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Our recent success in the long-term maintenance of lantern shark embryos in artificial uterine systems has provided a novel option for the medical treatment of premature embryos for captive viviparous elasmobranchs. The remaining issue with this system is that the embryos cannot survive the abrupt change in the chemical environment from artificial uterine fluid (AUF) to seawater during delivery. To overcome this issue, the present study developed a new protocol for seawater adaptation, which is characterized by a long-term and stepwise shift from AUF to seawater prior to delivery. This protocol was employed successfully, and the specimen survived for more than seven months after delivery, the longest captive record of the species. During the experiment, we unexpectedly detected bioluminescence of the embryonic lantern shark in the artificial uterus. This observation indicates that lantern sharks can produce luciferin, a substance for bioluminescence. This contradicts the recent hypothesis that lantern sharks lack the ability to produce luciferin and use luciferin obtained from food sources., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Tomita et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Examination of the uterus of a dead female white shark (Carcharodon carcharias), which contained the earliest known white shark embryos, revealed that the uterine wall produces lipid-rich secretion (histotroph or "uterine milk") for embryonic nutrition. Uterine tissue was processed for light and electron microscopy, and immunohistochemical techniques to identify its secretory mechanism. Our results indicate that the white shark uterus secretes lipids via holocrine secretion. This type of secretion is characterized by the release of large lipid droplets accumulated in the epithelial cells into the uterine lumen through cell disintegration. The secretory epithelium of the uterus is stratified, and new surface epithelial cells are continuously supplied from deeper epithelial layers to replace the dead secretory cells at the surface. This vertical replacement possibly facilitates the active renewal of the surface epithelium, which is necessary for maintaining holocrine secretory mechanisms. These secretory mechanisms are different from those of myliobatiform stingrays, another elasmobranch taxon that exhibits lipid histotrophy. This may reflect the different origins of lipid histotrophy between these taxa., (© 2022 American Association for Anatomy.)

The present study provides a noninvasive method to estimate the body volume of sharks (Elasmobranchii, Selachii) using a computational geometric model. This method allows the volume of sharks to be estimated from lateral and ventral photographs assuming an elliptical body cross-sectional geometry. A comparison of the estimated and actual body volumes of several shark species showed that the estimation error was < 0.5%. The accuracy of the model decreased if photographs that were inclined to the orthogonal plane were used, although this error was on average < 2.3% if the inclination angle was 10° or less. Applying this model to captive whale sharks (Rhincodon typus) that were 8.0 and 8.8 m in total length revealed that their body volumes were 3.5 and 4.5 m 3 , respectively. These estimates allowed for the quantitative evaluation of our hypothesis, that the whale shark uses suctioned air for buoyancy control during vertical feeding-a behavior unique to this species among elasmobranchs. The volume estimates of the captive whale sharks, together with the density estimates from their liver proportions, revealed that the air occupying a part of oro-pharyngeal and branchial cavities can help the whale sharks to keep their body floating. This hypothesis may explain how the whale shark sometimes stays at the water surface without fin motion during vertical feeding, even though their body density is greater than that of seawater., (Copyright © 2021. Published by Elsevier GmbH.)

This report elaborates on adaptations of the eyes of the whale shark Rhincodon typus (Elasmobranchii, Rhincodontidae), including the discovery that they are covered with dermal denticles, which is a novel mechanism of eye protection in vertebrates. The eye denticle differs in morphology from that of the dermal denticles distributed over the rest of the body, consistent with a different function (abrasion resistance). We also demonstrate that the whale shark has a strong ability to retract the eyeball into the eye socket. The retraction distance was calculated to be approximately half the diameter of the eye, which is comparable to those of other vertebrates that are known to have highly retractable eyes. These highly protective features of the whale shark eye seem to emphasize the importance of vision for environmental perception, which contradicts the general, though poorly established, notion of low reliance on vision in this species., Competing Interests: The authors have declared that no competing interests exist.

For benthic fishes, breathing motion (e.g., oral, pharyngeal, and branchial movements) can result in detection by both prey and predators. Here we investigate the respiratory behavior of the angelshark Squatina japonica (Pisces: Squatiniformes: Squatinidae) to reveal how benthic elasmobranchs minimize this risk of detection. Sonographic analyses showed that the angelshark does not utilize water-pumping in the oropharyngeal cavity during respiration. This behavior is in contrast with most benthic fishes, which use the rhythmical expansion/contraction of the oropharyngeal cavity as the main pump to generate the respiratory water current. In the angelshark, breathing motion is restricted to the gill flaps located on the ventral side of the body. We suspect that the gill flaps function as an active pump to eject water through the gill slits. This respiratory mode allows conspicuous breathing motion to be concealed under the body, thereby increasing crypsis capacity., (Copyright © 2018 Elsevier GmbH. All rights reserved.)

The lunate-shaped caudal fin in lamnid sharks is a morphological specialization for their thunniform mode of locomotion, but its developmental process during gestation has been poorly investigated. Observations of 21 embryonic specimens of the white shark (Carcharodon carcharias) revealed that their caudal fin morphology drastically changes from strongly heterocercal to lunate-shaped through ontogeny. This morphological change involves (1) rapid elongation of the ventral lobe, (2) increased upward curvature of the vertebra within the caudal fin, and (3) formation of keels at both lateral sides of the caudal fin base. These morphological changes are probably shared among the members of the family Lamnidae and are in contrast with the developmental process of the heterocercal tail in the lamniform Carcharias taurus, in which the caudal fin morphology is almost unchanged through the late gestation period. Anat Rec, 301:1068-1073, 2018. © 2018 Wiley Periodicals, Inc., (© 2018 Wiley Periodicals, Inc.)

Unlike most viviparous vertebrates, lamniform sharks develop functional teeth during early gestation. This feature is considered to be related to their unique reproductive mode where the embryo grows to a large size via feeding on nutritive eggs in utero. However, the developmental process of embryonic teeth is largely uninvestigated. We conducted X-ray microcomputed tomography to observe the dentitions of early-, mid-, and full-term embryos of the white shark Carcharodon carcharias (Lamniformes, Lamnidae). These data reveal the ontogenetic change of embryonic dentition of the species for the first time. Dentition of the early-term embryos (∼45 cm precaudal length, PCL) is distinguished from adult dentition by 1) the presence of microscopic teeth in the distalmost region of the paratoquadrate, 2) a fang-like crown morphology, and 3) a lack of basal concavity of the tooth root. The "intermediate tooth" of early-term embryos is almost the same size as the adjacent teeth, suggesting that lamnoid-type heterodonty (lamnoid tooth pattern) has not yet been established. We also discovered that mid-term embryos (∼80 cm PCL) lack functional dentition. Previous studies have shown that the maternal supply of nutritive eggs in lamnoid sharks ceases during mid- to late-gestation. Thus, discontinuation of functional tooth development is likely associated with the completion of the oophagous (egg-eating) phase. Replacement teeth in mid-term embryos include both embryonic and adult-type teeth, suggesting that the embryo to adult transition in dental morphology occurs during this period. J. Morphol. 278:215-227, 2017. © 2016 Wiley Periodicals,Inc., (© 2016 Wiley Periodicals, Inc.)

We provide an alternative interpretation for the "whitish film" that covered the young white shark observed with an aerial drone in a paper published recently (Gauna and Sternes, Environ Biol Fish 107:249–254, 2024). The paper claimed that this whitish film was a leftover intrauterine substance (e.g., uterine milk, which is a maternal secretion to nourish embryos), thus suggesting that the individual was a newborn. However, we are skeptical of this interpretation for the following reasons: (1) our previous studies showed that the secretion of intrauterine substances in white sharks ceases in mid-gestation and, (2) even if the secretion continues until birth, the substance is unlikely to be retained on the skin for a long time after birth. Based on data from Lamna ditropis, a close relative of the white shark, we hypothesize that the whitish film is an embryonic epithelium that covers the surface of skin denticles. This reinterpretation does not alter but reinforces the major conclusion of (Gauna and Sternes, Environ Biol Fish 107:249–254, 2024) that the individual sighted is a newborn. [ABSTRACT FROM AUTHOR]

The evolution of thunniform swimming in sharks involves many morphological modifications that enhance the energetic efficiency of aquatic locomotion. The present study describes a clasper pocket, a male-specific groove to "stow" claspers close to the body, as a novel example of such modifications. According to observations in two lamnoid species, Lamna ditropis and Isurus oxyrinchus, the groove becomes drastically deeper along with clasper elongation through ontogeny. The clasper pocket was found in sharks with a thunniform body plan, so far only observed in Lamnidae and Cetorhinidae. The number of acquisitions of clasper pockets during lamniform phylogeny assessment ranges from one to two, depending on the interpretation regarding the phylogenetic relationships between Lamnidae and Cetorhinidae in the phylogenetic tree is used in the analysis. [ABSTRACT FROM AUTHOR]