Basics of skin structure and function in elasmobranchs: a review

Histological comparison of shark dermis across various ecomorphologies

The integument plays essential roles in the structural support, protection, and hydrodynamic capability among fishes. Most research on shark skin has focused on the external epidermal layer, while the larger dermis anchoring the dermal denticles has been mostly ignored. Shark dermis is composed of two layers, the upper stratum laxum and the lower stratum compactum, holding supportive collagen and elastic fibers. There may be morphological and compositional differences in the dermis across various species of sharks that could relate to their different swimming modes and ecologies. The goal of this study was to characterize and describe the dermis among three shark species, Ginglymostoma cirratum, Sphyrna mokarran, and Isurus oxyrinchus, each representing a different swimming mode. Histological characterizations were performed at 16 locations along the body of each shark; variables such as dermal thickness, abundance of collagen and elastic fibers, and fiber size were quantified. Results showed G. cirratum has the thickest skin overall, and the largest fiber size for both collagen and elastic fibers, with overall patterns of increased amounts of collagen fibers and decreased amount of elastic fibers. At the opposite end of the spectrum, I. oxyrinchus showed the thinnest dermis along the flank region, with overall patterns of increased elastic fibers and decreased collagen fibers. These findings may challenge our original assumptions of a rigid body in fast moving sharks and a more flexible body in slower moving sharks and highlight the diversity of the shark integument.

Toward the validation of an alternative method for endocrine monitoring in sharks: insights from testosterone analyses in the skin of bycatch individuals

The present study presents a new technique for measuring steroid hormones in shark skin. Results reveal for the first time that shark skin contains measurable levels of testosterone and that levels can be reliably measured by enzyme immunoassay. We identify the mass threshold below which samples should not be used to avoid inconsistent hormone data and highlight the importance of considering body location when designing future collection protocols.

Characterization of shark skin properties and biomimetic replication

This review explores the present knowledge of the unique properties of shark skin and possible applications of its functionalities, including drag reduction and swimming efficiency. Tooth-like denticles, with varied morphologies, sizes, and densities across the shark's body, significantly influence the flow and interaction of fluids. Examining dermal denticle morphology, this study unveils the functional properties of real shark skin, including mechanical properties such as stiffness, stress-strain characteristics, and denticle density's impact on tensile properties. The adaptive capabilities of the Mako shark scales, especially in high-speed swimming, are explored, emphasizing their passive flow-actuated dynamic micro-roughness. This research contains an overview of various studies on real shark skin, categorizing them into skin properties, morphology, and hydrodynamics. The paper extends exploration into industrial applications, detailing fabrication techniques and potential uses in vessels, aircraft, and water pipes for friction reduction. Three manufacturing approaches, bio-replicated forming, direct fabrication, and indirect manufacturing, are examined, with 3D printing and photoconfiguration technology emerging as promising alternatives. Investigations into the mechanical properties of shark skin fabrics reveal the impact of denticle size on tensile strength, stress, and strain. Beyond drag reduction, the study highlights the shark skin's role in enhancing thrust and lift during locomotion. The paper identifies future research directions, emphasizing live shark testing and developing synthetic skin with the help of 3D printing incorporating the bristling effect.

Intermediate filaments spatially organize intracellular nanostructures to produce the bright structural blue of ribbontail stingrays across ontogeny

In animals, pigments but also nanostructures determine skin coloration, and many shades are produced by combining both mechanisms. Recently, we discovered a new mechanism for blue coloration in the ribbontail stingray Taeniura lymma, a species with electric blue spots on its yellow-brown skin. Here, we characterize finescale differences in cell composition and architecture distinguishing blue from non-blue regions, the first description of elasmobranch chromatophores and the nanostructures responsible for the stingray's novel structural blue, contrasting with other known mechanisms for making nature's rarest color. In blue regions, the upper dermis comprised a layer of chromatophore units -iridophores and melanophores entwined in compact clusters framed by collagen bundles- this structural stability perhaps the root of the skin color's robustness. Stingray iridophores were notably different from other vertebrate light-reflecting cells in having numerous fingerlike processes, which surrounded nearby melanophores like fists clenching a black stone. Iridophores contained spherical iridosomes enclosing guanine nanocrystals, suspended in a 3D quasi-order, linked by a cytoskeleton of intermediate filaments. We argue that intermediate filaments form a structural scaffold with a distinct optical role, providing the iridosome spacing critical to produce the blue color. In contrast, black-pigmented melanosomes within melanophores showed space-efficient packing, consistent with their hypothesized role as broadband-absorbers for enhancing blue color saturation. The chromatophore layer's ultrastructure was similar in juvenile and adult animals, indicating that skin color and perhaps its ecological role are likely consistent through ontogeny. In non-blue areas, iridophores were replaced by pale cells, resembling iridophores in some morphological and nanoscale features, but lacking guanine crystals, suggesting that the cell types arise from a common progenitor cell. The particular cellular associations and structural interactions we demonstrate in stingray skin suggest that pigment cells induce differentiation in the progenitor cells of iridophores, and that some features driving color production may be shared with bony fishes, although the lineages diverged hundreds of millions of years ago and the iridophores themselves differ drastically.

Effect of photobiomodulation therapy as a multimodal approach to treatment of dermal abrasions in cownose rays (Rhinoptera bonasus)

Photobiomodulation therapy (PBMT) has become increasingly utilized in the zoo and aquarium fields to treat ailments in various species; however, its use in teleosts and elasmobranchs is relatively underrepresented in the literature. This study examined the efficacy of PBMT as an adjunct treatment to reduce the healing time of dermal abrasions in cownose rays (Rhinoptera bonasus). Ten cownose rays were included in this study and separated into control (n = 5) and treatment groups (n = 5). Animals in both treatment and control groups received intramuscular injections of enrofloxacin every 72 h to provide broad-spectrum antibiotic coverage. Cownose rays in the treatment group additionally received PBMT using the Companion® CTC-12 unit using the large, noncontact head. Control group rays were subjected to identical handling without the use of PBMT. A total of 196 Joules were delivered at a power of 2 W based on a dose of 5 J/cm2 at each session. Observational analysis revealed a lack of appreciable difference in healing time between the treatment and control groups in the aforementioned settings; however, it is recommended that PBMT protocols should be re-evaluated if there is little to no response in healing after three to four sessions. PBMT use on acute superficial dermal abrasions in cownose rays was well-tolerated in this study and may have utility in a multimodal treatment approach to wound healing. This study also produced a wound scoring metric based on photographs captured at each treatment. Future studies should utilize increased fluence (J/cm2) and irradiance (W/cm2) and incorporate synchronous histological analysis and increased sample size.

Relationships in Shark Skin: Mechanical and Morphological Properties Vary between Sexes and among Species

Shark skin is a composite of mineralized dermal denticles embedded in an internal collagen fiber network and is sexually dimorphic. Female shark skin is thicker, has greater denticle density and denticle overlap compared to male shark skin, and denticle morphology differs between sexes. The skin behaves with mechanical anisotropy, extending farther when tested along the longitudinal (anteroposterior) axis but increasing in stiffness along the hoop (dorsoventral or circumferential) axis. As a result, shark skin has been hypothesized to function as an exotendon. This study aims to quantify sex differences in the mechanical properties and morphology of shark skin. We tested skin from two immature male and two immature female sharks from three species (bonnethead shark, Sphyrna tiburo; bull shark, Carcharhinus leucas; silky shark, Carcharhinus falciformis) along two orientations (longitudinal and hoop) in uniaxial tension with an Instron E1000 at a 2 mm s-1 strain rate. We found that male shark skin was significantly tougher than female skin, although females had significantly greater skin thickness compared to males. We found skin in the hoop direction was significantly stiffer than the longitudinal direction across sexes and species, while skin in the longitudinal direction was significantly more extensible than in the hoop direction. We found that shark skin mechanical behavior was impacted by sex, species, and direction, and related to morphological features of the skin.

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.

Morpho-functional changes of lungfish Protopterus dolloi skin in the shift from freshwater to aestivating conditions

African dipnoi (Protopterus sp.) are obligate air-breathing fish that, during dry season, may experience a period of dormancy named aestivation. Aestivation is characterized by complete reliance on pulmonary breathing, general decrease of metabolism and down-regulation of respiratory and cardiovascular functions. To date, little is known about morpho-functional rearrangements induced by aestivation in the skin of African lungfishes. Our study aims to identify, in the skin of P. dolloi, structural modifications and stress-induced molecules in response to short-term (6 days) and long-term (40 days) aestivation. Light microscopy showed that short-term aestivation induces major reorganization, with narrowing of epidermal layers and decrease of mucous cells; prolonged aestivation is characterized by regenerative processes and re-thickening of epidermal layers. Immunofluorescence reveals that aestivation correlates with an increased oxidative stress and changes of Heat Shock Proteins expression, suggesting a protective role for these chaperons. Our findings revealed that lungfish skin undergoes remarkable morphological and biochemical readjustments in response to stressful conditions associated with aestivation.

Gradients of Variation in the At-Vessel Mortality Rate between Twelve Species of Sharks and Skates Sampled through a Fishery-Independent Trawl Survey in the Asinara Gulf (NW Mediterranean Sea)

Elasmobranchs are priority species for conservation due to their rapid decline determined by the unbalanced struggle between a fragile bio-ecology and strong anthropogenic impacts, such as bycatch from professional fishing. In this context, measuring species resistance to catch of poorly selective gear is of paramount importance. During June–October 2022, five experimental fishing campaigns were carried out in the Asinara Gulf (northern Sardinia) through 35 geographically and bathymetrically representative hauls of an area between 30 and 600 m in depth. Skates prevailed over sharks in the number of species, with seven and five species, respectively. We first evaluated the status of each individual with respect to stress due to the trawl’s catch using a three-graded scale. We also recorded individual biometrics (total and disk length, weight and sex, and maturity for males) on board by implementing the best practices in manipulating individuals for physiological recovery and release at sea. After capture, skates resulted in generally better conditions than sharks, although deepwater species of both groups exhibited a worse state than coastal species. The estimated vitality rates also depended on the size of the individuals. This work provides standardized data on the intermingled effect of size, species type, and inhabited depth on the resistance response of some elasmobranch species against capture by trawl fishery activities.

The Epidermal Microbiome Within an Aggregation of Leopard Sharks (Triakis semifasciata) Has Taxonomic Flexibility with Gene Functional Stability Across Three Time-points

The epidermis of Chondrichthyan fishes consists of dermal denticles with production of minimal but protein-rich mucus that collectively, influence the attachment and biofilm development of microbes, facilitating a unique epidermal microbiome. Here, we use metagenomics to provide the taxonomic and functional characterization of the epidermal microbiome of the Triakis semifasciata (leopard shark) at three time-points collected across 4 years to identify links between microbial groups and host metabolism. Our aims include (1) describing the variation of microbiome taxa over time and identifying recurrent microbiome members (present across all time-points); (2) investigating the relationship between the recurrent and flexible taxa (those which are not found consistently across time-points); (3) describing the functional compositions of the microbiome which may suggest links with the host metabolism; and (4) identifying whether metabolic processes are shared across microbial genera or are unique to specific taxa. Microbial members of the microbiome showed high similarity between all individuals (Bray-Curtis similarity index = 82.7, where 0 = no overlap, 100 = total overlap) with the relative abundance of those members varying across sampling time-points, suggesting flexibility of taxa in the microbiome. One hundred and eighty-eight genera were identified as recurrent, including Pseudomonas, Erythrobacter, Alcanivorax, Marinobacter, and Sphingopxis being consistently abundant across time-points, while Limnobacter and Xyella exhibited switching patterns with high relative abundance in 2013, Sphingobium and Sphingomona in 2015, and Altermonas, Leeuwenhoekiella, Gramella, and Maribacter in 2017. Of the 188 genera identified as recurrent, the top 19 relatively abundant genera formed three recurrent groups. The microbiome also displayed high functional similarity between individuals (Bray-Curtis similarity index = 97.6) with gene function composition remaining consistent across all time-points. These results show that while the presence of microbial genera exhibits consistency across time-points, their abundances do fluctuate. Microbial functions however remain stable across time-points; thus, we suggest the leopard shark microbiomes exhibit functional redundancy. We show coexistence of microbes hosted in elasmobranch microbiomes that encode genes involved in utilizing nitrogen, but not fixing nitrogen, degrading urea, and resistant to heavy metal.

Stingray epidermal microbiomes are species-specific with local adaptations

Marine host-associated microbiomes are affected by a combination of species-specific (e.g., host ancestry, genotype) and habitat-specific features (e.g., environmental physiochemistry and microbial biogeography). The stingray epidermis provides a gradient of characteristics from high dermal denticles coverage with low mucus to reduce dermal denticles and high levels of mucus. Here we investigate the effects of host phylogeny and habitat by comparing the epidermal microbiomes of Myliobatis californica (bat rays) with a mucus rich epidermis, and Urobatis halleri (round rays) with a mucus reduced epidermis from two locations, Los Angeles and San Diego, California (a 150 km distance). We found that host microbiomes are species-specific and distinct from the water column, however composition of M. californica microbiomes showed more variability between individuals compared to U. halleri. The variability in the microbiome of M. californica caused the microbial taxa to be similar across locations, while U. halleri microbiomes were distinct across locations. Despite taxonomic differences, Shannon diversity is the same across the two locations in U. halleri microbiomes suggesting the taxonomic composition are locally adapted, but diversity is maintained by the host. Myliobatis californica and U. halleri microbiomes maintain functional similarity across Los Angeles and San Diego and each ray showed several unique functional genes. Myliobatis californica has a greater relative abundance of RNA Polymerase III-like genes in the microbiome than U. halleri, suggesting specific adaptations to a heavy mucus environment. Construction of Metagenome Assembled Genomes (MAGs) identified novel microbial species within Rhodobacteraceae, Moraxellaceae, Caulobacteraceae, Alcanivoracaceae and Gammaproteobacteria. All MAGs had a high abundance of active RNA processing genes, heavy metal, and antibiotic resistant genes, suggesting the stingray mucus supports high microbial growth rates, which may drive high levels of competition within the microbiomes increasing the antimicrobial properties of the microbes.

Epidermal Microbiomes of Leopard Sharks (Triakis semifasciata) Are Consistent across Captive and Wild Environments

Characterizations of shark-microbe systems in wild environments have outlined patterns of species-specific microbiomes; however, whether captivity affects these trends has yet to be determined. We used high-throughput shotgun sequencing to assess the epidermal microbiome belonging to leopard sharks (Triakis semifasciata) in captive (Birch Aquarium, La Jolla California born and held permanently in captivity), semi-captive (held in captivity for <1 year in duration and scheduled for release; Scripps Institute of Oceanography, San Diego, CA, USA) and wild environments (Moss Landing and La Jolla, CA, USA). Here, we report captive environments do not drive epidermal microbiome compositions of T. semifasciata to significantly diverge from wild counterparts as life-long captive sharks maintain a species-specific epidermal microbiome resembling those associated with semi-captive and wild populations. Major taxonomic composition shifts observed were inverse changes of top taxonomic contributors across captive duration, specifically an increase of Pseudoalteromonadaceae and consequent decrease of Pseudomonadaceae relative abundance as T. semifasciata increased duration in captive conditions. Moreover, we show captivity did not lead to significant losses in microbial α-diversity of shark epidermal communities. Finally, we present a novel association between T. semifasciata and the Muricauda genus as Metagenomes associated genomes revealed a consistent relationship across captive, semi-captive, and wild populations. Since changes in microbial communities is often associated with poor health outcomes, our report illustrates that epidermally associated microbes belonging to T. semifasciata are not suffering detrimental impacts from long or short-term captivity. Therefore, conservation programs which house sharks in aquariums are providing a healthy environment for the organisms on display. Our findings also expand on current understanding of shark epidermal microbiomes, explore the effects of ecologically different scenarios on benthic shark microbe associations, and highlight novel associations that are consistent across captive gradients.

Phage Diving: An Exploration of the Carcharhinid Shark Epidermal Virome

The epidermal microbiome is a critical element of marine organismal immunity, but the epidermal virome of marine organisms remains largely unexplored. The epidermis of sharks represents a unique viromic ecosystem. Sharks secrete a thin layer of mucus which harbors a diverse microbiome, while their hydrodynamic dermal denticles simultaneously repel environmental microbes. Here, we sampled the virome from the epidermis of three shark species in the family Carcharhinidae: the genetically and morphologically similar Carcharhinus obscurus (n = 6) and Carcharhinus galapagensis (n = 10) and the outgroup Galeocerdo cuvier (n = 15). Virome taxonomy was characterized using shotgun metagenomics and compared with a suite of multivariate analyses. All three sharks retain species-specific but highly similar epidermal viromes dominated by uncharacterized bacteriophages which vary slightly in proportional abundance within and among shark species. Intraspecific variation was lower among C. galapagensis than among C. obscurus and G. cuvier. Using both the annotated and unannotated reads, we were able to determine that the Carcharhinus galapagensis viromes were more similar to that of G. cuvier than they were to that of C. obscurus, suggesting that behavioral niche may be a more prominent driver of virome than host phylogeny.

Morphological study of the oral denticles of the porbeagle shark Lamna nasus

Oral denticles of sharks are composed by a crown, dentine covered by a layer of enameloid and pulp cavity, the same structure of the dermal denticles found across the body surface of most elasmobranchs. In addition, oral papillae and taste buds are distributed among denticles within the oropharyngeal cavity, playing a fundamental role for tasting as part of the chemosensory system of fishes. Scanning electron microscopy (SEM) has been employed as an important tool for the study of dermal denticles and other structures, as well as histology and more recently computed tomography (CT) scan analysis. Herein, the authors used two methods for the study of the morphology of the oropharyngeal cavity of Lamna nasus (Lamniformes), an oceanic and pelagic shark: SEM and CT scan. The general morphology of oral denticles studied herein is related to abrasion strength as they are diamond-shaped, lack lateral cusps and have less pronounced ridges. In addition, smooth ridges and broad rounded denticles could be related to prevent abrasion during food consumption and manipulation. Oral papillae had a round shape and were observed only under SEM. The densities of papillae were estimated in 100 per cm² , whereas denticles were 1760 and 1230 cm² over the dorsal and ventral regions, respectively. The high numbers of denticles are inversely proportional to papillae density; denticles seem to restrict papillae distribution. Regarding the differences between methodologies, under SEM, only the crown was visualized, as well the papillae, allowing the estimation of size and density of both structures. Nonetheless, under CT scan, the whole components of denticles were clearly visualized: different views of the crown, peduncle, basal plate, and pulp cavity. On the contrary, oral papillae were not visualized under CT due to the tissue preparation. Furthermore, both methods are complementary and were important to extract as much information as possible from denticles and papillae.

Ontogeny of a tessellated surface: Carapace growth of the longhorn cowfish Lactoria cornuta

Biological armors derive their mechanical integrity in part from their geometric architectures, often involving tessellations: individual structural elements tiled together to form surface shells. The carapace of boxfish, for example, is composed of mineralized polygonal plates, called scutes, arranged in a complex geometric pattern and nearly completely encasing the body. In contrast to artificial armors, the boxfish exoskeleton grows with the fish; the relationship between the tessellation and the gross structure of the armor is therefore critical to sustained protection throughout growth. To clarify whether or how the boxfish tessellation is maintained or altered with age, we quantify architectural aspects of the tessellated carapace of the longhorn cowfish Lactoria cornuta through ontogeny (across nearly an order of magnitude in standard length) and in a high‐throughput fashion, using high‐resolution microCT data and segmentation algorithms to characterize the hundreds of scutes that cover each individual. We show that carapace growth is canalized with little variability across individuals: rather than continually adding scutes to enlarge the carapace surface, the number of scutes is surprisingly constant, with scutes increasing in volume, thickness, and especially width with age. As cowfish and their scutes grow, scutes become comparatively thinner, with the scutes at the edges (weak points in a boxy architecture) being some of the thickest and most reinforced in younger animals and thinning most slowly across ontogeny. In contrast, smaller scutes with more variable curvature were found in the limited areas of more complex topology (e.g., around fin insertions, mouth, and anus). Measurements of Gaussian and mean curvature illustrate that cowfish are essentially tessellated boxes throughout life: predominantly zero curvature surfaces comprised of mostly flat scutes, and with scutes with sharp bends used sparingly to form box edges. Since growth of a curved, tiled surface with a fixed number of tiles would require tile restructuring to accommodate the surface's changing radius of curvature, our results therefore illustrate a previously unappreciated advantage of the odd boxfish morphology: by having predominantly flat surfaces, it is the box‐like body form that in fact permits a relatively straightforward growth system of this tessellated architecture (i.e., where material is added to scute edges). Our characterization of the ontogeny and maintenance of the carapace tessellation provides insights into the potentially conflicting mechanical, geometric, and developmental constraints of this species but also perspectives into natural strategies for constructing mutable tiled architectures.

The carapace of boxfish is composed of mineralized polygonal plates, called scutes, arranged in a complex geometric pattern and nearly completely encasing the body. To clarify whether or how this armor is maintained or altered with age, we quantify architectural aspects of the carapace of the longhorn cowfish Lactoria cornuta through ontogeny, using high‐resolution microCT data and segmentation algorithms to characterize the hundreds of scutes that cover each individual.

Grouping behaviour impacts on the parasitic pressure and squamation of sharks

The evolution of grouping behaviour involves a complex trade-off of benefits and costs. Among the latter, an increase in the risk of parasitic transmission is a well-documented phenomenon that has likely promoted the evolution of defensive mechanisms in aquatic vertebrates. Here, we explore the relationship between grouping behaviour, parasitic richness (∼parasitic pressure), and the evolution of potential defensive traits in the squamation of sharks through phylogenetic, standard and zero-inflation regression models. Our results demonstrate that sharks that frequently aggregate show increased parasitic pressure, which may constitute an agent of selection. Accordingly, their squamation is characterized by large-scale crown insertion angles and low-scale coverage, which are interpreted as traits that compromise parasite attachment and survival. These traits are less evident in regions of the body and ecological groups that are subjected to high abrasive stress or increased drag. Thus, the squamation of sharks responds to a compromise between various functions, where protective and hydrodynamic roles prevail over the rest (e.g. ectoparasitic defence and bioluminescence aiding). This work establishes a quantitative framework for inferring parasitic pressure and social interaction from squamation traits and provides an empirical basis from which to explore these phenomena through early vertebrate and chondrichthyan evolution.

Study of the integument that covering back and stinger of the freshwater stingray Potamotrygon rex (Chondricthyes, Potamotrygonidae)

Abstract The objective of this study was to describe the histology and histochemistry of the integument covering the back and stinger of the freshwater stingray Potamotrygon rex, endemic to the Middle Upper Tocantins River. The species has a dark back and yellowish circular spots that extend to the tail, which has one to two stings located in the median portion of the tail. Through histological study it was observed that the epithelia of the back and stinger are composed of non-keratinized stratified pavement epithelial tissue, and are organized in three layers: basal, intermediate and superficial. The differences between the tissues are related to the cell types that compose them. The back is organized with epithelial cells, mucus cells, granulocyte cells and chromatophores. The mucus cells are distributed in different layers along the animal's back, influencing the thickness of the tissue. The tissue that covers the stinger is composed of epithelial cells, chromatophores and specialized cells in protein synthesis. In the histochemistry, the stinger epithelial cells were stained with Bromophenol Blue, especially those of the intermediate layer, which were called specialized cells. In the back the epithelial cells were stained with Bromophenol Blue, Alcian Blue and PAS, and the mucous cells with PAS. In both tissues the presence of protein reserves was detected, areas so called because they are stained strongly with Bromophenol Blue. The results show that the stinger presents activity directed to the production of proteins, and that the back is organized to produce different components, which constitute the cuticle that covers the animal's body.

Bacterial community dynamics during embryonic development of the little skate (Leucoraja erinacea)

Background

Microbial transmission from parent to offspring is hypothesized to be widespread in vertebrates. However, evidence for this is limited as many evolutionarily important clades remain unexamined. There is currently no data on the microbiota associated with any Chondrichthyan species during embryonic development, despite the global distribution, ecological importance, and phylogenetic position of this clade. In this study, we take the first steps towards filling this gap by investigating the microbiota associated with embryonic development in the little skate, Leucoraja erinacea, a common North Atlantic species and popular system for chondrichthyan biology.

Methods

To assess the potential for bacterial transmission in an oviparous chondrichthyan, we used 16S rRNA amplicon sequencing to characterize the microbial communities associated with the skin, gill, and egg capsule of the little skate, at six points during ontogeny. Community composition was analyzed using the QIIME2 pipeline and microbial continuity between stages was tracked using FEAST.

Results

We identify site-specific and stage-specific microbiota dominated by the bacterial phyla Proteobacteria and Bacteroidetes. This composition is similar to, but distinct from, that of previously published data on the adult microbiota of other chondrichthyan species. Our data reveal that the skate egg capsule harbors a highly diverse bacterial community–particularly on the internal surface of the capsule–and facilitates intergenerational microbial transfer to the offspring. Embryonic skin and external gill tissues host similar bacterial communities; the skin and gill communities later diverge as the internal gills and skin denticles develop.

Conclusions

Our study is the first exploration of the chondrichthyan microbiota throughout ontogeny and provides the first evidence of vertical transmission in this group.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-021-00136-x.

General descriptions of the dermis structure of a juvenile whale shark Rhincodon typus from the Gulf of California

The aim of this study is to provide preliminary observations on the microanatomy of Rhincodon typus skin using histology and electron microscopy analyses. Skin biopsies were obtained from a deceased juvenile male shark (548 cm total length) stranded in La Paz, Mexico, during February 2018. The results of this study evidenced the basic structure of the dermal denticles in the epidermis of the trunk of the shark, as well as the composition of the connective tissue in the hypodermis. Histological images of the hypodermis showed a high concentration of collagen fibres, formed by a large number of fine and wavy fibres of compact shape and little intercellular substance.

Elasmobranch microbiomes: emerging patterns and implications for host health and ecology

Elasmobranchs (sharks, skates and rays) are of broad ecological, economic, and societal value. These globally important fishes are experiencing sharp population declines as a result of human activity in the oceans. Research to understand elasmobranch ecology and conservation is critical and has now begun to explore the role of body-associated microbiomes in shaping elasmobranch health. Here, we review the burgeoning efforts to understand elasmobranch microbiomes, highlighting microbiome variation among gastrointestinal, oral, skin, and blood-associated niches. We identify major bacterial lineages in the microbiome, challenges to the field, key unanswered questions, and avenues for future work. We argue for prioritizing research to determine how microbiomes interact mechanistically with the unique physiology of elasmobranchs, potentially identifying roles in host immunity, disease, nutrition, and waste processing. Understanding elasmobranch–microbiome interactions is critical for predicting how sharks and rays respond to a changing ocean and for managing healthy populations in managed care.

Putative Immunological Functions of Inducible Skin-Associated Lymphoid Tissue in the Context of Mucosa-Associated Lymphoid Tissue

Acquired immunity is orchestrated in various lymphoid organs, including bone marrow, thymus, spleen, and lymph nodes in humans. However, mucosa-associated lymphoid tissue (MALT) is evolutionally known to be emerged in the oldest vertebrates as an immunological tissue for acquired immunity, much earlier than the advent of lymph nodes which appeared in endotherms. Furthermore, the lymphocytes which developed in MALT are known to circulate within the limited anatomical areas. Thus, MALT is comprehended as not the structure but the immune network dedicated to local immunity. As for the skin, skin-associated lymphoid tissue (SALT) was previously postulated; however, its existence has not been proven. Our group recently showed that aggregations of dendritic cells, M2 macrophages, and high endothelial venules (HEVs) are essential components to activate effector T cells in the murine contact hypersensitivity model and termed it as inducible SALT (iSALT) since it was a transient entity that serves for acquired immunity of the skin. Furthermore, in various human skin diseases, we reported that the ectopic formation of lymphoid follicles that immunohistochemically analogous to MALT and regarded them as human counterparts of iSALT. These data raised the possibility that SALT can exist as an inducible form, namely iSALT, which shares the biological significance of MALT. In this article, we revisit the evolution of immunological organs and the related components among vertebrates to discuss the conserved functions of MALT. Furthermore, we also discuss the putative characteristics and functions of iSALT in the context of the MALT concept.

Manta-like planktivorous sharks in Late Cretaceous oceans

The ecomorphological diversity of extinct elasmobranchs is incompletely known. Here, we describe Aquilolamna milarcae, a bizarre probable planktivorous shark from early Late Cretaceous open marine deposits in Mexico. Aquilolamna, tentatively assigned to Lamniformes, is characterized by hypertrophied, slender pectoral fins. This previously unknown body plan represents an unexpected evolutionary experimentation with underwater flight among sharks, more than 30 million years before the rise of manta and devil rays (Mobulidae), and shows that winglike pectoral fins have evolved independently in two distantly related clades of filter-feeding elasmobranchs. This newly described group of highly specialized long-winged sharks (Aquilolamnidae) displays an aquilopelagic-like ecomorphotype and may have occupied, in late Mesozoic seas, the ecological niche filled by mobulids and other batoids after the Cretaceous-Paleogene boundary.

Ecomorphological, space, and mineral relations of dermal denticles in angular angel shark (Squatina guggenheim)

Shark skin is predominantly specialized for swimming and protection, with the dermal denticle being the main structure associated with these abilities. The dermal denticle is a mineral structure with a unique morphology for each species, which allows its use as a taxonomic tool. Few studies have investigated the microscopy aspects of skin and dermal denticles, considering the high diversity of sharks. Here, we investigated the three-dimensional morphoquantitative aspects and mineral composition of dermal denticles in different regions of the angular angel shark, Squatina guggenheim, using scanning electron microscopy and dispersive energy system. With the microscopy, we were able to observe that the dermal denticle morphology changes according to the area it is located. It was possible to describe the dermal denticles individually, from root to the crown, highlighting all of their individualities. Through the dispersive energy system, we showed the proportions of each mineral found in the denticle, by area, demonstrating the composition and the particularities of crown, body, and root, where whitlockite was described for the first time in elasmobranchs. In this way, the present study presented the specificities of the dermal denticles of S. guggenheim, as well sought to understand the different structure functions for the animal, thus assisting future research in animal morphology.

Scale performance and composition in a small Amazonian armored catfish, Corydoras trilineatus

The cory catfishes (Callichthyidae) are small, South American armored catfishes with a series of dermal scutes that run the length of the fish from posterior to the parieto-supraoccipital down to the caudal peduncle. In this study, we explore the anatomy and functional performance of the armored scutes in the three-striped cory catfish, Corydoras trilineatus. The lateral surface has a dorsal and a ventral row of scutes that interact at the horizontal septum. The scutes have little overlap with sequential posterior scutes (~33% overlap) and a deep ridge in the internal surface that connects to the underlying soft tissue. The internal surface of C. trilineatus scutes is stiffer than the external surface, contrary to the findings in a related species of cory catfish, C. aeneus, which documented a hypermineralized, enamel-like, non-collagenous, hyaloine layer along the external surface of the scute. Clearing and staining of C. trilineatus scutes revealed that the scutes have highly mineralized (~50% mineralization) regions embedded in between areas of low mineralization along the posterior margin. Puncture tests showed that posterior scutes were weaker than both anterior and middle scutes, and scutes attached to the body required 50% more energy to puncture than isolated scutes. Corydoras trilineatus has the strongest armor in areas critical for protecting vital organs and the external armored scute receives synergistic benefits from interactions to the soft underlying tissue, which combine to provide a tough protective armor that still allows for flexible mobility.

Epidermal Club Cells in Fishes: A Case for Ecoimmunological Analysis

Epidermal club cells (ECCs), along with mucus cells, are present in the skin of many fishes, particularly in the well-studied Ostariophysan family Cyprinidae. Most ECC-associated literature has focused on the potential role of ECCs as a component of chemical alarm cues released passively when a predator damages the skin of its prey, alerting nearby prey to the presence of an active predator. Because this warning system is maintained by receiver-side selection (senders are eaten), there is want of a mechanism to confer fitness benefits to the individual that invests in ECCs to explain their evolutionary origin and maintenance in this speciose group of fishes. In an attempt to understand the fitness benefits that accrue from investment in ECCs, we reviewed the phylogenetic distribution of ECCs and their histochemical properties. ECCs are found in various forms in all teleost superorders and in the chondrostei inferring either early or multiple independent origins over evolutionary time. We noted that ECCs respond to several environmental stressors/immunomodulators including parasites and pathogens, are suppressed by immunomodulators such as testosterone and cortisol, and their density covaries with food ration, demonstrating a dynamic metabolic cost to maintaining these cells. ECC density varies widely among and within fish populations, suggesting that ECCs may be a convenient tool with which to assay ecoimmunological tradeoffs between immune stress and foraging activity, reproductive state, and predator-prey interactions. Here, we review the case for ECC immune function, immune functions in fishes generally, and encourage future work describing the precise role of ECCs in the immune system and life history evolution in fishes.

Comparative study of cartilaginous fish divulges insights into the early evolution of primary, secondary and mucosal lymphoid tissue architecture

Cartilaginous fish are located at a pivotal point in phylogeny where the adaptive immune system begins to resemble that of other, more-derived jawed vertebrates, including mammals. For this reason, sharks and other cartilaginous fish are ideal models for studying the natural history of immunity. Insights from such studies may include distinguishing the (evolutionarily conserved) fundamental aspects of adaptive immunity from the (more recent) accessory. Some lymphoid tissues of sharks, including the thymus and spleen, resemble those of mammals in both appearance and function. The cartilaginous skeleton of sharks has no bone marrow, which is also absent in bony fish despite calcified bone, but cartilaginous fish have other Leydig's and epigonal organs that function to provide hematopoiesis analogous to mammalian bone marrow. Conserved across all vertebrate phylogeny in some form is gut-associated lymphoid tissues, or GALT, which is seen from agnathans to mammals. Though it takes many forms, from typhlosole in lamprey to Peyer's patches in mammals, the GALT serves as a site of antigen concentration and exposure to lymphocytes in the digestive tract. Though more complex lymphoid organs are not present in agnathans, they have several primitive tissues, such as the thymoid and supraneural body, that appear to serve their variable lymphocyte receptor-based adaptive immune system. There are several similarities between the adaptive immune structures in cartilaginous and bony fish, such as the thymus and spleen, but there are mechanisms employed in bony fish that in some instances bridge their adaptive immune systems to that of tetrapods. This review summarizes what we know of lymphoid tissues in cartilaginous fishes and uses these data to compare primary and secondary tissues in jawless, cartilaginous, and bony fishes to contextualize the early natural history of vertebrate mucosal immune tissues.

Living naked: first case of lack of skin-related structures in an elasmobranch, the blackmouth catshark (Galeus melastomus)

As far as is known, in this paper the first case of lacking of skin-related structures (epidermis, stratum laxum, dermal denticles and teeth) in a free-swimming elasmobranch, the blackmouth catshark, Galeus melastomus, is reported. The individual was caught by trawl in Sardinian waters (central-western Mediterranean) in July 2019 at a depth of 500 m. Although this kind of morphological abnormality is potentially fatal, the observations suggested that the specimen was in good health and well developed.

The skin microbiome of elasmobranchs follows phylosymbiosis, but in teleost fishes, the microbiomes converge

BACKGROUND

The vertebrate clade diverged into Chondrichthyes (sharks, rays, and chimeras) and Osteichthyes fishes (bony fishes) approximately 420 mya, with each group accumulating vast anatomical and physiological differences, including skin properties. The skin of Chondrichthyes fishes is covered in dermal denticles, whereas Osteichthyes fishes are covered in scales and are mucous rich. The divergence time among these two fish groups is hypothesized to result in predictable variation among symbionts. Here, using shotgun metagenomics, we test if patterns of diversity in the skin surface microbiome across the two fish clades match predictions made by phylosymbiosis theory. We hypothesize (1) the skin microbiome will be host and clade-specific, (2) evolutionary difference in elasmobranch and teleost will correspond with a concomitant increase in host-microbiome dissimilarity, and (3) the skin structure of the two groups will affect the taxonomic and functional composition of the microbiomes.

RESULTS

We show that the taxonomic and functional composition of the microbiomes is host-specific. Teleost fish had lower average microbiome within clade similarity compared to among clade comparison, but their composition is not different among clade in a null based model. Elasmobranch's average similarity within clade was not different than across clade and not different in a null based model of comparison. In the comparison of host distance with microbiome distance, we found that the taxonomic composition of the microbiome was related to host distance for the elasmobranchs, but not the teleost fishes. In comparison, the gene function composition was not related to the host-organism distance for elasmobranchs but was negatively correlated with host distance for teleost fishes.

CONCLUSION

Our results show the patterns of phylosymbiosis are not consistent across both fish clades, with the elasmobranchs showing phylosymbiosis, while the teleost fish are not. The discrepancy may be linked to alternative processes underpinning microbiome assemblage, including possible historical host-microbiome evolution of the elasmobranchs and convergent evolution in the teleost which filter specific microbial groups. Our comparison of the microbiomes among fishes represents an investigation into the microbial relationships of the oldest divergence of extant vertebrate hosts and reveals that microbial relationships are not consistent across evolutionary timescales. Video abstract.

Potential Use of Seaweed Bioactive Compounds in Skincare-A Review

Modern lifestyles have developed new attention on appearance and personal care which attract a huge number of consumers towards cosmetic products. The demand for a skincare product with natural ingredients is rapidly increasing. Seaweeds are major resources for in-demand active compounds with a wide variety of applications. The use of seaweed-derived ingredients in cosmetic products has increased in recent years as many scientific studies have proved the potential skincare properties of seaweed bioactive compounds. This review emphasizes possible skincare properties of seaweed bioactive compounds. The review outlines the mechanism involved in skin problems including hyperpigmentation, premature skin aging, and acne in the first part while the second part focuses on the promising application of seaweeds in skin protection by highlighting the bioactive compound responsible for their bioactivity.

A Comparison of the Innate and Adaptive Immune Systems in Cartilaginous Fish, Ray-Finned Fish, and Lobe-Finned Fish

The immune system is composed of two subsystems-the innate immune system and the adaptive immune system. The innate immune system is the first to respond to pathogens and does not retain memory of previous responses. Innate immune responses are evolutionarily older than adaptive responses and elements of innate immunity can be found in all multicellular organisms. If a pathogen persists, the adaptive immune system will engage the pathogen with specificity and memory. Several components of the adaptive system including immunoglobulins (Igs), T cell receptors (TCR), and major histocompatibility complex (MHC), are assumed to have arisen in the first jawed vertebrates-the Gnathostomata. This review will discuss and compare components of both the innate and adaptive immune systems in Gnathostomes, particularly in Chondrichthyes (cartilaginous fish) and in Osteichthyes [bony fish: the Actinopterygii (ray-finned fish) and the Sarcopterygii (lobe-finned fish)]. While many elements of both the innate and adaptive immune systems are conserved within these species and with higher level vertebrates, some elements have marked differences. Components of the innate immune system covered here include physical barriers, such as the skin and gastrointestinal tract, cellular components, such as pattern recognition receptors and immune cells including macrophages and neutrophils, and humoral components, such as the complement system. Components of the adaptive system covered include the fundamental cells and molecules of adaptive immunity: B lymphocytes (B cells), T lymphocytes (T cells), immunoglobulins (Igs), and major histocompatibility complex (MHC). Comparative studies in fish such as those discussed here are essential for developing a comprehensive understanding of the evolution of the immune system.

Biochemical and biological characterization of the Hypanus americanus mucus: A perspective on stingray immunity and toxins

Stingrays skin secretions are largely studied due to the human envenoming medical relevance of the sting puncture that evolves to inflammatory events, including necrosis. Such toxic effects can be correlated to the biochemical composition of the sting mucus, according to the literature. Fish skin plays important biological roles, such as the control of the osmotic pressure gradient, protection against mechanical forces and microorganism infections. The mucus, on the other hand, is a rich and complex fluid, acting on swimming, nutrition and the innate immune system. The elasmobranch's epidermis is a tissue composed mainly by mucus secretory cells, and marine stingrays have already been described to present secretory glands spread throughout the body. Little is known about the biochemical composition of the stingray mucus, but recent studies have corroborated the importance of mucus in the envenomation process. Aiming to assess the mucus composition, a new non-invasive mucus collection method was developed that focused on peptides and proteins, and biological assays were performed to analyze the toxic and immune activities of the Hypanus americanus mucus. Pathophysiological characterization showed the presence of peptidases on the mucus, as well as the induction of edema and leukocyte recruitment in mice. The fractionated mucus improved phagocytosis on macrophages and showed antimicrobial activity against T. rubrumç. neoformans and C. albicans in vitro. The proteomic analyses showed the presence of immune-related proteins like actin, histones, hemoglobin, and ribosomal proteins. This protein pattern is similar to those reported for other fish mucus and stingray venoms. This is the first report depicting the Hypanus stingray mucus composition, highlighting its biochemical composition and importance for the stingray immune system and the possible role on the envenomation process.

Passive bristling of mako shark scales in reversing flows

Shark skin has been shown to reduce drag in turbulent boundary layer flows, but the flow control mechanisms by which it does so are not well understood. Drag reduction has generally been attributed to static effects of scale surface morphology, but possible drag reduction effects of passive or active scale actuation, or 'bristling', have been recognized more recently. Here, we provide the first direct documentation of passive scale bristling due to reversing, turbulent boundary layer flows. We recorded and analysed high-speed videos of flow over the skin of a shortfin mako shark, Isurus oxyrinchus These videos revealed rapid scale bristling events with mean durations of approximately 2 ms. Passive bristling occurred under flow conditions representative of cruise swimming speeds and was associated with two flow features. The first was a downward backflow that pushed a scale-up from below. The second was a vortex just upstream of the scale that created a negative pressure region, which pulled up a scale without requiring backflow. Both flow conditions initiated bristling at lower velocities than those required for a straight backflow. These results provide further support for the role of shark scale bristling in drag reduction.

First insights into the function of the sawshark rostrum through examination of rostral tooth microwear

Potential roles of the rostrum of sawsharks (Pristiophoridae), including predation and self-defence, were assessed through a variety of inferential methods. Comparison of microwear on the surface of the rostral teeth of sawsharks and sawfishes (Pristidae) show that microwear patterns are alike and suggest that the elongate rostra in these two elasmobranch families are used for a similar purpose (predation). Raman spectroscopy indicates that the rostral teeth of both sawsharks and sawfishes are composed of hydroxyapatite, but differ in their collagen content. Sawfishes possess collagen throughout their rostral teeth whereas collagen is present only in the centre of the rostral teeth of sawsharks, which may relate to differences in ecological use. The ratio of rostrum length to total length in the common sawshark Pristiophorus cirratus was found to be similar to the largetooth sawfish Pristis pristis but not the knifetooth sawfish Anoxypristis cuspidata. Analysis of the stomach contents of P. cirratus indicates that the diet consists of demersal fishes and crustaceans, with shrimp from the family Pandalidae being the most important dietary component. No prey item showed evidence of wounds inflicted by the rostral teeth. In light of the similarities in microwear patterns, rostral tooth chemistry and diet with sawfishes, it is hypothesised that sawsharks use their rostrum in a similar manner for predation (sensing and capturing prey) and possibly for self-defence.

The skin microbiome of the common thresher shark (Alopias vulpinus) has low taxonomic and gene function β-diversity

The health of sharks, like all organisms, is linked to their microbiome. At the skin interface, sharks have dermal denticles that protrude above the mucus, which may affect the types of microbes that occur here. We characterized the microbiome from the skin of the common thresher shark (Alopias vulpinus) to investigate the structure and composition of the skin microbiome. On average 618 812 (80.9% ± S.D. 0.44%) reads per metagenomic library contained open reading frames; of those, between 7.6% and 12.8% matched known protein sequences. Genera distinguishing the A. vulpinus microbiome from the water column included, Pseudoalteromonas (12.8% ± 4.7 of sequences), Erythrobacter (5. 3% ± 0.5) and Idiomarina (4.2% ± 1.2) and distinguishing gene pathways included, cobalt, zinc and cadmium resistance (2.2% ± 0.1); iron acquisition (1.2% ± 0.1) and ton/tol transport (1.3% ± 0.08). Taxonomic community overlap (100 - dissimilarity index) was greater in the skin microbiome (77.6), relative to the water column microbiome (70.6) and a reference host-associated microbiome (algae: 71.5). We conclude the A. vulpinus skin microbiome is influenced by filtering processes, including biochemical and biophysical components of the shark skin and result in a structured microbiome.

Morphometry and microanatomy of the barbels of the common sawshark Pristiophorus cirratus (Pristiophoridae): implications for pristiophorid behaviour

The internal anatomy of the barbels of the common sawshark Pristiophorus cirratus was examined with light microscopy to clarify their sensory role. No sensory structures such as taste buds (chemoreception), ampullae of Lorenzini (electroreception) or free neuromasts (lateral line mechanoreception) could be located in the barbels. The presence of bundles of nerve fibres, however, indicates a tactile function for the barbels. Conveyance of information regarding potentially damaging stimuli (nociception) and temperature (thermoception) cannot be excluded at this stage. It is hypothesized that the barbels are used by P. cirratus to locate prey in both the water column and on the substratum via wake detection and sensing changes in surface texture. The barbels may also be involved in the detection of water currents for rheotaxis. Regression analyses on P. cirratus morphometric data showed that the width of the rostrum at two sections (the barbels and the rostrum tip) does not significantly correlate with total length. The regression analyses also suggested that the barbels of P. cirratus may be lateralised.

Thorns and dermal denticles of skates Atlantoraja cyclophora and A. castelnaui: Microscopic features and functional implications

Some batoid species are covered with dermal denticles (or placoid scales) that occasionally develop into thorns. In sexually mature males, sharp teeth and alar thorns found on the apex of the lateral disc are used to hold the female during copulation. This study set out to analyze microscopic features of modified dermal denticles and thorns and to investigate sexual dimorphism in Atlantoraja cyclophora and A. castelnaui species. Skin samples collected from areas covered with thorns were fixed in 10% formaldehyde, processed and analyzed using scanning electron microscopy. Alar thorn morphology varied within species, while caudal thorn, rostral and caudal dermal denticle morphology varied within and between species. These structures play an important role in the protection and reproduction of the species studied and constitute important taxonomic information, given they are often the only elements preserved in archaeological sites and fossil records.

Cutaneous malignant melanoma in a Haller's round ray Urobatis halleri

Multiple black raised nodular masses were noted on the dorsal surface of an adult male Haller's round ray Urobatis halleri. Biopsy of 2 masses was performed, and histopathology revealed proliferative sheets of melanocytes exhibiting mild anisocytosis and anisokaryosis, supporting a diagnosis of malignant melanoma. Approximately 2 mo following the biopsy procedure, the round ray became acutely anorexic and was found dead in its enclosure. A full necropsy was performed, and tissues were submitted for histopathology. The black raised nodular masses again exhibited histologic features of a melanoma. In addition to the nodular masses present, multiple flat areas of increased pigmentation were also present throughout the course of the case and were not suggestive of neoplasia histologically. The transformation of benign to malignant neoplasia has been well described in other species and may have played a role in the development of multiple tumors in this case.

Hydrodynamic function of biomimetic shark skin: effect of denticle pattern and spacing

The structure of shark skin has been the subject of numerous studies and recently biomimetic shark skin has been fabricated with rigid denticles (scales) on a flexible substrate. This artificial skin can bend and generate thrust when attached to a mechanical controller. The ability to control the manufacture of biomimetic shark skin facilitates manipulation of surface parameters and understanding the effects of changing denticle patterns on locomotion. In this paper we investigate the effect of changing the spacing and arrangement of denticles on the surface of biomimetic shark skin on both static and dynamic locomotor performance. We designed 3D-printed flexible membranes with different denticle patterns and spacings: (1) staggered-overlapped, (2) linear-overlapped, and (3) linear-non-overlapped, and compared these to a 3D-printed smooth-surfaced control. These 3D printed shark skin models were then tested in a flow tank with a mechanical flapping device that allowed us to either hold the models in a stationary position or move them dynamically. We swam the membranes at a frequency of 1 Hz with different heave amplitudes (from ±1 cm to ±3 cm) while measuring forces, torques, self-propelled swimming speed, and cost of transport (COT). Static tests revealed drag reduction of denticle patterns compared to a smooth control at low speeds, but increased drag at speeds above 25 cm s(-1). However, during dynamic (swimming) tests, the staggered-overlapped pattern produced the fastest swimming speeds with no significant increase in the COT at lower heave values. For instance, at a heave frequency of 1 Hz and amplitude of ±1 cm, swimming speed of the staggered-overlapped pattern increased by 25.2% over the smooth control. At higher heave amplitudes, significantly faster self-propelled swimming speeds were achieved by the staggered-overlapped pattern, but with higher COT. Only the staggered-overlapped pattern provides a significant swimming performance advantage over the smooth control and the other two denticle patterns. Quantitative hydrodynamic comparisons among skin models where control over manufacture allows alteration of design parameters provides a useful experimental tool for future work on the considerable natural diversity of shark skin denticles both among species and on different body locations.

Denticle-embedded ampullary organs in a Cretaceous shark provide unique insight into the evolution of elasmobranch electroreceptors

Here, we report a novel type of dermal denticle (or placoid scale), unknown among both living and fossil chondrichthyan fishes, in a Cretaceous lamniform shark. By their morphology and location, these dermal denticles, grouped into clusters in the cephalic region, appear to have been directly associated with the electrosensory ampullary system. These denticles have a relatively enlarged (∼350 μm in diameter), ornamented crown with a small (∼100 μm) asterisk- or cross-shaped central perforation connected to a multi-alveolate internal cavity. The formation of such a complex structure can be explained by the annular coalescence and fusion, around an ampullary vesicle, of several developmental units still at papillary stage (i.e. before mineralization), leading to a single denticle embedding an alveolar ampulla devoid of canal. This differs from larger typical ampullae of Lorenzini with a well-developed canal opening in a pore of the skin and may represent another adaptive response to low skin resistance. Since it has been recently demonstrated that ampullary organs arise from lateral line placodes in chondrichthyans, this highly specialized type of dermal denticle (most likely non-deciduous) may be derived from the modified placoid scales covering the superficial neuromasts (pit organs) of the mechanosensory lateral line system of many modern sharks.

Biological characterization of the skin of shortfin mako shark Isurus oxyrinchus and preliminary study of the hydrodynamic behaviour through computational fluid dynamics

This study characterized the morphology, density and orientation of the dermal denticles along the body of a shortfin mako shark Isurus oxyrinchus and identified the hydrodynamic parameters of its body through a computational fluid-dynamics model. The study showed a great variability in the morphology, size, shape, orientation and density of dermal denticles along the body of I. oxyrinchus. There was a significant higher density in dorsal and ventral areas of the body and their highest angular deviations were found in the lower part of the mouth and in the areas between the pre-caudal pit and the second dorsal and pelvic fins. A detailed three-dimensional geometry from a scanned body of a shark was carried out to evaluate the hydrodynamic properties such as drag coefficient, lift coefficient and superficial (skin) friction coefficient of the skin together with flow velocity field, according to different roughness coefficients simulating the effect of the dermal denticles. This preliminary approach contributed to detailed information of the denticle interactions. As the height of the denticles was increased, flow velocity and the effect of lift decreased whereas drag increased. The highest peaks of skin friction coefficient were observed around the pectoral fins.