Squalid Research Articles

Asymmetric fluid flow in helical pipes inspired by shark intestines

Unlike human intestines, which are long, hollow tubes, the intestines of sharks and rays contain interior helical structures surrounding a cylindrical hole. One function of these structures may be to create asymmetric flow, favoring passage of fluid down the digestive tract, from anterior to posterior. Here, we design and 3D print biomimetic models of shark intestines, in both rigid and deformable materials. We use the rigid models to test which physical parameters of the interior helices (the pitch, the hole radius, the tilt angle, and the number of turns) yield the largest flow asymmetries. These asymmetries exceed those of traditional Tesla valves, structures specifically designed to create flow asymmetry without any moving parts. When we print the biomimetic models in elastomeric materials so that flow can couple to the structure's shape, flow asymmetry is significantly amplified; it is sevenfold larger in deformable structures than in rigid structures. Last, we 3D-print deformable versions of the intestine of a dogfish shark, based on a tomogram of a biological sample. This biomimic produces flow asymmetry comparable to traditional Tesla valves. The ability to influence the direction of a flow through a structure has applications in biological tissues and artificial devices across many scales, from large industrial pipelines to small microfluidic devices.

Thermal sensitivity of metabolic performance in Squalus acanthias: efficacy of aerobic scope as a predictor of viable thermal habitat

Ocean warming due to climate change can affect the metabolism, performance, and survival of ectothermic marine species. On the US Northeast continental shelf (US NES), waters are warming faster than the global average, leading to elevated mean temperatures and an increased risk of marine heatwave exposure in the region. Thus, it is critical to understand the effects of warming on the region’s living marine resources. Here, we quantified the acute temperature sensitivity of metabolic traits to evaluate their role as possible drivers of acute thermal tolerance and viable habitat in the spiny dogfish shark Squalus acanthias on the US NES. From 10-23°C, the standard metabolic rate increased more rapidly than the maximum metabolic rate, resulting in a reduction in factorial aerobic scope at warmer temperatures. However, the oxygen supply capacity increased with temperature in proportion to maximum metabolic rate, and neither metric declined at the warmest temperatures, suggesting oxygen supply capacity does not limit performance within the tested range. Although behavioral observations revealed overt thermal stress via loss of equilibrium at ≥20°C and estimated lethal temperature at ∼24°C, sharks retained the ability to regulate their resting metabolic rate, achieve maximum activity, and peak absolute aerobic scope at warm temperatures. Results suggest that factors other than oxygen supply or aerobic scope are constraining thermal tolerance in S. acanthias and support the notion that aerobic scope cannot be universally applied to determine optimal or viable metabolic habitat.

Learning Dogfish Shark Anatomy Using 3D-Printed Models: A Feasibility Study

3D-printed models (3DPMs) are being increasingly utilized as learning aids in medical and comparative anatomy education. Research suggests that 3DPMs can significantly improve students’ accuracy in recognizing important anatomical landmarks and provide a low-cost alternative to expensive or fragile specimens. The use of 3DPMs can also alleviate conservation concerns for certain endangered species. Additionally, 3DPMs provide a unique interactive experience in viewing structures that may otherwise be difficult to observe or handle directly by students. A novel 3DPM has been developed to help biology students learn the anatomy of the Squalus acanthias (S. acanthias), or dogfish shark, chondrocranium and brain. This feasibility study evaluated the perceived utility of these new 3DPMs in an undergraduate-level comparative chordate anatomy lab (BIOL 351) at Iowa State University in Spring 2023. Students responded to a questionnaire comprising Likert and open-ended long-form questions that uncovered their perceptions of and experience interacting with the 3DPMs. Two separate surveys were administered, one for the chondrocranium (29 responses) and one for the brain (16 responses). Students indicated a strong preference for using the 3DPMs as compared to the dissected and preserved specimens, citing the 3DPMs’ size, durability, and the ability to handle and rotate them as beneficial for understanding relevant anatomy. Further investigation is required to understand how the 3DPM improves students’ learning outcomes; however, this study confirms the model’s utility and biology students’ desire to have access to additional 3DPMs in the comparative chordate anatomy lab.

Mass Spectrometry Analysis of Shark Skin Proteins.

The mucus layer covering the skin of fish has several roles, including protection against pathogens and mechanical damage in which proteins play a key role. While proteins in the skin mucus layer of various common bony fish species have been explored, the proteins of shark skin mucus remain unexplored. In this pilot study, we examine the protein composition of the skin mucus in spiny dogfish sharks and chain catsharks through mass spectrometry (NanoLC-MS/MS). Overall, we identified 206 and 72 proteins in spiny dogfish (Squalus acanthias) and chain catsharks (Scyliorhinus retifer), respectively. Categorization showed that the proteins belonged to diverse biological processes and that most proteins were cellular albeit a significant minority were secreted, indicative of mucosal immune roles. The secreted proteins are reviewed in detail with emphasis on their immune potentials. Moreover, STRING protein-protein association network analysis showed that proteins of closely related shark species were more similar as compared to a more distantly related shark and a bony fish, although there were also significant overlaps. This study contributes to the growing field of molecular shark studies and provides a foundation for further research into the functional roles and potential human biomedical implications of shark skin mucus proteins.

Identification of Novel Glycans in the Mucus Layer of Shark and Skate Skin.

The mucus layer covering the skin of fish has several roles, including protection against pathogens and mechanical damage. While the mucus layers of various bony fish species have been investigated, the composition and glycan profiles of shark skin mucus remain relatively unexplored. In this pilot study, we aimed to explore the structure and composition of shark skin mucus through histological analysis and glycan profiling. Histological examination of skin samples from Atlantic spiny dogfish (Squalus acanthias) sharks and chain catsharks (Scyliorhinus retifer) revealed distinct mucin-producing cells and a mucus layer, indicating the presence of a functional mucus layer similar to bony fish mucus albeit thinner. Glycan profiling using liquid chromatography-electrospray ionization tandem mass spectrometry unveiled a diverse repertoire of mostly O-glycans in the mucus of the two sharks as well as little skate (Leucoraja erinacea). Elasmobranch glycans differ significantly from bony fish, especially in being more sulfated, and some bear resemblance to human glycans, such as gastric mucin O-glycans and H blood group-type glycans. This study contributes to the concept of shark skin having unique properties and provides a foundation for further research into the functional roles and potential biomedical implications of shark skin mucus glycans.

Market demand of conventional and non-conventional fish species and price comparison between BFDC landing center and two popular fish markets, the Baharchora and Borobazar, in Cox’s Bazar, Bangladesh

The study of conventional (common commercial fish species which is consumed by people in Bangladesh like Seabass,Ribbon fish, Hilsha, Bombay ducketc.) and non-conventional (fish species which is consumed by people in Bangladesh like Milk shark, Stingrayetc.) fish species at BFDC landing center was conducted to know the current condition of fish, their market demand, prices and price difference between BFDC landing center and two popular fish market Baharchora and Borobazar. There were about 32 varietiesof conventional fish species and most common were Seabass,Ribbon fish, Hilsha,Bombay duck, Mackerel, Silver pomfret, Red snapper, etc. The average price for most demandable fish were Hilsha (TK. 600-1000 kg), Silver pomfret(TK. 550-900 kg), Seabass (TK.500- 650 kg),Bombay duck(TK. 100 kg), Ribbon fish (TK. 300 kg) which varied with size and season. There were about 20 species of non- conventional fish includingMilk shark, Stingray,Long Tung Sole,Mud spiny lobster, Mud crab etc. observed. Milk sharkand Dog shark was common at the landing center. Their average size and weight were 1-2 feet and 1-3 kg which were sold for TK. 120-140 kg. The regular price of Stingray was TK.180-220 kg at the landing center with average weight of 2-8 kg. Normally marketing channel of both fish was maintained by middle men. A large number of fish from the landing center was sold to them by auction. The landing center is the common place from where local fish seller collects fish and sell by for around 15-25% of the profit. Usually, unsold raw fishes were preserved in the freezer and rests were sold to dry fish processors mainly in Nazirartek. The study shows a variation of conventional and non-conventional fish species, their price, and market demand in Cox’s Bazar region.

A new record of Squalus montalbani (Chondrichthyes: Squaliformes: Squalidae) from the Nansha (Spratly) Islands, South China Sea

The Indonesian greeneye spurdog (or a dogfish shark), Squalus montalbani Whitley, 1931, is widely distributed in the warm temperate to tropical waters of Indonesia, Philippines, the island of Taiwan, and Australia. Previous studies suggested that the distribution of dogfish shark species in the South China Sea is composed of two species, Squalus mitsukurii Jordan et Snyder, 1903 and Squalus brevirostris Tanaka, 1917. In March 2020 a dogfish shark specimen was collected from the Nansha (Spratly) Islands, South China Sea. We identified it as S. montalbani based on morphology and mitochondrial DNA barcoding. Our results confirmed the presence of S. montalbani in the South China Sea, leading us to conclude that it represents a new species record of the genus Squalus in the region. Furthermore, our findings demonstrate that the combined approach is highly effective in identifying Squalus species that share similar morphological characteristics.

AFM reveals the interaction and nanoscale effects imposed by squalamine on Staphylococcus epidermidis

The Gram-positive bacterium Staphylococcus epidermidis is responsible for important nosocomial infections. With the continuous emergence of antibiotic-resistant strains, the search for new treatments has been amplified in the last decades. A potential candidate against multidrug-resistant bacteria is squalamine, a natural aminosterol discovered in dogfish sharks. Despite its broad-spectrum efficiency, little is known about squalamine mode of action. Here, we used atomic force microscopy (AFM) imaging to decipher the effect of squalamine on S. epidermidis morphology, revealing the peptidoglycan structure at the bacterial surface after the drug action. Single-molecule force spectroscopy with squalamine-decorated tips shows that squalamine binds to the cell surface via the spermidine motif, most likely through electrostatic interactions between the amine groups of the molecule and the negatively-charged bacterial cell wall. We demonstrated that - although spermidine is sufficient for the initial attachment of squalamine to S. epidermidis – the integrity of the molecule needs to be conserved for its antimicrobial action. A deeper analysis of the AFM force-distance signatures suggests the implication of the accumulation-associated protein (Aap), one of the main adhesins of S. epidermidis, in the initial binding of squalamine to the bacterial cell wall. This work highlights that AFM -combined with microbiological assays at the bacterial suspension scale- is a valuable approach to better understand the molecular mechanisms behind the efficiency of squalamine antibacterial activity.

Untangling the systematic dilemma behind the roughskin spurdog Cirrhigaleus asper (Merrett, 1973) (Chondrichthyes: Squaliformes), with phylogeny of Squalidae and a key to Cirrhigaleus species.

Cirrhigaleus comprises a small genus of rare barbel-bearing dogfish sharks with distributions in limited regions of all oceans. Generic validity and taxonomic status of some species are upon controversies by morphological and molecular evidence that often suggest reallocation of Cirrhigaleus species into the genus Squalus. Particularly, the roughskin spurdog C. asper exhibits intermediary morphological characteristics within Squalidae that requires clarification. In the present study, a phylogenetic approach was undertaken to test the correct generic placement of C. asper using novel and revised morphological characters. We performed maximum parsimony analysis of 51 morphological characters of the internal (e.g., neurocranium, clasper cartilages, pectoral and pelvic girdles) and external anatomy applied to 13 terminal taxa. Cirrhigaleus represents a valid genus and it is supported by eight synapomorphies: high number of monospondylous vertebrae; medial nasal lobe supported by fleshy core and innervated by the buccopharyngeal branch of the facial nerve; neurocranium with greatest width across nasal capsules; one facet and one condyle in the puboischiadic bar for articulating with the basipterygium; two intermediate segments between the basipterygium of the pelvic fin and the axial cartilage of the claspers; five terminal clasper cartilages; and posterior medial process of the puboischiadic bar absent. Cirrhigaleus asper is sister-species to a small clade comprising C. barbifer and C. australis which is supported by one synapomorphy, presence of conspicuous cusplets in the dermal denticles. Cirrhigaleus barbifer, C. asper and C. australis are redescribed herein and the neotype of C. barbifer is designated. A key to Cirrhigaleus species is also given and the inner relationships within Squalus is tentatively discussed.

Revision of the Cretaceous shark Protoxynotus (Chondrichthyes, Squaliformes) and early evolution of somniosid sharks

Due to the peculiar combination of dental features characteristic for different squaliform families, the position of the Late Cretaceous genera Protoxynotus and Paraphorosoides within Squaliformes has long been controversial. In this study, we revise these genera based on previously known fossil teeth and new dental material. The phylogenetic placement of Protoxynotus and Paraphorosoides among other extant and extinct squaliforms is discussed based on morphological characters combined with DNA sequence data of extant species. Our results suggest that Protoxynotus and Paraphorosoides should be included in the Somniosidae and that Paraphorosoides is a junior synonym of Protoxynotus. New dental material from the Campanian of Germany and the Maastrichtian of Austria enabled the description of a new species Protoxynotus mayrmelnhofi sp. nov. In addition, the evolution and origin of the characteristic squaliform tooth morphology are discussed, indicating that the elongated lower jaw teeth with erected cusp and distinct dignathic heterodonty of Protoxynotus represents a novel functional adaptation in its cutting-clutching type dentition among early squaliform sharks. Furthermore, the depositional environment of the tooth bearing horizons allows for an interpretation of the preferred habitat of this extinct dogfish shark, which exclusively occupied shelf environments of the Boreal- and northern Tethyan realms during the Late Cretaceous.

Diving Lessons

Diving Lessons

Using Compound-Specific Carbon Stable Isotope Analysis of Squalene to Establish Provenance and Ensure Sustainability for the Deep-Water Shark Liver Oil Industry

Deep-water dogfish (sharks) are caught on Australia’s continental shelf as by-products to other deep-water species with revenue derived from fillets for human consumption and from the livers which are sold for their oil content. Deep-water dogfish utilise a large oil-rich liver for buoyancy, which may account for 20–25% of their body weight. An important constituent of certain dogfish liver oil is squalene, a highly unsaturated triterpenoid (C30H50) hydrocarbon which in some species can be up to 90% of the oil, though in the Australian commercial species it is typically around 50%. Squalene (and deep-water dogfish liver oil in general) has a long-standing high value in products, such as cosmetics and nutraceuticals. Manufacturers are increasingly required to demonstrate the sustainability of products, and this is integral to the importance of demonstrating product provenance. Australia’s mid-slope deep-water dogfish fishery is recognised globally as well managed and sustainable; therefore, it is important to be able to distinguish products derived from these regions from other unregulated, unsustainable and cheaper sources in order to protect Australia’s competitive advantages and ensure sustainability. In this study, we have sourced deep-water dogfish liver oil samples originating from Southeast Australia, New Zealand, India, Northeast Africa and the Arabian Sea. The squalene was isolated by commercial or laboratory processing. A compound-specific carbon stable isotope analysis of the derived squalene was then used to determine isotopic resolution and assign the likely origins of a variety of commonly available off-the-shelf nutraceuticals in Australian outlets. Squalene sourced and produced from Southeast Australian and New Zealand dogfish liver oils showed δ13C values in the range of −22.1 to −24.2‰, with all other squalene samples distinguishable at −19.9 to −20.7‰. Many of the off-the-shelf squalene products claiming to be of Australian origin showed δ13C values very distinct from the range of the genuine Southeast Australian- and New Zealand-produced squalene.

Osmorespiratory compromise in an elasmobranch: oxygen consumption, ventilation and nitrogen metabolism during recovery from exhaustive exercise in dogfish sharks (Squalus suckleyi).

The functional trade-off between respiratory gas exchange versus osmolyte and water balance that occurs at the thin, highly vascularized gills of fishes has been termed the osmorespiratory compromise. Increases in gas exchange capacity for meeting elevated oxygen demands can end up favoring the passive movement of osmolytes and water, potentially causing a disturbance in osmotic balance. This phenomenon has been studied only sparsely in marine elasmobranchs. Our goal was to evaluate the effects of exhaustive exercise (as a modulator of oxygen demand) on oxygen consumption (MO2), branchial losses of nitrogenous products (ammonia and urea-N), diffusive water exchange rates, and gill ventilation (frequency and amplitude), in the Pacific spiny dogfish (Squalus suckleyi). To that end, MO2, osmolyte fluxes, diffusive water exchange rate, and ventilation dynamics were first measured under resting control conditions, then sharks were exercised until exhaustion (20min), and the same parameters were monitored for the subsequent 4h of recovery. While MO2 nearly doubled immediately after exercise and remained elevated for 2h, ventilation dynamics did not change, suggesting that fish were increasing oxygen extraction efficiency at the gills. Diffusive water flux rates (measured over 0-2h of recovery) were not affected. Ammonia losses were elevated by 7.6-fold immediately after exercise and remained elevated for 3h into recovery, while urea-N losses were elevated only 1.75-fold and returned to control levels after 1h. These results are consistent with previous investigations using different challenges (hypoxia, high temperature) and point to a tighter regulation of urea-N conservation mechanisms at the gills, likely due to the use of urea as a prized osmolyte in elasmobranchs. Environmental hyperoxia offered no relief from the osmorespiratory compromise, as there were no effects on any of the parameters measured during recovery from exhaustive exercise.

DNA Barcoding and Species Delimitation for Dogfish Sharks Belonging to the Squalus Genus (Squaliformes: Squalidae)

The Squalus genus comprises a group of small demersal sharks occurring circumglobally, popularly known as dogfish sharks. This genus exhibits a conserved morphology, thus making correct morphological identification difficult. Considering these taxonomic problems and the scarcity of molecular data, the present study aimed to identify Squalus genus MOTUs, using DNA barcoding for species delimitation via ABGD (automatic barcode gap discovery), PTP (Poisson tree process), and GMYC (general mixed Yule coalescent) employing the mitochondrial COI gene. A total of 69 sequences were generated from samples obtained from the American coast in both the Atlantic and Pacific Oceans. The ABGD analysis was the most conservative among the three applied delimitations, indicating three taxonomic units, while the PTP analysis revealed nine MOTUs, with two conflicting units noted between S. clarkae + S. mitsukurii and S. albicaudus + S. cubensis. The GMYC analysis indicated an excessive division, with S. acanthias and S. mitsukurii subdivided into six MOTUs each and S. blainville, into four. These findings demonstrated that Squalus presents a complex of previously defined species, with misidentified samples deposited in databases leading to difficulties in analyzing the real distribution and diversity of species belonging to this genus. Thus, further efforts to highlight possible new species are recommended.

Squalamine and Its Aminosterol Derivatives: Overview of Biological Effects and Mechanisms of Action of Compounds with Multiple Therapeutic Applications.

Squalamine is a natural aminosterol that has been discovered in the tissues of the dogfish shark (Squalus acanthias). Studies have previously demonstrated that this promoter compound and its derivatives exhibit potent bactericidal activity against Gram-negative, Gram-positive bacteria, and multidrug-resistant bacteria. The antibacterial activity of squalamine was found to correlate with that of other antibiotics, such as colistin and polymyxins. Still, in the field of microbiology, evidence has shown that squalamine and its derivatives have antifungal activity, antiprotozoa effect against a limited list of protozoa, and could exhibit antiviral activity against both RNA- and DNA-enveloped viruses. Furthermore, squalamine and its derivatives have been identified as being antiangiogenic compounds in the case of several types of cancers and induce a potential positive effect in the case of other diseases such as experimental retinopathy and Parkinson’s disease. Given the diverse effects of the squalamine and its derivatives, in this review we provide the different advances in our understanding of the various effects of these promising molecules and try to draw up a non-exhaustive list of the different mechanisms of actions of squalamine and its derivatives on the human organism and on different pathogens.

Shark bellies flow like Tesla valves

Sharks, skates and rays are known for their unique intestines, which are spiral shaped and located behind the stomach. This unusual spiral intestine naturally slows digestion and expands the intestine's surface area to improve nutrient absorption. The inside of the spiral intestine can vary greatly in shape, but had previously only been studied through two-dimensional illustrations, losing the complexity of the structure's three-dimensional shape and preventing researchers from fully understanding the role that these designs may play in digestion. Samantha Leigh from California State University Dominguez Hills, USA, and colleagues from the University of Washington, Biomark Inc., and the University of California Irvine, USA, wanted to compare the three-dimensional (3D) shape and function of the spiral intestines from 22 shark families to better understand whether corkscrew shaped intestines could predict a shark's diet type and how food moves through these unconventional intestines.After obtaining museum specimens of species ranging from the small and rare combtooth dogfish (Centroscyllium nigrum) to the large and charismatic smooth hammerhead (Sphyrna zygaena), the team removed the sharks’ spiral intestines and CT scanned them at Friday Harbor Laboratories, USA, and the University of California Irvine. Analysing the structures of the intestines in 3D, the team realised that the digestive tracts came in four distinct forms: column-shaped, funnel-shaped toward the end, funnel-shaped toward the beginning and scroll-shaped. The team then selected four sharks, each representing one of the four intestine designs, and pushed three different solutions, ranging in viscosity from water to 25% glycerol, through the intestines to measure their flow rates. Lastly, the team injected a hormone that stimulates muscles to contract into the intestines of five dogfish sharks, to measure how long it takes for food to move through these spiral intestines as the animals digest dinner.The researchers found that some shapes, such as the funnel-shaped spiral intestines, have a slower flow rate than others, suggesting that sharks with these shapes likely have slower digestion rates. This may also increase the water and nutrient absorption in the spiral intestine, so that sharks can harvest the most energy from their food. Though spiral intestines vary in shape, these shapes did not predict what the sharks chose to eat. In addition, when the team analysed how fluids flow through the pulsating tissue, they realised that spiral intestines work like a Tesla valve, which only allows fluid to move in one direction without the benefit of additional moving components, such as flap valves. In short, spiral intestines prevent food from moving backward, in the wrong direction. The team also realised that the pulsing contractions that propel food through these intestines can be concentrated in just a few locations and could help to mix and move food better as it progresses through the digestive system.Most incredibly, this corkscrew intestine design evolved in a shark ancestor around 450 million years ago, so this structure is older than land animals! Yet, despite its success at sticking around for so long, it has only evolved in a few groups of fishes and occurs most in the cartilaginous fishes (such as the sharks, skates and rays). The main benefit of the curly design is that it naturally prevents blockages and food from accidentally flowing backward, maintaining a slow progress that is ideal for digestion. While the four different spiral intestine shapes may not be directly related to the diets that these animals consume, they do affect how fast food moves through. Future studies might uncover why these different shapes evolved in the first place and what role the difference in food flow speed may have on a shark's digestion.

A Highly Complex, MHC-Linked, 350 Million-Year-Old Shark Nonclassical Class I Lineage.

Cartilaginous fish, or Chondrichthyes, are the oldest extant vertebrates to possess the MHC and the Ig superfamily-based Ag receptors, the defining genes of the gnathostome adaptive immune system. In this work, we have identified a novel MHC lineage, UEA, a complex multigene nonclassical class I family found in sharks (division Selachii) but not detected in chimaeras (subclass Holocephali) or rays (division Batoidea). This new lineage is distantly related to the previously reported nonclassical class I lineage UCA, which appears to be present only in dogfish sharks (order Squaliformes). UEA lacks conservation of the nine invariant residues in the peptide (ligand)-binding regions (PBR) that bind to the N and C termini of bound peptide in most vertebrate classical class I proteins, which are replaced by relatively hydrophobic residues compared with the classical UAA. In fact, UEA and UCA proteins have the most hydrophobic-predicted PBR of all identified chondrichthyan class I molecules. UEA genes detected in the whale shark and bamboo shark genome projects are MHC linked. Consistent with UEA comprising a very large gene family, we detected weak expression in different tissues of the nurse shark via Northern blotting and RNA sequencing. UEA genes fall into three sublineages with unique characteristics in the PBR. UEA shares structural and genetic features with certain nonclassical class I genes in other vertebrates, such as the highly complex XNC nonclassical class I genes in Xenopus, and we anticipate that each shark gene, or at least each sublineage, will have a unique function, perhaps in bacterial defense.

Day/night patterns of habitat use by dogfish sharks (Squalidae) at photic and subphotic warm-temperate reefs: evidence for diel movements and size- and sex-segregation

Dogfish sharks (genus Squalus) demonstrate complex distribution patterns that may increase their vulnerability to selective overfishing. This study investigated the day/night reef-use patterns in a population of dogfish (presumably Squalus acutipinnis) on shallow photic (13–35 m) and deep subphotic (51–99 m) nearshore rocky reefs in South Africa. Using baited remote underwater stereo-video systems, immature male dogfish were commonly recorded at subphotic deep reefs. At shallow photic reefs, dogfish were essentially absent during the day; however, a significant increase in the abundance of large male dogfish was observed at night. The size class of dogfish that moved onto photic reefs at night was not recorded at deep subphotic reefs, suggesting that they make use of a different habitat during daytime. The observed differences in depth use by cohorts of small and large male dogfish, and the absence of females on the reefs, provided strong evidence for size- and sex-segregation within the surveyed population. While the potential biotic and abiotic drivers were not directly tested, the results suggest that reef-use patterns may be linked to photic or temperature preferences and/or to competition- and mating-avoidance strategies. This new information about the use of nearshore rocky reefs by dogfish in South Africa raises important questions relating to the distribution and habitat use of females and the daytime habitats of mature males. With dogfish extensively caught in longline and trawl fisheries in South Africa, further research is needed to address the current knowledge gaps.

Deep-sea biodiversity at the extremes of the Salas y Gómez and Nazca ridges with implications for conservation.

The Salas y Gómez and Nazca ridges are underwater mountain chains that stretch across 2,900 km in the southeastern Pacific and are recognized for their high biodiversity value and unique ecological characteristics. Explorations of deep-water ecosystems have been limited in this region, and elsewhere globally. To characterize community composition of mesophotic and deep-sea demersal fauna at seamounts in the region, we conducted expeditions to Rapa Nui (RN) and Salas y Gómez (SyG) islands in 2011 and Desventuradas Islands in 2013. Remote autonomous baited-cameras were used to conduct stationary video surveys between 150–1,850 m at RN/SyG (N = 20) and 75–2,363 m at Desventuradas (N = 27). Individual organisms were identified to the lowest possible taxonomic level and relative abundance was quantified with the maximum number of individuals per frame. Deployments were attributed with associated environmental variables (temperature, salinity, dissolved oxygen, nitrate, silicate, phosphate, chlorophyll-a, seamount age, and bathymetric position index [BPI]). We identified 55 unique invertebrate taxa and 66 unique fish taxa. Faunal community structure was highly dissimilar between and within subregions both for invertebrate (p < 0.001) and fish taxa (p = 0.022). For fishes, dogfish sharks (Squalidae) accounted for the greatest dissimilarity between subregions (18.27%), with mean abundances of 2.26 ± 2.49 at Desventuradas, an order of magnitude greater than at RN/SyG (0.21 ± 0.54). Depth, seamount age, broad-scale BPI, and nitrate explained most of the variation in both invertebrate (R2 = 0.475) and fish (R2 = 0.419) assemblages. Slightly more than half the deployments at Desventuradas (N = 14) recorded vulnerable marine ecosystem taxa such as corals and sponges. Our study supports mounting evidence that the Salas y Gómez and Nazca ridges are areas of high biodiversity and high conservation value. While Chile and Peru have recently established or proposed marine protected areas in this region, the majority of these ridges lie outside of national jurisdictions and are under threat from overfishing, plastic pollution, climate change, and potential deep-sea mining. Given its intrinsic value, this region should be comprehensively protected using the best available conservation measures to ensure that the Salas y Gómez and Nazca ridges remain a globally unique biodiversity hotspot.

Size Changes within a Southeastern United States Coastal Shark Assemblage: 1975–2018

AbstractHarvest may have myriad effects on target species, including a change in population size structure. To assess whether size shifts have occurred among managed coastal species of shark (superorder Selachimorpha), we examined the population size structure of 12 species caught during a nearly five‐decade‐long fishery‐independent survey conducted in Onslow Bay, North Carolina, using standardized longline gear. We evaluated trends in mean fork length (FL), median FL, and index of maximum FL (L90%) for each species separately across time using linear regression models. We also examined trends in size‐classes (200‐mm bins) and catch per unit effort for each species over time. For 10 of the 12 species (excluding sample‐size‐constrained Tiger Shark Galeocerdo cuvier and Bull Shark Carcharhinus leucas), size structure metrics indicated decreasing sizes over time, although statistical confidence for these patterns varied across species and metrics. Strongest statistical support for declining sizes was observed for Blacknose Shark Carcharhinus acronotus (mean FL, median FL, L90%), Dusky Shark Carcharhinus obscurus (L90%), Smooth Dogfish Mustelus canis (L90%), and Atlantic Sharpnose Shark Rhizoprionodon terraenovae (L90%). Magnitude of decreases in L90% among these 10 species during the survey ranged from roughly 9% (Silky Shark Carcharhinus falciformis; 83‐mm decrease) to 35% (Sandbar Shark Carcharhinus plumbeus; 541‐mm decrease). Our findings indicate a potential for fishing pressure to exert directional selection on these coastal shark species, although further research is needed regarding the nature of size‐dependent catchability and species‐specific vital rates to adequately evaluate these dynamics. Furthermore, in addition to the removal of “great sharks,” decreasing sizes of small coastal sharks, such as Blacknose Shark, Smooth Dogfish, and Atlantic Sharpnose Shark (i.e., “mesopredators”), suggest that harvest may have pervasive effects on species throughout this assemblage.