, lack physical defences, and are subject to both visually and non-visually mediated detection. Here, we report a novel cryptic mechanism in S. officinalis in which bioelectric cues are reduced via a behavioural freeze response to a predator stimulus. The reduction of bioelectric fields created by the freeze-simulating stimulus resulted in a possible decrease in shark predation risk by reducing detectability. The freeze response may also facilitate other non-visual cryptic mechanisms to lower predation risk from a wide range of predator types.

The coelacanth rostral organ is a unique low-resolution electro-detector that facilitates the feeding strike.

The cartilaginous and non-neopterygian bony fishes have an electric sense typically comprised of hundreds or thousands of sensory canals distributed in broad clusters over the head. This morphology facilitates neural encoding of local electric field intensity, orientation, and polarity, used for determining the position of nearby prey. The coelacanth rostral organ electric sense, however, is unique in having only three paired sensory canals with distribution restricted to the dorsal snout, raising questions about its function. To address this, we employed magnetic resonance imaging methods to map electrosensory canal morphology in the extant coelacanth, Latimeria chalumnae, and a simple dipole 'rabbit ears' antennae model with toroidal gain function to approximate their directional sensitivity. This identified a unique focal region of electrosensitivity directly in front of the mouth, and is the first evidence of a low-resolution electro-detector that solely facilitates prey ingestion.

Pelvic girdle shape predicts locomotion and phylogeny in batoids.

In terrestrial vertebrates, the pelvic girdle can reliably predict locomotor mode. Because of the diminished gravitational effects on positively buoyant bony fish, the same relationship does not appear to exist. However, within the negatively buoyant elasmobranch fishes, benthic batoids employ pelvic fin bottom-walking and punting as primary or supplementary forms of locomotion. Therefore, in this study, we employed geometric and linear morphometrics to investigate if their pelvic girdles exhibit shape characteristics similar to those of sprawling terrestrial vertebrates. We tested for correlates of pelvic girdle shape with 1) Order, 2) Family, 3) Swim Mode, and/or 4) Punt Mode. Landmarks and semilandmarks were placed along outlines of dorsal views of 61 batoid pelvic girdles (3/3 orders, 10/13 families, 35/72 genera). The first three relative warps explained 88.45% of the variation among individuals (P < 0.01%). Only Order and Punt Mode contained groups that were all significantly different from each other (P < 0.01%). Discriminant function analyses indicated that the majority of variation within each category was due to differences in extension of lateral and prepelvic processes and puboischiac bar angle. Over 60% of the original specimens and 55% of the cross-validated specimens were correctly classified. The neutral angle of the propterygium, which articulates with the pelvic girdle, was significantly different among punt modes, whereas only pectoral fin oscillators had differently shaped pelvic girdles when compared with batoids that perform other swimming modes (P < 0.01). Pelvic girdles of batoids vary greatly, and therefore, likely function in ways not previously described in teleost fishes. This study illustrates that pelvic girdle shape is a good predictor of punt mode, some forms of swimming mode, and a species' Order. Such correlation between locomotor style and pelvic girdle shape provides evidence for the convergent evolution of morphological features that support both sprawled-gait terrestrial walking and aquatic bottom-walking.

Behavioral responses of batoid elasmobranchs to prey-simulating electric fields are correlated to peripheral sensory morphology and ecology.

Electrosensory pore number, distribution, and sensitivity to prey-simulating electric fields have been described for many shark species. Electrosensory systems in batoids have received much less attention. Pore number and distribution have yet to be correlated to differences in sensitivity. However, pore number, pore distribution and sensitivity have been linked to behavior, diet, and morphology and follow species-specific trends. We report here that cownose rays have a greater number of pores than the yellow stingray, most of which are concentrated on the anterior ventral surface for both species. However, yellow stingrays have a broader arrangement of pores on both their dorsal and ventral surfaces than the cownose rays. Yellow stingrays demonstrated a median behavioral sensitivity to weak electric fields of 22nVcm(-1) and are among the most highly sensitive batoids studied to date. Cownose rays are less sensitive than all other elasmobranch species with a median sensitivity of 107nVcm(-1). As reported in previous studies, a higher pore number did not result in greater sensitivity. Cownose rays are benthopelagic schooling rays and may benefit from reduced sensitivity to bioelectric fields when they are surrounded by the bioelectric fields of conspecifics. Yellow stingrays, on the other hand, are typically solitary and bury in the substrate. A greater number of pores on their dorsal surface might improve detection of predators above them. Also, increased sensitivity and a broader distribution of pores may be beneficial as small prey items move past a buried ray.

Are you positive? Electric dipole polarity discrimination in the yellow stingray, Urobatis jamaicensis.

It is well established that elasmobranchs can detect dipole electric fields. However, it is unclear whether they can discriminate between the anode and cathode. To investigate this subject, we employed a behavioral assay to determine the discriminatory ability of the yellow stingray, Urobatis jamaicensis. We conditioned stingrays with food rewards to bite either the anode (n=5) or the cathode (n=6) of a direct-current dipole located on the floor of an experimental tank. All individuals successfully performed the task after 18 to 22 days. Stingrays were then tested in experimental sessions when they were rewarded only after they identified the correct pole. Stingrays successfully discriminated between the poles at a rate greater than chance, ranging among individuals from a mean of 66% to 93% correct. During experimental sessions, stingrays conditioned to distinguish the anode performed similarly to those conditioned to distinguish the cathode. We hypothesize that the ability to discriminate anode from cathode is physiologically encoded, but its utility in providing spatial information under natural conditions remains to be demonstrated. The ability to discriminate polarity may eliminate ambiguity in induction-based magnetoreception and facilitate navigation with respect to the geomagnetic field.

A physiological analysis of color vision in batoid elasmobranchs.

The potential for color vision in elasmobranchs has been studied in detail; however, a high degree of variation exists among the group. Evidence for ultraviolet (UV) vision is lacking, despite the presence of UV vision in every other vertebrate class. An integrative physiological approach was used to investigate color and ultraviolet vision in cownose rays and yellow stingrays, two batoids that inhabit different spectral environments. Both species had peaks in UV, short, medium, and long wavelength spectral regions in dark-, light-, and chromatic-adapted electroretinograms. Although no UV cones were found with microspectrophotometric analysis, both rays had multiple cone visual pigments with λ max at 470 and 551 nm in cownose rays (Rhinoptera bonasus) and 475, 533, and 562 nm in yellow stingrays (Urobatis jamaicensis). The same analysis demonstrated that both species had rod λ max at 500 and 499 nm, respectively. The lens and cornea of cownose rays maximally transmitted wavelengths greater than 350 nm and greater than 376 nm in yellow stingrays. These results support the potential for color vision in these species and future investigations should reveal the extent to which color discrimination is significant in a behavioral context.

Comparative visual function in predatory fishes from the Indian River Lagoon.

Visual temporal resolution and spectral sensitivity of three coastal teleost species (common snook [Centropomus undecimalis], gray snapper [Lutjanus griseus], and pinfish [Lagodon rhomboides]) were investigated by electroretinogram. Temporal resolution was quantified under photopic and scotopic conditions using response waveform dynamics and maximum critical flicker fusion frequency (CFFmax). Photopic CFFmax was significantly higher than scotopic CFFmax in all species. The snapper had the shortest photoreceptor response latency time (26.7 ms) and the highest CFFmax (47 Hz), suggesting that its eyes are adapted for a brighter photic environment. In contrast, the snook had the longest response latency time (36.8 ms) and lowest CFFmax (40 Hz), indicating that its eyes are adapted for a dimmer environment or nocturnal lifestyle. Species spectral responses ranged from 360 to 620 nm and revealed the presence of rods sensitive to dim and twilight conditions, as well as multiple cone visual pigments providing the basis for color and contrast discrimination. Collectively, our results demonstrate differences in visual function among species inhabiting the Indian River Lagoon system, representative of their unique ecology and life histories.

Bioelectric fields of marine organisms: voltage and frequency contributions to detectability by electroreceptive predators.

Behavioral responses of elasmobranch fishes to weak electric fields have been well studied. These studies typically employ a stimulator that produces a dipole electric field intended to simulate the natural electric field of prey items. However, the characteristics of bioelectric fields have not been well described. The magnitude and frequency of the electric field produced by 11 families of marine organisms were quantified in this study. Invertebrate electric potentials ranged from 14 to 28 µV and did not differ from those of elasmobranchs, which ranged from 18 to 30 µV. Invertebrates and elasmobranchs produced electric potentials smaller than those of teleost fishes, which ranged from 39 to 319 µV. All species produced electric fields within the frequency range that is detectable by elasmobranch predators (<16 Hz), with the highest frequencies produced by the penaeids (10.3 Hz) and the gerreids (4.6 Hz). Although voltage differed by family, there was no relationship between voltage and mass or length of prey. Differences in prey voltage may be related to osmoregulatory strategies; invertebrates and elasmobranchs are osmoconformers and have less ion exchange with the surrounding seawater than teleosts species, which are hyposmotic. As predicted, voltage production was greatest at the mucous membrane-lined mouth and gills, which are sites of direct ion exchange with the environment.

The somatotopic organization of the olfactory bulb in elasmobranchs.

The olfactory bulbs (OBs) are bilaterally paired structures in the vertebrate forebrain that receive and process odor information from the olfactory receptor neurons (ORNs) in the periphery. Virtually all vertebrate OBs are arranged chemotopically, with different regions of the OB processing different types of odorants. However, there is some evidence that elasmobranch fishes (sharks, rays, and skates) may possess a gross somatotopic organization instead. To test this hypothesis, we used histological staining and retrograde tracing techniques to examine the morphology and organization of ORN projections from the olfactory epithelium (OE) to the OB in three elasmobranch species with varying OB morphologies. In all three species, glomeruli in the OB received projections from ORNs located on only the three to five lamellae situated immediately anterior within the OE. These results support that the gross arrangement of the elasmobranch OB is somatotopic, an organization unique among fishes and most other vertebrates. In addition, certain elasmobranch species possess a unique OB morphology in which each OB is physically subdivided into two or more "hemi-olfactory bulbs." Somatotopy could provide a preadaptation which facilitated the evolution of olfactory hemibulbs in these species.

Electric field detection in sawfish and shovelnose rays.

In the aquatic environment, living organisms emit weak dipole electric fields, which spread in the surrounding water. Elasmobranchs detect these dipole electric fields with their highly sensitive electroreceptors, the ampullae of Lorenzini. Freshwater sawfish, Pristis microdon, and two species of shovelnose rays, Glaucostegus typus and Aptychotrema rostrata were tested for their reactions towards weak artificial electric dipole fields. The comparison of sawfishes and shovelnose rays sheds light on the evolution and function of the elongated rostrum ('saw') of sawfish, as both groups evolved from a shovelnose ray-like ancestor. Electric stimuli were presented both on the substrate (to mimic benthic prey) and suspended in the water column (to mimic free-swimming prey). Analysis of around 480 behavioural sequences shows that all three species are highly sensitive towards weak electric dipole fields, and initiate behavioural responses at median field strengths between 5.15 and 79.6 nV cm(-1). The response behaviours used by sawfish and shovelnose rays depended on the location of the dipoles. The elongation of the sawfish's rostrum clearly expanded their electroreceptive search area into the water column and enables them to target free-swimming prey.