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Outline of a representative shortfin mako shark Isurus oxyrinchus showing the 16 regions sampled for scale photography, angle measurement, and histology. The mean erection angle for five scales in each region is given beside the nomenclature for each region. Flank scales (B2, B5, A2) are capable of greater erection angles than dorsal (B1, B4, A1) and ventral (B3, B6, A3) regions. All regions on the pectoral fin (P 1, P 2, P 3) differed in the degree of erection possible with the leading edge scales (P 1) incapable of erection. Erection angle did not differ among the three regions on the caudal fin (C1, C2, C3).
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An investigation into the separation control mechanisms found on the skin of fast-swimming sharks, with a particular focus on the shortfin mako (Isurus oxyrinchus) which is considered to be one of the fastest pelagic shark species, was carried out. Previous researchers have reported a bristling capability of the scales, or denticles, in certain spe...
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Citations
... The other more popular perspective indicates that the vortices (they are called boundary vortices in this paper) within transverse grooves change the sliding friction into the rolling friction at the solid-liquid interface, which is also named the "micro air-bearing phenomenon" [9][10][11][12][13][14][15]. In addition, the studies of Mariotti et al. [16,17], Pasqualetto et al. [18], Howard et al. [19], and Lang et al. [3,20] showed that the vortices formed in the transverse grooves (and small cavities) increase the momentum in the boundary layer near the wall, thus effectively controlling the flow separation. However, none of the qualitative descriptions of the drag-reduction is qualitatively analyzed to prove that it is also a necessary condition for obtaining the optimal drag-reduction rate. ...
... 20 show the evolution of the instantaneous spanwise vorticity in the grooves with different ARs at Re = 5.44E4, where the ARs = 0.5 and 8 (Figure 18) at region III, the ARs = 1 and 5 (Figure 19) belonging to region II, and the AR = 2 (Figure 20) at region I were predicted theoretically as shown inFigure 12. The 3D isometric of the instantaneous coherent structures with indicator λ ci . ...
Previous studies have implied that the AR (aspect ratio) of the transverse groove significantly affects the stability of the boundary vortex within the groove and thus drives the variation in the drag-reduction rate. However, there is no theoretical model describing the relationship between the AR and the stability of the boundary vortex, resulting in difficulty in developing a forward method to obtain the optimum AR. In this paper, the velocity potential of the groove sidewalls to the boundary vortex is innovatively described by an image vortex model, thus establishing the relationship between the AR and the induced velocity. Secondly, the velocity profile of the migration flow is obtained by decomposing the total velocity inside the groove, by which the relationship between the AR and the migration velocity is established. Finally, the analytical solution of the optimal AR (ARopt = 2.15) is obtained based on the kinematic condition for boundary vortex stability, i.e., the induced velocity equals the migration velocity, and the forms of boundary vortex motion at other ARs are discussed. Furthermore, the stability of the boundary vortex at the optimal AR and the corresponding optimal drag-reduction rate are verified by the large eddy simulations method. At other ARs, the motion forms of the boundary vortex are characterized by “vortex shedding” and “vortex sloshing,” respectively, and the corresponding drag-reduction rates are smaller than those for vortex stability.
... One function that has been an important focus for research is the role of denticles in support of locomotion, as many extant shark species have denticles with morphologies that improve swimming performance. Fluid dynamic studies have revealed that denticles can improve swimming performance by enhancing thrust, reducing hydrodynamic drag, as well as changing the boundary layer characteristics of water flow over the body (DuClos et al., 2018;Lang et al., 2012; Lauder et al., 2016;Oeffner and Lauder, 2012). Hydrodynamic studies using foils covered in real pieces of shark skin or 3D-printed shark skin models have offered an additional understanding of how denticles may function in flow (Domel et al., 2018;Lauder et al., 2016;Oeffner and Lauder, 2012;Wen et al., 2014;Wen et al., 2015). ...
Synopsis
Shark skin is covered in dermal denticles—tooth-like structures consisting of enameloid, dentine, and a central pulp cavity. Previous studies have demonstrated differences in denticle morphology both among species and across different body regions within a species, including one report of extreme morphological variation within a 1 cm distance on the skin covering the branchial pouches, a region termed “interbranchial skin.” We used gel-based profilometry, histology, and scanning electron microscopy to quantify differences in denticle morphology and surface topography of interbranchial skin denticles among 13 species of sharks to better understand the surface structure of this region. We show that (1) interbranchial skin denticles differ across shark species, and (2) denticles on the leading edge of the skin covering each gill pouch have different morphology and surface topography compared with denticles on the trailing edge. Across all species studied, there were significant differences in denticle length (P = 0.01) and width (P = 0.002), with shorter and wider leading edge denticles compared with trailing edge denticles. Surface skew was also higher in leading edge denticles (P = 0.009), though most values were still negative, indicating a surface texture more dominated by valleys than peaks. Overall, leading edge denticles were smoother-edged than trailing edge denticles in all of the species studied. These data suggest two hypotheses: (1) smoother-edged leading edge denticles protect the previous gill flap from abrasion during respiration, and (2) ridged denticle morphology at the trailing edge might alter water turbulence exiting branchial pouches after passing over the gills. Future studies will focus on determining the relationship between denticle morphology and water flow by visualizing fluid motion over interbranchial denticles during in vivo respiration.
... While teleost skin generally has scales, elasmobranch skin is covered with toothlike projections called dermal denticles. Also known as placoid scales, dermal denticles act as body armor and reduce drag during swimming [86,92]. In most sharks, dermal denticles cover the entire body but vary in size and structure among species with different swimming modes (e.g., slow vs. fast; [93]). ...
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.
... They reported that the change of size and shape of the denticles affect the boundary layer and hence the reduce the drag. Amy Lang et al. [11] studied short fin mako (Isurus oxyrinchus) in shark skin and reported that scales act as a passive, flow-actuated mechanism that controls the flow separation. Martínez-Filgueira et al. [12] conducted parametric study on low profile vortex generator that has vane incident angle of 18.5°. ...
Fluid flow control is vital in submarine operations to prevent drag of crossflow separation and boundary layer separation. This research attempts to use bristled shark skin vortex generator to control the fluid around the submarine and to reduce drag during operation of submarine. The conventional vortex generator is delta wing shaped which unbaled to generate vortices efficiently. The effect of using biologically inspired vortex generator on hydrodynamic behaviour of submarine is studied through Computational Fluid Dynamic analysis. The analysis is carried out for linear movement, 30° yaw movement and 30° pitch movement of the submarine. Further wind tunnel experiments are conducted to validate the numerical simulation results. The numerical analysis and experiments reveal that the use of bristled shark skin vortex generator in the lateral side of submarine reduces the drag drastically in linear, yaw and pitch movements of the submarine. It assists in crossflow separation and moves the fluid away from the body of the submarine. During yaw and pitch movement, it moves the fluid towards the same direction as the submarine turns, which subsequently assists in generating additional torque during the turning.
... To solve the problem of flow separation, researchers have studied nature's approach to flow control. A non-exhaustive list of biological samples investigated as a means of passive flow control include bird feathers [4,5], butterfly wings [6], fish fins [7,8], and shark denticles [9][10][11][12]. ...
... The over-actuation of the flap, defined when the optimal bristling angle range is exceeded to an extent where the flaps worsen the flow conditions, is interesting to note since on the shortfin mako shark the bristling angle is physically limited to 50°, thus making overactuation impossible. Building off this study, it was theorized that the shark scales work in a similar manner to the movable flaps but on a micro-scale level [9,10]. ...
... This geometry is reproduced on a larger scale for the models used in the turbulent boundary layer study, but a simplified version is used in the laminar boundary layer study. The shark scales have been theorized to work in a similar manner to bird feather-inspired flaps on a micro-scale level [9,10] by impeding the reversing flow in the region. It is likely that the actuation height of movable flaps relative to the boundary layer thickness is an important parameter for the effectiveness of flow control. ...
The skin of fast-swimming sharks has been shown to have mechanisms able to reduce flow separation in both laminar and turbulent flows. This study analyzes arrays of bio-inspired microflaps and scales in a separated region generated by an adverse pressure gradient in a water tunnel environment. In the laminar boundary layer case, the microflap model bristles due to vortex interaction. This bristling controls the separation downstream of the model, reducing overall reversing flow by up to 59%. This investigation finds that the height of the protrusion into the boundary layer is an crucial factor in controlling the reversing flow. For the turbulent boundary layer, arrays of manufactured scales are tested in weak and strong adverse pressure gradients, controlled by a rotating cylinder. It has been found that the scales are ineffective at controlling separation in the weaker adverse pressure gradient case and increase the separation. However, in the stronger adverse pressure gradient conditions, the scales are highly effective at controlling separation, reducing the reversing flow by up to 70%. Additionally, the models are able to reattach the flow in extreme separation conditions.
... Comparing pennation angles allowed for comparisons of potential force transmission to the tendons in the forelimb muscles (Brown et al. 2003). Within the tapir limb itself, one-way analyses of variance (ANOVAs) and Tukey-B post-hoc tests were performed to test for significant differences between pennation angles (following successful normality testing using Shapiro-Wilk test; Lang et al. 2012;Hady et al. 2015;Arruda et al. 2018). Significant differences in pennation angle were tested for between the lower forelimb flexors (FDS + P, FCU, FCR) and extensors (UL, ECR). ...
The absence of preserved soft tissues in the fossil record is frequently a hindrance for palaeontologists wishing to investigate morphological shifts in key skeletal systems, such as the limbs. Understanding the soft tissue composition of modern species can aid in understanding changes in musculoskeletal features through evolution, including those pertaining to locomotion. Establishing anatomical differences in soft tissues utilising an extant phylogenetic bracket can, in turn, assist in interpreting morphological changes in hard tissues and modelling musculoskeletal movements during evolutionary transitions (e.g. digit reduction in perissodactyls). Perissodactyls (horses, rhinoceroses, tapirs and their relatives) are known to have originated with a four‐toed (tetradactyl) forelimb condition. Equids proceeded to reduce all but their central digit, resulting in monodactyly, whereas tapirs retained the ancestral tetradactyl state. The modern Malayan tapir (Tapirus indicus) has been shown to exhibit fully functional tetradactyly in its forelimb, more so than any other tapir, and represents an ideal case‐study for muscular arrangement and architectural comparison with the highly derived monodactyl Equus. Here, we present the first quantification of muscular architecture of a tetradactyl perissodactyl (T. indicus), and compare it to measurements from modern monodactyl caballine horse (Equus ferus caballus). Each muscle of the tapir forelimb was dissected out from a cadaver and measured for architectural properties: muscle‐tendon unit (MTU) length, MTU mass, muscle mass, pennation angle, and resting fibre length. Comparative parameters [physiological cross‐sectional area (PCSA), muscle volume, and % muscle mass] were then calculated from the raw measurements. In the shoulder region, the infraspinatus of T. indicus exhibits dual origination sites on either side of the deflected scapular spine. Within ungulates, this condition has only been previously reported in suids. Differences in relative contribution to limb muscle mass between T. indicus and Equus highlight forelimb muscles that affect mobility in the lateral and medial digits (e.g. extensor digitorum lateralis). These muscles were likely reduced in equids during their evolutionary transition from tetradactyl forest‐dwellers to monodactyl, open‐habitat specialists. Patterns of PCSA across the forelimb were similar between T. indicus and Equus, with the notable exceptions of the biceps brachii and flexor carpi ulnaris, which were much larger in Equus. The differences observed in PCSA between the tapir and horse forelimb muscles highlight muscles that are essential for maintaining stability in the monodactyl limb while moving at high speeds. This quantitative dataset of muscle architecture in a functionally tetradactyl perissodactyl is a pivotal first step towards reconstructing the locomotor capabilities of extinct, four‐toed ancestors of modern perissodactyls, and providing further insights into the equid locomotor transition.
... More recent work by the authors [11][12][13] has investigated the mechanism by which shark skin bristling may lead to the development of a passive, flowactuated mechanism for separation control. The current working hypothesis is that the passive, flexible scales of the shark work as microflaps to locally control flow separation as needed on crucial regions of the body where it most often occurs during swimming maneuvers. ...
... Because the flank scales are very loosely attached to the skin and highly mobile, these observations led us to infer that the scales are most likely bristled by reverse flow actuation. Sixteen body locations [12,13] were considered, six on fins and ten on the body. Scales were manipulated by a fine acupuncture needle and remained bristled once manipulated as shown in Fig. 4. ...
... By responding to the pressure fluctuations across the surface, a compliant material on the surface of an object in a fluid flow has been shown to be beneficial. Studies have reported 7% drag reduction [12]. ...
In sharks, the skin is a biological composite with mineralized denticles embedded within a collagenous matrix. Swimming performance is enhanced by the dermal denticles on the skin, which have drag reducing properties produced by regional morphological variations and changes in density along the body. We used mechanical testing to quantify the effect of embedded mineralized denticles on the quasi-static tensile properties of shark skin to failure in four coastal species. We investigated regional differences in denticle density and skin properties by dissecting skin from the underlying fascia and muscle at 10 anatomical landmarks. Hourglass-shaped skin samples were extracted in the cranial to caudal orientation. Denticle density was quantified and varied significantly among both regions and species. We observed the greatest denticle densities in the cranial region of the body for the bonnethead, scalloped hammerhead, and bull sharks. Skin samples were then tested in tension until failure, stress strain curves were generated, and mechanical properties calculated. We found significant species and region effects for all three tensile mechanical properties. We report the greatest ultimate tensile strength, stiffness, and toughness near the cranial and lateral regions of the body for all 4 of the coastal species. We also report that denticle density increases with skin stiffness but decreases with toughness.
The Shortfin Mako shark (Isurus oxyrinchus) is a fast swimmer and has incredible turning agility, and has flexible scales known to bristle up to 50° in the flank regions. It is purported that this bristling capability of the scales may result in a unique pass flow control method to control flow separation and reduce drag. It appears that the scales have evolved to be only actuated when the flow over the body is reversed; thereby inducing a method of inhibiting flow reversal close to the surface. In addition, bristled scales form cavities which could induce boundary layer mixing and further assist in delaying flow separation. To substantiate the hypothesis, samples of skin from the flank region of the mako have been tested in a water tunnel facility under various strengths of adverse pressure gradient (APG). Laminar and turbulent separation over the skin was studied experimentally using time-resolved digital particle image velocimetry, where the APG was generated and varied using a rotating cylinder. Shark skin results were compared with that of a smooth plate data for a given amount of APG. Both the instantaneous and time-averaged results reveal that shark skin is capable of controlling laminar as well as turbulent separation. Under laminar conditions, the shark skin also induces an early transition to turbulence and reduces the degree of laminar separation. For turbulent separation, the presence of the shark skin reduces the amount of backflow and size of the separation region. Under both flow conditions, the shark skin also delayed the point of separation as compared to a smooth wall.
