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A sleeper shark (Somniosus sp.) was captured over the insular slope at Glover’s Reef, a coral atoll in Belize. The estimated total length of the individual was ca. 3.0–3.5 m and multiple morphological features diagnostic of a somniosid shark (short snout, presence of an ectoparasitic copepod on the eye, low and rounded dorsal and pectoral fins, and small eyes with posteriorly situated large spiracles) were observed. While the exact species could not be confirmed, it is most likely a Greenland shark (Somniosus microcephalus) or a hybrid between the Greenland shark and the Pacific sleeper shark (Somniosus pacificus). This is the first record of a sleeper shark in the western Caribbean region and further supports the hypothesis that these sharks, best known from polar and subpolar latitudes, occur at depth in tropical regions.

We conducted an experiment of planting a dead cow and a metal-framed cage with cameras on the 1629 m deep sea floor off the southeast coast of Hainan Island in the northwestern South China Sea, using ROV diving and setting up a video camera on the cage to observe animals who came to eat the bait. The deep-sea cameras captured footage of eight Pacific sleeper sharks (Somniosus pacificus) swimming and feeding around the dead cow. To our knowledge, this is the first time the occurrence of such a shark species has been reported in the South China Sea. Eight individuals were differentiated based on the characteristic differences displayed in the images, with lengths of 1.9 to 5.1 m estimated. The video camera also recorded the predators’ behavior of tearing at the dead cow on the seabed. It was discovered that Pacific sleeper sharks are not strictly solitary and exhibit queue-feeding behavior. This study is significant as it documents a record of a data-scarce shark species, for which little information is available in the literature. It also documents an expansion of the species’ known habitat from the north Pacific Ocean into the South China Sea. Such sharks diving into the deep sea to predate on dead animals also suggests that occurrences of large chunks of dead organic bodies falling onto the deep sea might have been more frequent than we previously thought in the South China Sea. The findings have implications for understanding the geographic connectivity of large swimming animals between the South China Sea and the Pacific Ocean and provide scientific evidence for formulating conservation and management strategies for sharks and other large animals in the oceans.

Remote polar and deepwater fish faunas are under pressure from ongoing climate change and increasing fishing effort. However, these fish communities are difficult to monitor for logistic and financial reasons. Currently, monitoring of marine fishes largely relies on invasive techniques such as bottom trawling, and on official reporting of global catches, which can be unreliable. Thus, there is need for alternative and non-invasive techniques for qualitative and quantitative oceanic fish surveys. Here we report environmental DNA (eDNA) metabarcoding of seawater samples from continental slope depths in Southwest Greenland. We collected seawater samples at depths of 188-918 m and compared seawater eDNA to catch data from trawling. We used Illumina sequencing of PCR products to demonstrate that eDNA reads show equivalence to fishing catch data obtained from trawling. Twenty-six families were found with both trawling and eDNA, while three families were found only with eDNA and two families were found only with trawling. Key commercial fish species for Greenland were the most abundant species in both eDNA reads and biomass catch, and interpolation of eDNA abundances between sampling sites showed good correspondence with catch sizes. Environmental DNA sequence reads from the fish assemblages correlated with biomass and abundance data obtained from trawling. Interestingly, the Greenland shark (Somniosus microcephalus) showed high abundance of eDNA reads despite only a single specimen being caught, demonstrating the relevance of the eDNA approach for large species that can probably avoid bottom trawls in most cases. Quantitative detection of marine fish using eDNA remains to be tested further to ascertain whether this technique is able to yield credible results for routine application in fisheries. Nevertheless, our study demonstrates that eDNA reads can be used as a qualitative and quantitative proxy for marine fish assemblages in deepwater oceanic habitats. This relates directly to applied fisheries as well as to monitoring effects of ongoing climate change on marine biodiversity-especially in polar ecosystems.

The Greenland shark (Somniosus microcephalus), an iconic species of the Arctic Seas, grows slowly and reaches >500 centimeters (cm) in total length, suggesting a life span well beyond those of other vertebrates. Radiocarbon dating of eye lens nuclei from 28 female Greenland sharks (81 to 502 cm in total length) revealed a life span of at least 272 years. Only the smallest sharks (220 cm or less) showed signs of the radiocarbon bomb pulse, a time marker of the early 1960s. The age ranges of prebomb sharks (reported as midpoint and extent of the 95.4% probability range) revealed the age at sexual maturity to be at least 156 ± 22 years, and the largest animal (502 cm) to be 392 ± 120 years old. Our results show that the Greenland shark is the longest-lived vertebrate known, and they raise concerns about species conservation.

Few published studies have measured the oxygen consumption rates of elasmobranchs larger than 1.5 m, with only one measured at temperatures below 10 °C. This study provides initial measurements of the metabolic rate of three juvenile Pacific sleeper sharks, SP1904, SP1908, and SP2005 (199 cm, 162 cm, 144 cm), which were caught and temporarily housed at the Alaska SeaLife Center in Seward, Alaska. While at the Center, multiple respirometry trials were performed utilizing an intermittent-closed system respirometer to measure oxygen consumption. The average routine metabolic rates for each individual were calculated at 17.8 ± 1.3 mgO2kg−1 h−1, 20.5 ± 1.6 mgO2kg−1 h−1, and 4.0 ± NA mgO2kg−1 h−1, respectively, while the resting metabolic rate of SP1908 was calculated at 14.3 ± 2.4 mgO2kg−1 h−1. Oxygen consumption rates were measured between 6.0 and 8.8 °C. Both the active and resting metabolic rates for the Pacific sleeper shark were like those of the previously measured and closely related Greenland shark (Somniosus microcephalus) and the temperate zebra shark (Stegostoma fasciatum) when scaled to the temperature in this study. Therefore, Pacific sleeper shark metabolic rate is not unusually low compared to other shark species. These initial measurements of the metabolic rate of Pacific sleeper sharks enhance deep-sea and polar physiology knowledge by filling gaps in respirometry research for large, Arctic elasmobranchs.

The Pacific sleeper shark Somniosus pacificus is one of the largest predators in deep Suruga Bay, Japan. A single individual of the sleeper shark (female, ~300 cm in total length) was observed with two baited camera systems deployed simultaneously on the deep seafloor in the bay. The first arrival was recorded 43 min after the deployment of camera #1 on 21 July 2016 at a depth of 609 m. The shark had several remarkable features, including the snout tangled in a broken fishing line, two torn anteriormost left-gill septums, and a parasitic copepod attached to each eye. The same individual appeared at camera #2, which was deployed at a depth of 603 m, ~37 min after it disappeared from camera #1 view. Finally, the same shark returned to camera #1 ~31 min after leaving camera #2. The distance between the two cameras was 436 m, and the average groundspeed and waterspeed of the shark were 0.21 and 0.25 m s−1, respectively, which were comparable with those of the Greenland shark Somniosus microcephalus (0.22–0.34 m s−1) exhibiting the slowest comparative swimming speed among fish species adjusted for size. The ambient water temperature of the Pacific sleeper shark was 5.3 °C, which is considerably higher than that of the Greenland shark (~2 °C). Such a low swimming speed might be explained by the ‘visual interactions hypothesis’, but it is not a consequence of the negative effects of cold water on their locomotor organs.

Our knowledge regarding the role of the microbiome in fish health has been steadily increasing in the last decade, especially for species of commercial interest. Conversely, relatively few studies focus on the microbiomes of wild fish, especially apex predators like sharks, due to lower economic interest and greater difficulty in obtaining samples. Studies investigating microbiome differences between diverse anatomical locations of sharks are limited, and the majority of the available studies are focused on the microbial diversity present on shark teeth, with the aim of preventing infections due to bites of these animals or evaluating the presence of certain pathogens in healthy or diseased specimens. Here, we investigated the skin, mouth, gills, and cloaca microbiomes of five individuals of two phylogenetically distant species of sharks (Prionace glauca and Somniosus rostratus) to obtain a better understanding of the diversity regarding the microbiomes of these animals, how they change throughout different body parts, and how much they are influenced and determined by the ecology and evolutionary relationship between host and microbiome. To confirm the taxonomy of the sharks under study, we barcoded the specimens by sequencing the mtDNA COI from a biopsy of their skin. Microbial diversity based on the 16S rRNA gene reveals that partially overlapping microbiomes inhabit different body parts of each shark species, while the communities are distinct between the two species. Our results suggest that sharks’ microbiome species-specific differences are controlled by the ecology of the shark species. This is the first study comparatively analyzing the microbiome diversity of different anatomical locations in two shark species of the Mediterranean Sea.

This study investigates the distribution and morphometrics of the little sleeper shark, Somniosus rostratus (Risso, 1827), in the Mediterranean Sea. Ten sharks caught as bycatch between 2009 and 2019 in the northern sectors of the Strait of Sicily, the south Tyrrhenian Sea, the northwestern Ionian Sea, and the south Adriatic Sea using drifting longlines, as well as five retrieved from the Tripoli (Libya) marketplace, were morphologically confirmed to represent S. rostratus. The sharks exhibited typical characteristics for this species and were all caught from deep waters, indicating a potential mesopelagic habit. The study also utilized literature reviews and global databases for a comprehensive mapping of S. rostratus distribution in the Mediterranean Sea, which revealed sporadic occurrences in the eastern Mediterranean and an absence in the north Adriatic Sea. Morphometric data provided insights into the reproductive characteristics of S. rostratus . The study highlights the ecological significance of the Strait of Sicily (Central Mediterranean Sea) for the species, indicating it as a likely spawning area, and underscores the impact of the interactions between sharks and pelagic drifting swordfish fisheries in the Mediterranean, which result in increased mortality rates for threatened shark and ray species. Prioritizing conservation measures for endangered elasmobranch populations is crucial for maintaining marine ecosystem balance and ensuring fishery resource sustainability.

Cystic spermatogenesis in the subadult, maturing and adult Greenland shark ( Somniosus microcephalus ) displays multiple novel features, characterized early on by an unorganized internal cellular environment of the spermatocysts (anatomically discrete follicle-like units containing a single germ cell stage and its complement of co-developing Sertoli cells). These typically show polar asymmetries due to asymmetrically distributed germ and Sertoli cells. These arise from several novel cellular rearrangements at the immature pole, including fusion of a cluster of somatic cells with newly formed cysts containing only one to three spermatogonia and that already display an excess of Sertoli cells. The subadult’s germinative zone revealed an additional novelty, namely numerous previously formed somatic cell-lined rings into which spermatogonia were incorporated. A striking finding was the conspicuous rarity of the routinely discernible Sertoli mitotic figures in the hallmark cyst stage of diametric elasmobranch spermatogenesis that is known for the peak display of the latter. Scrutiny of sequentially unfolding phenomena in the linearly arranged spermatogonial generations revealed that the cellular developments at the most common type of cyst–duct transition area (comprising slender to spindle-like basophilic cells with pointed ends) were concurrent with the discreet appearance of a second dark Sertoli nucleus, a development that persisted in spermiated cysts. Spermatogenically active mature males displayed vigorous meiotic divisions. However, a scattering of their spermatid cysts also displayed shark-atypical asynchronous passage through spermiogenesis, phenomena which were exacerbated as arrested spermiogenesis in an archival collection of tissues from 13 maturing specimens. Subadult specimens revealed meiotic arrest, and foci of infiltration of leukocytes that originate from a mass of eosinophilic, granule-laden immune cells dorsally under the testis capsule. This tissue was identical to the testis-affixed bone marrow equivalent in other shark species. This tissue is likely developmentally regulated in the Greenland shark as it is absent in adults.

Metabolic rate is intricately linked to the ecology of organisms and can provide a framework to study the behaviour, life history, population dynamics, and trophic impact of a species. Acquiring measures of metabolic rate, however, has proven difficult for large water-breathing animals such as sharks, greatly limiting our understanding of the energetic lives of these highly threatened and ecologically important fish. Here, we provide the first estimates of resting and active routine metabolic rate for the longest lived vertebrate, the Greenland shark ( Somniosus microcephalus ). Estimates were acquired through field respirometry conducted on relatively large-bodied sharks (33–126 kg), including the largest individual shark studied via respirometry. We show that despite recording very low whole-animal resting metabolic rates for this species, estimates are within the confidence intervals predicted by derived interspecies allometric and temperature scaling relationships, suggesting this species may not be unique among sharks in this respect. Additionally, our results do not support the theory of metabolic cold adaptation which assumes that polar species maintain elevated metabolic rates to cope with the challenges of life at extreme cold temperatures.