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Walking is a locomotion mode in which animals move over the ground using their appendages. Walking is observed in both terrestrial and aquatic animals, but the morphology and diversity of appendages in the latter group have been less extensively studied. The present paper reports on the "adhesive areas," which may represent morphological and physiological adaptations for stable aquatic walking, in the paintpot cuttlefish, Ascarosepion tullbergi. This animal employs arm IV as a forelimb and an ambulatory flap as a hindlimb for walking, resulting in a gait‐like manner of movement. The structure of the adhesive area is exclusively located on the ventral skin surface of arm IV and the ambulatory flap, which are in contact with the ground during walking. The "adhesive areas" are characterized by a dense population of adhesive mucus‐secreting cells and the development of numerous wrinkles on the surface. These features may enhance the gripping and sticking capacity of the ground‐contact area, thus improving walking stability. The use of adhesive areas for walking is a unique feature of A. tullbergi, as other cuttlefish with adhesive areas primarily use them for attaching to substrata in strong currents. Our results contribute to the understanding of the locomotion strategy of cuttlefish. [ABSTRACT FROM AUTHOR]

Underwater ultrasound, a new tool for observing the internal body parts of aquatic animals by scuba divers, allowed us long‐term and frequent observations of the embryos of captive aquatic vertebrates. New ultrasound data of captive tawny nurse sharks (Nebrius ferrugineus) revealed that their embryos frequently migrate between the right and left uteri during gestation. This report is the first reliable evidence of active embryonic locomotion in live‐bearing vertebrates and is contradictory to the concept of "sedentary embryo" which has mainly arisen from studies of mammals. The tawny nurse shark is unique among orectolobiform sharks, in which the embryo develops by feeding on sibling eggs in utero. Thus, we hypothesized that swimming aids in an efficient search and capture of these eggs in the uterine environment. [ABSTRACT FROM AUTHOR]

The megatooth shark, † Otodus megalodon , which likely reached at least 15 m in total length, is an iconic extinct shark represented primarily by its gigantic teeth in the Neogene fossil record. As one of the largest marine carnivores to ever exist, understanding the biology, evolution, and extinction of † O. megalodon is important because it had a significant impact on the ecology and evolution of marine ecosystems that shaped the present-day oceans. Some attempts inferring the body form of † O. megalodon have been carried out, but they are all speculative due to the lack of any complete skeleton. Here we highlight the fact that the previous total body length estimated from vertebral diameters of the extant white shark (Carcharodon carcharias) for an † O. megalodon individual represented by an incomplete vertebral column is much shorter than the sum of anteroposterior lengths of those fossil vertebrae. This factual evidence indicates that † O. megalodon had an elongated body relative to the body of the modern white shark. Although its exact body form remains unknown, this proposition represents the most parsimonious empirical evidence, which is a significant step towards deciphering the body form of † O. megalodon .

In this study, we examined the structure of the heart of the whale shark, Rhincodon typus, using a plastination technique and three-dimensional X-ray computer tomography (3DCT). Inspection of the atrium revealed a symmetric distribution of the pectinate muscles attached to the commissures of the sino-atrial valve, suggesting some functional advantages. The majority of the ventricular wall comprised spongiosa, and compacta accounted for only ~3% of the entire thickness. There were three major fiber orientations in the spongiosa: the fibers on the endocardial side formed trabeculae that were aligned with the blood flow tract, whereas those on the epicardial side formed a circular pattern around the flow tract. Transmural myofibers connected the inner and outer layers in the spongiosa, which may serve as an intraventricular conduction pathway. Plastination and 3DCT is a powerful combination that allowed for multifaceted visualization of the internal structure of rare heart specimens in a nondestructive manner. Anat Rec, 301:1801-1808, 2018. © 2018 Wiley Periodicals, Inc., (© 2018 Wiley Periodicals, Inc.)

The spiracle of elasmobranchs (sharks, skates, and rays) is a gill-slit-derived tube located behind the eye. Its inner structure was well studied in the late nineteenth to early twentieth century, but its entire morphology has rarely been characterized and is poorly understood. The present study shows the three-dimensional morphology of the spiracular tube for the first time, using resin injection and CT scanning, in the Japanese bullhead shark. The spiracular tube is characterized by the presence of two caeca (dorsal and ventral spiracular caeca) on the medial wall of the spiracular tube and the presence of a pseudobranch on the anterior wall. This study provides a basis for further studies on the morphological diversity, function, and evolution of spiracles in elasmobranch fishes. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc., (© 2018 Wiley Periodicals, Inc.)