Feeding strategy of the megamouth shark Megachasma pelagios (Lamniformes: Megachasmidae)

The feeding biology of the planktivorous megamouth shark Megachasma pelagios was investigated. Morphological examination disclosed that the megamouth has a suite of unique characteristics among sharks, such as large mouth, large bucco-pharyngeal cavity, elongate jaw cartilages, long palatoquadrate levator and preorbital muscles, long ethmopalatine ligament and elastic skin around the pharynx. The combination of these characters suggests that the megamouth shark performs engulfment feeding that is typically seen in the rorqual and humpback whales. Engulfment is a new feeding method for sharks, and the detailed mechanism of the engulfment feeding is discussed.     

Use of nursery areas by the extinct megatooth shark Otodus megalodon (Chondrichthyes: Lamniformes)

Nursery areas are fundamental for the success of many marine species, particularly for large, slow-growing taxa with low fecundity and high age of maturity. Here, we examine the population size-class structure of the extinct gigantic shark Otodus megalodon in a newly described middle Miocene locality from Northeastern Spain, as well as in eight previously known formations (Temblor, Calvert, Pisco, Gatún, Chucunaque, Bahía Inglesa, Yorktown and Bone Valley). In all cases, body lengths of all individuals were inferred from dental parameters and the size-class structure was estimated from kernel probability density functions and Gaussian mixture models. Our analyses support the presence of five potential nurseries ranging from the Langhian (middle Miocene) to the Zanclean (Pliocene), with higher densities of individuals with estimated body lengths within the typical range of neonates and young juveniles. These results reveal, for the first time, that nursery areas were commonly used by O. megalodon over large temporal and spatial scales, reducing early mortality and playing a key role in maintaining viable adult populations. Ultimately, the presumed reliance of O. megalodon on the presence of suitable nursery grounds might have also been determinant in the demise of this iconic top predatory shark.     

Feeding Habits of Four Species of Istiophoridae (Pisces: Perciformes) from Northeastern Brazil

Billfishes from the Brazilian oceanic northeastern region feed on a large food spectrum composed mainly of epipelagic species of fish and cephalopods, with occasional occurrences of mesopelagic organisms and crustaceans. The oceanic pomfret, Brama brama, and the squid, Ornithoteuthis antillarum, were the main prey items in the diet of the four species, occurring in at least 50% of the stomachs containing food. There was no correlation between body length of prey and predator, or between mass of stomach content and body mass in the individuals from 100 to 330cm fork length. The feeding of the four species was characterized by constant ingestion of small quantities of food, and constant presence of food in the stomachs, with a predominance of epipelagic organisms of small sizes with an average of 8.0cm. The reduced stomach size forces the predators to feed constantly on several meals during the day, and consequently provide constant energy for migration.     

Huffmanela selachii n. sp. (Nematoda: Trichosomoididae: Huffmanelinae): A new species infecting the skin of the great hammerhead shark (Sphyrna mokarran) and the blacktip reef shark (Carcharhinus melanopterus) in the Arabian Gulf,off-shore Saudi Arabia

From January 2017 - December 2019, 75 out of 850 (8.8 %) great hammerhead sharks from the Arabian Gulf had skin lesions of black irregular discolorations on the ventral surface of the head. The lesions consisted of pencil-like lineations often advancing forward by about 2 mm in back-and-forth looped scribbles often forming a relatively linear bands of about 5–7 cm wide. Similar lesions were also found in the blacktip reef shark from the same area within the same period, and consisted of straight to irregular black lines, extended indiscriminately across the skin of the sharks. Microscopic examination of the skin revealed the presence of dark-brown eggs exhibiting the spindle or ellipsoidal eggs characteristic of Huffmanela sp. The morphometrics of eggs from both hosts were similar (62.9–89.9 μm long and 29.3–56.1 μm wide). The eggshells were smooth with polar plugs protruding or not, with an abruptly truncated crown-like or shoulder-like collar surrounding the plug. The eggs were only found in the epidermal layer of the skin. Based on the unique morphometrics of the eggs, we report a new species, named: Huffmanela selachii n. sp.. This appears to be the first report of Huffmanela from either the great hammerhead shark or the blacktip reef shark, and the third reported Huffmanela in sharks from the Arabian Gulf. It is also one of few species reported from connecting waters of the greater Indian Ocean. This new finding contributes to our understanding of the diversity and ubiquity of Huffmanela sp. in marine creatures.     

Quantification of red myotomal muscle volume and geometry in the shortfin mako shark (Isurus oxyrinchus) and the salmon shark (Lamna ditropis) using T1-weighted magnetic resonance imaging

T1-weighted magnetic resonance imaging (MRI) in conjunction with image and segmentation analysis (i.e., the process of digitally partitioning tissues based on specified MR image characteristics) was evaluated as a noninvasive alternative for differentiating muscle fiber types and quantifying the amounts of slow, red aerobic muscle in the shortfin mako shark (Isurus oxyrinchus) and the salmon shark (Lamna ditropis). MRI-determinations of red muscle quantity and position made for the mid-body sections of three mako sharks (73.5-110 cm fork length, FL) are in close agreement (within the 95% confidence intervals) with data obtained for the same sections by the conventional dissection method involving serial cross-sectioning and volumetric analyses, and with previously reported findings for this species. The overall distribution of salmon shark red muscle as a function of body fork length was also found to be consistent with previously acquired serial dissection data for this species; however, MR imaging revealed an anterior shift in peak red muscle cross-sectional area corresponding to an increase in body mass. Moreover, MRI facilitated visualization of the intact and anatomically correct relationship of tendon linking the red muscle and the caudal peduncle. This study thus demonstrates that MRI is effective in acquiring high-resolution three-dimensional digital data with high contrast between different fish tissue types. Relative to serial dissection, MRI allows more precise quantification of the position, volume, and other details about the types of muscle within the fish myotome, while conserving specimen structural integrity.     

Observations of coastal aggregations of the broadnose sevengill shark (Notorynchus cepedianus) in Chilean waters

The presence of four sharks was documented in coastal waters of Antofagasta (Chile) using an unmanned aerial video camera. Fishers took advantage of this aggregation to catch and sold three adult broadnose sevengill sharks Notorynchus cepedianus. Species identity was determined by using the cox1 gene. One additional video was later recorded 3000 km south of Antofagasta, and shows a large female interacting with a salmon farming facility. Shallow water records of N. cepedianus were previously undocumented in Chilean waters, yet historically have provided an opportunistic event to fishers in Chile.     

A predatory use of counterillumination by the squaloid shark, Isistius brasiliensis

A number of very unusual morphological and behavioral characteristics attributed to the cookie-cutter shark, Isistius brasiliensis, may be explained by a novel use of counterillumination. Specifically, it is proposed that a band of pigment, located beneath the jaw and bounded by ventrally directed bioluminescence, acts as a lure which mimics the search image of many upward-looking pelagic predators.     

Growth trajectories of prenatal embryos of the deep-sea shark Chlamydoselachus anguineus (Chondrichthyes)

Chlamydoselachus anguineus, Garman 1884, commonly called the frilled shark, is a deep-sea shark species occurring up to depths of 1300 m. It is assumed to represent an ancient morphotype of sharks (e.g., terminal mouth opening, more than five gill slits) and thus is often considered to represent plesiomorphic traits for sharks. Therefore, its early ontogenetic developmental traits are important for understanding the evolution of its particular phenotype. Here, we established six stages for prenatal embryos and used linear measurements and geometric morphometrics to analyse changes in shape and size as well as their timing during different embryonic stages. Our results show a change in head shape and a relocation of the mouth opening at a late stage of development. We also detected a negative allometric growth of the head and especially the eye compared to the rest of the body and a sexual dimorphism in total body length, which differs from the known data for adults. A multivariate analysis of covariance shows a significant interaction of shape related to the logarithm of centroid size and developmental stage. Geometric morphometrics results indicate that the head shape changes as a covariate of body size while not accounting for differences between sexes. The growth pattern of stages 32 and 33 indicates a shift in head shape, thus highlighting the moment in development when the jaws start to elongate anteriorly to finally achieve the adult condition of terminal mouth opening rather than retaining the early embryonic subterminal position as is typical for sharks. Thus, the antero-terminal mouth opening of the frilled shark has to be considered a derived feature.     

Development and microstructure of tooth histotypes in the blue shark,Prionace glauca (Carcharhiniformes: Carcharhinidae) and the great white shark,Carcharodon carcharias (Lamniformes: Lamnidae)

Elasmobranchs exhibit two distinct arrangements of mineralized tissues in the teeth that are known as orthodont and osteodont histotypes. Traditionally, it has been said that orthodont teeth maintain a pulp cavity throughout tooth development whereas osteodont teeth are filled with osteodentine and lack a pulp cavity when fully developed. We used light microscopy, scanning electron microscopy, and high‐resolution micro‐computed tomography to compare the structure and development of elasmobranch teeth representing the two histotypes. As an example of the orthodont histotype, we studied teeth of the blue shark, Prionace glauca (Carcharhiniformes: Carcharhinidae). For the osteodont histotype, we studied teeth of the great white shark, Carcharodon carcharias (Lamniformes: Lamnidae). We document similarities and differences in tooth development and the microstructure of tissues in these two species and review the history of definitions and interpretations of elasmobranch tooth histotypes. We discuss a possible correlation between tooth histotype and tooth replacement and review the history of histotype differentiation in sharks. We find that contrary to a long held misconception, there is no orthodentine in the osteodont teeth of C. carcharias. J. Morphol. 276:797–817, 2015. © 2015 Wiley Periodicals, Inc.     

A scanning electron microscopic study of germ cell maturation in the reproductive tract of the male soupfin shark (Galeorhinus galeus)

Germ cell maturation in the reproductive tract of the soupfin shark (Galeorhinus galeus) was studied using scanning electron microscopy (SEM). The SEM showed changes in Sertoli cytoplasm volume during spermatogenic development. In immature spermatocysts in the germinal zone, spermatogonia were embedded in Sertoli cytoplasm. In spermatogonial spermatocysts, Sertoli cells were adluminally located in the spermatocyst, with spermatogonia enveloped in the basal portions of the cytoplasm. During the round spermatid stage, Sertoli cytoplasm was very scanty. Spermatid elongation was accompanied by a progressive increase in the volume of Sertoli cytoplasm, notably around the spermatid heads. In the mature spermatocyst, bundles of spermatozoa are totally enveloped by Sertoli cytoplasm. Spermatozoa occurred randomly in the epididymis. However, in the ampulla ductus deferentis, spermatozoa reaggregated and were embedded in a mucoid substance to form highly ordered spherical bundles. In the sperm bundle, the spermatozoa heads were arranged such that the helical turns of adjacent spermatozoa were precisely aligned, and all the heads in the bundle formed a distinct apex. This study demonstrates the utility of exploring the relationship between germ cells and Sertoli cells in an evolutionarily ancient vertebrate, such as the shark.     

Observations on the biological traits of the rare shark Oxynotus centrina (Chondrichthyes: Oxynotidae) in the Hellenic Seas

New records of the rare angular rough shark Oxynotus centrina from the Hellenic Seas are presented. Its occurrence is reported for the first time in the Corinthian Gulf. Some aspects of the species' biology are described and compared with previous studies.     

Panbiogeographical analysis of the shark genus Rhizoprionodon (Chondrichthyes,Carcharhiniformes, Carcharhinidae)

The distributional patterns of the seven species of Rhizoprionodon were analysed using the panbiogeographical method of track analysis. The individual tracks of Rhizoprionodon suggest that the genus is mainly an Indian–Atlantic Ocean group. Five generalized tracks were found: (1) Caribbean, defined by R. porosus and R. terraenovae; (2) eastern coast of South America, defined by R. porosus and R. lalandei; (3) Indian Ocean, defined by R. acutus and R. oligolinx; (4) north‐western Australia, defined by R. acutus, R. oligolinx and R. taylori; (5) north‐north‐eastern Australia, defined by R. acutus and R. taylori. Only R. longurio was not included in any generalized track, and its distribution is restricted to the eastern Pacific Ocean. Two biogeographical nodes were found at the intersection of the generalized tracks 1 and 2 (Caribbean Sea) and generalized tracks 4 and 5 (north Australia). The generalized tracks overlap with those found in several unrelated marine taxa. Overall, the generalized tracks are associated with warm currents. The biogeographical nodes found (Caribbean and Australian) are coincident with the global distribution of mangroves.     

New record of a sharpnose sevengill shark Heptranchias perlo (Elasmobranchii,Hexanchidae) from the Balearic Sea,western Mediterranean Sea

In March 2018, an immature female sharpnose sevengill shark Heptranchias perlo was caught by a commercial bottom trawler in the Ibiza Channel. This represents the first substantiated record of the species around the Balearic Islands (GSA05) and the second record from off the Spanish Levantine coasts. This paper includes a review of the records of H. perlo in the Mediterranean as an aid to future conservation assessments.     

Morphological arrangement of the coronary vasculature in a shark (Squalus sucklei) and a teleost (Oncorhynchus mykiss)

The coronary circulation is of great importance in maintaining cardiovascular function and consequently it has been extensively studied in many mammalian species. However, much less attention has been paid to the coronary circulation in other vertebrates. For example, while elasmobranch fishes are of special interest as they are the most ancient lineage of vertebrates to possess a coronary circulation, only qualitative studies exist on their coronary circulation and most concern the architecture of the large arteries. Our study tested the prediction that the coronary circulation of sharks is better developed than previously thought. However, to test this idea, a methodology was needed to quantify vascularity, vessel morphology and oxygen diffusion distances in a heart with predominantly spongy myocardium. Here, we describe this methodology using dogfish and rainbow trout and suggest that the dogfish spongy myocardium appears to rely predominantly on the coronary circulation for its oxygen supply, an arrangement that contrasts with the spongy myocardial tissue of rainbow trout. In support of this suggestion, the density of the microvasculature of the spongy myocardial tissue of dogfish exceeded that of their compact tissue. Although vascularity in the compact myocardium of dogfish was significantly lower than trout, intervascular distances were similar on account of a significantly larger vessel diameter in dogfish, which corresponds to a larger red blood cell size of the dogfish when compared to trout. J. Morphol. 277:896–905, 2016. © 2016 Wiley Periodicals, Inc.     

Characterization of 12 polymorphic microsatellite loci for the whitespotted bamboo shark (Chiloscyllium plagiosum Bennett)

Twelve polymorphic microsatellite loci without linkage disequilibrium were isolated and characterized from a microsatellite DNA-enriched DNA library for the whitespotted bamboo shark (Chiloscyllium plagiosum Bennett, 1830). These loci show polymorphism information content ranging from 0.384 to 0.885, allele number ranging from 4 to 12, effective allele number ranging from 1.686 to 9.438, and observed and expected heterozygosities from 0.229 to 1.000 and from 0.407 to 0.894 respectively. Four loci show strong Hardy-Weinberg deviations. We expect that these markers would be useful for population genetic and breeding studies of the whitespotted bamboo shark.     

Possible range extension in the endangered and understudied winghead shark (Eusphyra blochii)

Here the authors report on a possible range extension in the rare and understudied winghead shark (Eusphyra blochii). A specimen was captured by recreational fishermen in Moreton Bay, Queensland, Australia, c. 800 km south of its current distribution. As winghead sharks show a clumped distribution in Australia associated with river outflow, Moreton Bay, with its large catchment area, may represent a suitable habitat for the species and previous occurrence may have gone undetected. Alternatively, climate change may have caused a recent southward shift in winghead sharks, as observed in other elasmobranch species along Australia's East Coast.     

The Effects of Satiation on Strike Mode and Prey Capture Kinematics in the Largemouth bass, Micropterus salmoides

The feeding mechanism and kinematics of prey capture have been studied in many fishes. However, the effects of satiation on the strike mode and prey capture kinematics have never been directly measured. We analyze 12 kinematic variables to determine the effects of satiation on prey capture in five largemouth bass, Micropterus salmoides, by using high speed videography. We also present the first experimental test for modulatory capabilities in response to satiation, by using the ram-suction index. Significant changes in the kinematic variables of maximum lower jaw depression, maximum gape distance, maximum hyoid depression, time to maximum hyoid depression, and time from maximum hyoid depression to recovery were seen with the effects of satiation. Change in the kinematic variables imply a decrease in jaw opening velocity and the magnitude of suction velocity created during repetitive strikes by M. salmoides with increasing satiation. The bass primarily uses a ram strike mode, with some suction bites occasionally. Ram-suction index analyses suggests that M. salmoides does not modulate strike mode in response to satiation. However, the bass modulate prey capture kinematics without altering strike mode with the effects of satiation. Prey capture success decreases in each bass, as the probability of a successful prey capture event becomes lower, with increasing satiation. These findings demonstrate that satiation can have major effects on prey capture kinematics and future studies of feeding kinematics should account for satiation in their analyses.     

Origin of the white shark Carcharodon (Lamniformes: Lamnidae) based on recalibration of the Upper Neogene Pisco Formation of Peru

Abstract: The taxonomic origin of the white shark, Carcharodon, is a highly debated subject. New fossil evidence presented in this study suggests that the genus is derived from the broad‐toothed ‘mako’, Carcharodon (Cosmopolitodus) hastalis, and includes the new species C. hubbelli sp. nov. – a taxon that demonstrates a transition between C. hastalis and Carcharodon carcharias. Specimens from the Pisco Formation clearly demonstrate an evolutionary mosaic of characters of both recent C. carcharias and fossil C. hastalis. Characters diagnostic to C. carcharias include the presence tooth serrations and a symmetrical first upper anterior tooth that is the largest in the tooth row, while those indicative of C. hastalis include a mesially slanted third anterior (intermediate) tooth. We also provide a recalibration of critical fossil horizons within the Pisco Formation, Peru using zircon U‐Pb dating and strontium‐ratio isotopic analysis. The recalibration of the absolute dates suggests that Carcharodon hubbelli sp. nov. is Late Miocene (6–8 Ma) in age. This research revises and elucidates lamnid shark evolution based on the calibration of the Neogene Pisco Formation.