Convergence of undulatory swimming kinematics across a diversity of fishes.

Fishes exhibit an astounding diversity of locomotor behaviors from classic swimming with their body and fins to jumping, flying, walking, and burrowing. Fishes that use their body and caudal fin (BCF) during undulatory swimming have been traditionally divided into modes based on the length of the propulsive body wave and the ratio of head:tail oscillation amplitude: anguilliform, subcarangiform, carangiform, and thunniform. This classification was first proposed based on key morphological traits, such as body stiffness and elongation, to group fishes based on their expected swimming mechanics. Here, we present a comparative study of 44 diverse species quantifying the kinematics and morphology of BCF-swimming fishes. Our results reveal that most species we studied share similar oscillation amplitude during steady locomotion that can be modeled using a second-degree order polynomial. The length of the propulsive body wave was shorter for species classified as anguilliform and longer for those classified as thunniform, although substantial variability existed both within and among species. Moreover, there was no decrease in head:tail amplitude from the anguilliform to thunniform mode of locomotion as we expected from the traditional classification. While the expected swimming modes correlated with morphological traits, they did not accurately represent the kinematics of BCF locomotion. These results indicate that even fish species differing as substantially in morphology as tuna and eel exhibit statistically similar two-dimensional midline kinematics and point toward unifying locomotor hydrodynamic mechanisms that can serve as the basis for understanding aquatic locomotion and controlling biomimetic aquatic robots.

Sticky, stickier and stickiest - a comparison of adhesive performance in clingfish, lumpsuckers and snailfish.

The coastal waters of the North Pacific are home to the northern clingfish (Gobiesox maeandricus), Pacific spiny lumpsucker (Eumicrotremus orbis) and marbled snailfish (Liparis dennyi) - three fishes that have evolved ventral adhesive discs. Clingfish adhesive performance has been studied extensively, but relatively little is known about the performance of other sticky fishes. Here, we compared the peak adhesive forces and work to detachment of clingfish, lumpsuckers and snailfish on surfaces of varying roughness and over ontogeny. We also investigated the morphology of their adhesive discs through micro-computed tomography scanning and scanning electron microscopy. We found evidence that adhesive performance is tied to the intensity and variability of flow regimes in the fishes' habitats. The northern clingfish generates the highest adhesive forces and lives in the rocky intertidal zone where it must resist exposure to crashing waves. Lumpsuckers and snailfish both generate only a fraction of the clingfish's adhesive force, but live more subtidal where currents are slower and less variable. However, lumpsuckers generate more adhesive force relative to their body weight than snailfish, which we attribute to their higher-drag body shape and frequent bouts into the intertidal zone. Even so, the performance and morphology data suggest that snailfish adhesive discs are stiffer and built more efficiently than lumpsucker discs. Future studies should focus on sampling additional diversity and designing more ecologically relevant experiments when investigating differences in adhesive performance.

Ecologically driven selection of nonstructural carbohydrate storage in oak trees.

Leaf habit is a major axis of plant diversity that has consequences for carbon balance since the leaf is the primary site of photosynthesis. Nonstructural carbohydrates (NSCs) produced by photosynthesis can be allocated to storage and serve as a resiliency mechanism to future abiotic and biotic stress. However, how leaf habit affects NSC storage in an evolutionary context has not been shown. Using a comparative physiological framework and an analysis of evolutionary model fitting, we examined if variation in NSC storage is explained by leaf habit. We measured sugar and starch concentrations in 51 oak species (Quercus spp.) growing in a common garden and representing multiple evolutions of three different leaf habits (deciduous, brevideciduous and evergreen). The best fitting evolutionary models indicated that deciduous oak species are evolving towards higher NSC concentrations than their brevideciduous and evergreen relatives. Notably, this was observed for starch (the primary storage molecule) in the stem (a long-term C storage organ). Overall, our work provides insight into the evolutionary drivers of NSC storage and suggests that a deciduous strategy may confer an advantage against stress associated with a changing world. Future work should examine additional clades to further corroborate this idea.

Ontogenetic scaling patterns of lizard skin surface structure as revealed by gel-based stereo-profilometry.

The skin surface structure of squamate reptiles varies greatly among species, likely because it plays a key role in a range of tasks, such as camouflage, locomotion, self-cleaning, mitigation of water loss and protection from physical damage. Although we have foundational knowledge about squamate skin morphology, we still know remarkably little about how intraspecific variation in skin surface structure translates to functional variation. This gap in our understanding can be in part traced back to: (i) our lack of knowledge on how body size determines skin surface structure; and (ii) the lack of means to perform high-throughput and detailed analysis of the three-dimensional (3D) anatomy of reptilian skin surfaces in a non-destructive manner. To fill this gap, we explored the possibilities of a new imaging technique, termed gel-based stereo-profilometry, to visualize and quantify the 3D topography of reptilian skin surface structure. Using this novel approach, we investigated intra-specific and intra-individual variation in the skin surface morphology of a focal lizard species, Anolis cristatellus. We assessed how various characteristics of surface topography (roughness, skew and kurtosis) and scale morphology (area, height, width and shape) scale with body size across different body regions. Based on an ontogenetic series of A. cristatellus males, we show that skin roughness increases with body size. Skin patches on the ventral body region of lizards were rougher than on the dorsum, but this was a consequence of ventral scales being larger than dorsal scales. Dorsal surface skew and kurtosis varied with body size, but surfaces on the ventral skin showed no such relationship. Scale size scaled isometrically with body size, and while ventral scales differed in shape from dorsal scales, scale shape did not change with ontogeny. Overall, this study demonstrates that gel-based stereo-profilometry is a promising method to rapidly assess the 3D surface structure of reptilian skin at the microscopic level. Additionally, our findings of the explanatory power of body size on skin surface diversity provide a foundation for future studies to disentangle the relationships among morphological, functional and ecological diversity in squamate reptile skin surfaces.

How smooth is a dolphin? The ridged skin of odontocetes.

Dolphin skin has long been an inspiration for research on drag reduction mechanisms due to the presence of skin ridges that could reduce fluid resistance. We gathered in vivo three-dimensional surface data on the skin from five species of odontocetes to quantitatively examine skin texture, including the presence and size of ridges. We used these data to calculate k+ values, which relate surface geometry to changes in boundary layer flow. Our results showed that while ridge size differs among species, odontocete skin was surprisingly smooth compared to the skin of other swimmers (average roughness = 5.3 µm). In addition, the presence of ridges was variable among individuals of the same species. We predict that odontocete skin ridges do not alter boundary layer flows at cruising swimming speeds. By combining k+ values and morphological data, our work provides evidence that skin ridges are unlikely to be an adaptation for drag reduction and that odontocete skin is exceptionally smooth compared to other pelagic swimmers.

Attachment to challenging substrates--fouling, roughness and limits of adhesion in the northern clingfish (Gobiesox maeandricus).

Northern clingfish use a ventral suction disc to stick to rough substrates in the intertidal zone. Bacteria, algae and invertebrates grow on these surfaces (fouling) and change the surface properties of the primary substrate, and therefore the attachment conditions for benthic organisms. In this study, we investigate the influence of fouling and surface roughness on the adhesive strength of northern clingfish, Gobiesox maeandricus. We measured clingfish tenacity on unfouled and fouled substrates over four surface roughnesses. We exposed surfaces for 6 weeks in the Pacific Ocean, until they were covered with periphyton. Clingfish tenacity is equivalent on both fouled and unfouled smooth substrates; however, tenacity on fouled rough surfaces is less compared with tenacity on unfouled ones. We hypothesize that parts of biofilm may act as a lubricant and decrease friction of the disc margin, thereby making disc margins slip inwards and fail at lower tenacities. Nevertheless, even on fouled surfaces the adhesive forces are approximately 150 times the body weight of the fish. To identify the upper threshold of surface roughness the fish can cling to, we tested seven unfouled substrates of increasing surface roughness. The threshold roughness at which northern clingfish failed increased with specimen size. We hypothesize that because of the elastic properties of the disc margin, a larger disc can adapt to larger surface irregularities. The largest specimens (length 10-12 cm) were able to cling to surfaces with 2-4 mm grain size. The fish can attach to surfaces with roughness between 2 and 9% of the suction disc width.

Hydrodynamic function of the slimy and scaly surfaces of teleost fishes.

The scales and skin mucus of bony fishes are both proposed to have a role in beneficially modifying the hydrodynamics of water flow over the body surface. However, it has been challenging to provide direct experimental evidence that tests how mucus and fish scales change the boundary layer in part due to the difficulties in working with live animal tissue and difficulty directly imaging the boundary layer. In this manuscript we use direct imaging and flow tracking within the boundary layer to compare boundary layer dynamics over surfaces of fish skin with mucus, without mucus, and a flat control surface. Our direct measurements of boundary layer flows for these three different conditions are repeated for two different species, bluegill sunfish (Lepomis macrochirus) and blue tilapia (Oreochromis aureus). Our goals are to understand if mucus and scales reduce drag, shed light on mechanisms underlying drag reduction, compare these results between species, and evaluate the relative contributions to hydrodynamic function for both mucus and scales. We use our measurements of boundary layer flow to calculate shear stress (proportional to friction drag), and we find that mucus reduces drag overall by reducing the velocity gradient near the skin surface. Both bluegill and tilapia showed similar patterns of surface velocity reduction. We also note that scales alone do not appear to reduce drag, but that mucus may reduce friction drag up to 50% compared to scaled surfaces without mucus or flat controls.

Stick tight: suction adhesion on irregular surfaces in the northern clingfish.

The northern clingfish, Gobiesox maeandricus, is able to adhere to slippery, fouled and irregular surfaces in the marine intertidal environment. We have found that the fish can adhere equally well to surfaces with a broad range of surface roughness, from the finest sandpaper (R(a) = 15 µm) to textures suitable for removing finish from flooring (R(a) = 269 µm). The fishes outperform man-made suction cups, which only adhere to the smoothest surfaces. The adhesive forces of clingfish correspond to pressures 0.2-0.5 atm below ambient and are 80-230 times the body weight of the fish. The tenacity appears related to hierarchically structured microvilli around the edges of the adhesive disc that are similar in size and aspect ratio to the setae found on the feet of geckoes, spiders and insects. This points to a possible biomimetic solution to the problem of reversibly adhering to irregular, submerged surfaces.

Lacustrine speciation associated with chromosomal inversion in a lineage of riverine fishes.

Geographic isolation is the primary driver of speciation in many vertebrate lineages. This trend is exemplified by North American darters, a clade of freshwater fishes where nearly all sister species pairs are allopatric and separated by millions of years of divergence. One of the only exceptions is the Lake Waccamaw endemic Etheostoma perlongum and its riverine sister species Etheostoma maculaticeps, which have no physical barriers to gene flow. Here we show that lacustrine speciation of E. perlongum is characterized by morphological and ecological divergence likely facilitated by a large chromosomal inversion. While E. perlongum is phylogenetically nested within the geographically widespread E. maculaticeps, there is a sharp genetic and morphological break coinciding with the lake-river boundary in the Waccamaw River system. Despite recent divergence, an active hybrid zone, and ongoing gene flow, analyses using a de novo reference genome reveal a 9 Mb chromosomal inversion with elevated divergence between E. perlongum and E. maculaticeps. This region exhibits striking synteny with known inversion supergenes in two distantly related fish lineages, suggesting deep evolutionary convergence of genomic architecture. Our results illustrate that rapid, ecological speciation with gene flow is possible even in lineages where geographic isolation is the dominant mechanism of speciation.

Fish-inspired segment models for undulatory steady swimming.

Many aquatic animals swim by undulatory body movements and understanding the diversity of these movements could unlock the potential for designing better underwater robots. Here, we analyzed the steady swimming kinematics of a diverse group of fish species to investigate whether their undulatory movements can be represented using a series of interconnected multi-segment models, and if so, to identify the key factors driving the segment configuration of the models. Our results show that the steady swimming kinematics of fishes can be described successfully using parsimonious models, 83% of which had fewer than five segments. In these models, the anterior segments were significantly longer than the posterior segments, and there was a direct link between segment configuration and swimming kinematics, body shape, and Reynolds number. The models representing eel-like fishes with elongated bodies and fishes swimming at high Reynolds numbers had more segments and less segment length variability along the body than the models representing other fishes. These fishes recruited their anterior bodies to a greater extent, initiating the undulatory wave more anteriorly. Two shape parameters, related to axial and overall body thickness, predicted segment configuration with moderate to high success rate. We found that head morphology was a good predictor of its segment length. While there was a large variation in head segments, the length of tail segments was similar across all models. Given that fishes exhibited variable caudal fin shapes, the consistency of tail segments could be a result of an evolutionary constraint tuned for high propulsive efficiency. The bio-inspired multi-segment models presented in this study highlight the key bending points along the body and can be used to decide on the placement of actuators in fish-inspired robots, to model hydrodynamic forces in theoretical and computational studies, or for predicting muscle activation patterns during swimming.

Dermal Denticle Diversity in Sharks: Novel Patterns on the Interbranchial Skin.

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.

La piel de los tiburones está cubierta de dentículos dérmicos, estructuras similares a los dientes que constan de un tejido esmaltado, una dentina y una cavidad pulpar central. Estudios anteriores han demostrado diferencias en la morfología de los dentículos tanto entre especies como entre diferentes regiones del cuerpo dentro de una misma especie, incluyendo un informe sobre la extrema variación morfológica dentro de una distancia de 1 cm en la piel que cubre las bolsas branquiales, una región denominada "piel interbranquial." Hemos utilizado perfilometría basada en gel, histología y microscopía electrónica de barrido, para cuantificar las diferencias en la morfología de los dentículos y la topografía de la superficie de la piel interbranquial de los dentículos en 13 especies de tiburones, para comprender mejor la estructura de la superficie de esta región. Demostramos que (1) los dentículos de la piel interbranquial difieren entre las especies de tiburones, y (2) los dentículos del borde anterior de la piel que cubre cada bolsa branquial tienen una morfología y una topografía superficial diferentes en comparación con los dentículos del borde posterior. En todas las especies estudiadas, hubo diferencias significativas en la longitud (P = 0.01) y en el ancho (P = 0.002), con dentículos del borde anterior más cortos y anchos que los del borde posterior. La inclinación de la superficie también era mayor en los dentículos del borde anterior (P = 0.009) aunque la mayoría de los valores seguían siendo negativos, lo que indicaba más valles que picos. En general, los dentículos de la parte anterior tenian los bordes mas lisos que los de la parte posterior en todas las especies estudiadas. Estos datos sugieren dos hipótesis: (1) los dentículos del borde anterior con bordes más lisos protegen la aleta branquial previa de la abrasión durante la respiración, y (2) la morfología de los dentículos con crestas en el borde posterior podría alterar la turbulencia del agua que sale de las bolsas branquiales después de pasar por las branquias. Futuros estudios se centrarán en determinar la relación entre la morfología de los dentículos y el flujo de agua mediante la visualización del movimiento del fluido sobre los dentículos interbranquiales durante la respiración in vivo.Translated by Laura Paez, Ph.D. student­Swiss Federal Institute of Technology Lausanne.

 ìƒì–´ì˜ 비늘은 유법랑질, 상아질, 치수강으로 이루어졌으며, 낱개로 ë³´ë©´ 생김새가 이빨을 닮았다. 기존의 연구에서는 상어 비늘의 형태학적 구조를 종 간 및 동종 내 다른 어체 부위의 다양한 각도에서 분석했는데, ê·¸ 중에서도 아가미구멍 사이의 1cm에 불과한 범위에서 예상 외의 표면미세구조 다양성을 발견한 연구가 주목된다. 상어 비늘의 형태학적 이해를 돕기 위하여, 본 연구에서는 젤을 사용한 프로필로메트리(gel-based profilometry), 조직학적 기법 및 주사전자현미경법을 통하여, 상어류 13종에서 아가미구멍 사이 피부의 비늘 형태와 표면미세구조를 분석했다. 본 연구의 결과는 (1) 아가미구멍 사이 피부의 표면형태에는 종 간 차이가 있고, (2) 아가미구멍 사이 피부의 전방 (머리 방향) 비늘은 후방 (꼬리 방향) 의 비늘에 비하여 훨씬 더 큰 수준의 다양성을 보였다는 것이다. 분석한 13종 모두를 통틀어, 아가미구멍 사이 피부의 전방 비늘은 후방의 비늘 보다 훨씬 넓고 (P = 0.01) ê¸¸ì—ˆë‹¤ (P = 0.002). 본 연구에서는 (1) 부드러운 머리 쪽 비늘이 상어가 숨 쉴 때마다 아가미구멍을 통해 빠져나온 물 흐름의 저항을 줄여준다는 것, 그리고 (2) 꼬리 쪽 가장자리의 비늘에서 두드러지는 들쭉날쭉한 가장자리는 아마도 비슷한 원리에서 아가미구멍을 통해 빠져나온 물의 소용돌이를 줄여 준다는 가설을 세울 수 있었다. 미래의 연구에서는 실험실 내의 조건에서 아가미구멍을 통해 흐르는 물의 역학적인 측면을 상어 비늘의 형태학적 측면과 연관지어 접근해야 할 것이다.Translated by Daemin Kim, Ph.D. student­Yale University.

Die Haut von Haien ist mit dermalen Dentikeln bedeckt - zahnähnlichen Strukturen, die aus Schmelz, Dentin und einer zentralen Pulpahöhle bestehen. Vorhergehende Studien haben Unterschiede in der Morphologie der Dentikel sowohl zwischen den Arten als auch zwischen verschiedenen Körperregionen innerhalb einer Art gezeigt, einschließlich eines Berichts über extreme morphologische Variationen innerhalb eines Abstands von 1 cm auf der Haut, die die Kiementaschen bedeckt, eine Region, die als "Interbranchialhaut" bezeichnet wird. Um die Oberflächenstruktur dieser Region besser zu versteshen, haben wir die Unterschiede in der Morphologie und Oberflächentopographie der Dentikel der Interbranchialhaut in 13 Haiarten mit Hilfe von Gel-Profilometrie, Histologie und Rasterelektronenmikroskopie quantifiziert. Wir konnten zeigen, dass (1) sich die Dentikel der Interbranchialhaut zwischen den Haiarten unterscheiden und (2) die Dentikel an der Vorderkante der Haut, die jede Kiementasche bedeckt, eine andere Morphologie und Oberflächentopographie aufweisen als die Dentikel an der Hinterkante. Bei allen untersuchten Arten gab es signifikante Unterschiede in der Länge (P = 0.01) und Breite (P = 0.002) der Dentikel, wobei die Dentikel an der Vorderkante kürzer und breiter waren als die Dentikel an der Hinterkante. Auch die Oberflächenschiefe war bei den Dentikeln der Vorderkante höher (P = 0.009), obwohl die meisten Werte immer noch negativ waren, was auf mehr Täler als Spitzen hinweist. Insgesamt waren die Vorderkanten-Dentikel bei allen untersuchten Arten glatter als die Hinterkanten-Dentikel. Diese Daten legen zwei Hypothesen nahe: (1) Glattere Vorderkantenzähne schützen den vorhergehenden Kiemenlappen vor Abrieb während der Atmung, und (2) die Morphologie der gezackten Zähne an der Hinterkante könnte die Wasserturbulenz beim Austritt aus den Kiementaschen nach dem Passieren der Kiemen verändern. Zukünftige Studien werden sich darauf konzentrieren, die Beziehung zwischen der Morphologie der Dentikel und der Wasserströmung zu bestimmen, indem die Flüssigkeitsbewegung über die Interbranchialdentikel während der In-vivo-Atmung sichtbar gemacht wird.Translated by Robin Thandiackal, postdoctoral fellow­Harvard University.

La peau des requins est recouverte de denticules dermiques - des structures semblables à des dents composées d'émail, de dentine et d'une cavité pulpaire centrale. Des études précédentes ont démontré que la morphologie des denticules diffère entre les espèces, mais également entre les différentes régions du corps au sein d'une même espèce. Il existe notamment une variation morphologique extrême sur une distance de 1 cm dans la région appelée "peau interbranchiale," soit la peau peau couvrant les poches branchiales. Nous avons utilisé la profilométrie à base de gel, l'histologie et la microscopie électronique à balayage pour quantifier les différences morphologiques et topographiques des denticules de la peau interbranchiale chez 13 espèces de requins, ceci afin de mieux comprendre la structure de la surface de cette région. Nos résultats montrent que (1) les denticules de la peau interbranchiale diffèrent selon les espèces de requins, et (2) les denticules situées sur le bord d'attaque de la peau couvrant chaque poche branchiale ont une morphologie et une topographie de surface différentes de celles des denticules situées sur le bord de fuite. Chez toutes les espèces étudiées, il y avait des différences significatives dans la longueur (P = 0.01) et la largeur (P = 0.002) des denticules, avec les denticules du bord antérieur plus courtes et plus larges que celles du bord postérieur. L'asymétrie de la surface était également plus élevée dans les denticules antérieures (P = 0.009), bien que la plupart des valeurs soient négatives, indiquant plus de vallées que de sommets.Par ailleurs, , les denticules du bord antérieur étaient plus lisses que celles du bord postérieur. Dans l'ensemble, ces données suggèrent deux hypothèses: (1) les denticules situées sur le bord d'attaque et possédant une surface plus lisse protègent le volet branchial précédent de l'abrasion pendant la respiration, et (2) la morphologie plutôt striée des denticules situées sur le bord de fuite pourrait modifier les caractéristiques turbulentes de l'écoulement sortant des poches branchiales après être passé sur les branchies. Les études futures se concentreront sur la détermination de la relation entre la morphologie des denticules et l'écoulement de l'eau en visualisant le mouvement du fluide sur les denticules interbranchiaux pendant la respiration in vivo.Translated by Elsa Goerig, postdoctoral fellow­Harvard University.

Tunas as a high-performance fish platform for inspiring the next generation of autonomous underwater vehicles.

Tunas of the genus Thunnus are a group of high-performance pelagic fishes with many locomotor traits that are convergently shared with other high-performance fish groups. Because of their swimming abilities, tunas continue to be an inspiration for both comparative biomechanics and the design of biomimetic autonomous underwater vehicles (AUVs). Despite the strong history of studies in tuna physiology and current interest in tuna biomechanics and bio-inspired design, we lack quantitative data on the function of many features of tunas. Here we present data on the morphology, behavior, and function of tunas, focusing especially on experimentally examining the function of tuna lateral keels, finlets, and pectoral fins by using simple physical models. We find that both triangular lateral keels and flexible finlets decrease power requirements during swimming, likely by reducing lateral forces and yaw torques (compared to models either without keels or with rectangular keels, and models with stiff finlets or strip fins of equal area, respectively). However, both triangular keels and flexible finlets generate less thrust than other models either without these features or with modified keels or finlets, leading to a tradeoff between power consumption and thrust. In addition, we use micro computed tomography (µCT) to show that the flexible lateral keels possess a lateral line canal, suggesting these keels have a sensory function. The curved and fully-attached base of tuna pectoral fins provides high lift-to-drag ratio at low angles of attack, and generates the highest torques across speeds and angles of attack. Therefore, curved, fully-attached pectoral fins grant both better gliding and maneuvering performance compared to flat or curved, partially-attached designs. We provide both 3D models of tuna morphology derived from µCT scans and conclusions about the performance effects of tuna-like features as a resource for future biological and engineering work for next-generation tuna-inspired AUV designs.

Tuna locomotion: a computational hydrodynamic analysis of finlet function.

Finlets are a series of small non-retractable fins common to scombrid fishes (mackerels, bonitos and tunas), which are known for their high swimming speed. It is hypothesized that these small fins could potentially affect propulsive performance. Here, we combine experimental and computational approaches to investigate the hydrodynamics of finlets in yellowfin tuna (Thunnus albacares) during steady swimming. High-speed videos were obtained to provide kinematic data on the in vivo motion of finlets. High-fidelity simulations were then carried out to examine the hydrodynamic performance and vortex dynamics of a biologically realistic multiple-finlet model with reconstructed kinematics. It was found that finlets undergo both heaving and pitching motion and are delayed in phase from anterior to posterior along the body. Simulation results show that finlets were drag producing and did not produce thrust. The interactions among finlets helped reduce total finlet drag by 21.5%. Pitching motions of finlets helped reduce the power consumed by finlets during swimming by 20.8% compared with non-pitching finlets. Moreover, the pitching finlets created constructive forces to facilitate posterior body flapping. Wake dynamics analysis revealed a unique vortex tube matrix structure and cross-flow streams redirected by the pitching finlets, which supports their hydrodynamic function in scombrid fishes. Limitations on modelling and the generality of results are also discussed.

The denticle surface of thresher shark tails: Three-dimensional structure and comparison to other pelagic species.

Shark skin denticles (scales) are diverse in morphology both among species and across the body of single individuals, although the function of this diversity is poorly understood. The extremely elongate and highly flexible tail of thresher sharks provides an opportunity to characterize gradients in denticle surface characteristics along the length of the tail and assess correlations between denticle morphology and tail kinematics. We measured denticle morphology on the caudal fin of three mature and two embryo common thresher sharks (Alopias vulpinus), and we compared thresher tail denticles to those of eleven other shark species. Using surface profilometry, we quantified 3D-denticle patterning and texture along the tail of threshers (27 regions in adults, and 16 regions in embryos). We report that tails of thresher embryos have a membrane that covers the denticles and reduces surface roughness. In mature thresher tails, surfaces have an average roughness of 5.6 µm which is smoother than some other pelagic shark species, but similar in roughness to blacktip, porbeagle, and bonnethead shark tails. There is no gradient down the tail in roughness for the middle or trailing edge regions and hence no correlation with kinematic amplitude or inferred magnitude of flow separation along the tail during locomotion. Along the length of the tail there is a leading-to-trailing-edge gradient with larger leading edge denticles that lack ridges (average roughness = 9.6 µm), and smaller trailing edge denticles with 5 ridges (average roughness = 5.7 µm). Thresher shark tails have many missing denticles visible as gaps in the surface, and we present evidence that these denticles are being replaced by new denticles that emerge from the skin below.

Scale diversity in bigeye tuna (Thunnus obesus): Fat-filled trabecular scales made of cellular bone.

Tunas of the genus Thunnus possess many morphological and physiological adaptations for their high-performance epipelagic ecology. Although Thunnus anatomy has been studied, there are no quantitative studies on the structure of their scales. We investigated the scales of bigeye tuna (Thunnus obesus) from ten regions of the body using micro computed tomography (µCT)-scanning and histology to quantitatively and qualitatively compare regional scale morphology. We found a diversity of scale sizes and shapes across the body of bigeye tuna and discriminant function analysis on variables derived from µCT-data showed that scales across the body differ quantitatively in shape and size. We also report the discovery of a novel scale type in corselet, tail, and cheek regions. These modified scales are ossified shells supported by internal trabeculae, filled with fat, and possessing an internal blood supply. Histological analysis showed that the outer lamellar layers of these thickened scales are composed of cellular bone, unexpected for a perciform fish in which bone is typically acellular. In the fairing region of the anterior body, these fat-filled scales are stacked in layers up to five scales deep, forming a thickened bony casing. Cheek scales also possess a fat-filled internal trabecular structure, while most posterior body scales are more plate-like and similar to typical teleost scales. While the function of these novel fat-filled scales is unknown, we explore several possible hypotheses for their function.

Diversity of dermal denticle structure in sharks: Skin surface roughness and three-dimensional morphology.

Shark skin is covered with numerous placoid scales or dermal denticles. While previous research has used scanning electron microscopy and histology to demonstrate that denticles vary both around the body of a shark and among species, no previous study has quantified three-dimensional (3D) denticle structure and surface roughness to provide a quantitative analysis of skin surface texture. We quantified differences in denticle shape and size on the skin of three individual smooth dogfish sharks (Mustelus canis) using micro-CT scanning, gel-based surface profilometry, and histology. On each smooth dogfish, we imaged between 8 and 20 distinct areas on the body and fins, and obtained further comparative skin surface data from leopard, Atlantic sharpnose, shortfin mako, spiny dogfish, gulper, angel, and white sharks. We generated 3D images of individual denticles and measured denticle volume, surface area, and crown angle from the micro-CT scans. Surface profilometry was used to quantify metrology variables such as roughness, skew, kurtosis, and the height and spacing of surface features. These measurements confirmed that denticles on different body areas of smooth dogfish varied widely in size, shape, and spacing. Denticles near the snout are smooth, paver-like, and large relative to denticles on the body. Body denticles on smooth dogfish generally have between one and three distinct ridges, a diamond-like surface shape, and a dorsoventral gradient in spacing and roughness. Ridges were spaced on average 56 µm apart, and had a mean height of 6.5 µm, comparable to denticles from shortfin mako sharks, and with narrower spacing and lower heights than other species measured. We observed considerable variation in denticle structure among regions on the pectoral, dorsal, and caudal fins, including a leading-to-trailing edge gradient in roughness for each region. Surface roughness in smooth dogfish varied around the body from 3 to 42 microns.

Morphology of the core fibrous layer of the cetacean tail fluke.

The cetacean tail fluke blades are not supported by any vertebral elements. Instead, the majority of the blades are composed of a densely packed collagenous fiber matrix known as the core layer. Fluke blades from six species of odontocete cetaceans were examined to compare the morphology and orientation of fibers at different locations along the spanwise and chordwise fluke blade axes. The general fiber morphology was consistent with a three-dimensional structure comprised of two-dimensional sheets of fibers aligned tightly in a laminated configuration along the spanwise axis. The laminated configuration of the fluke blades helps to maintain spanwise rigidity while allowing partial flexibility during swimming. When viewing the chordwise sectional face at the leading edge and mid-chord regions, fibers displayed a crossing pattern. This configuration relates to bending and structural support of the fluke blade. The trailing edge core was found to have parallel fibers arranged more dorso-ventrally. The fiber morphology of the fluke blades was dorso-ventrally symmetrical and similar in all species except the pygmy sperm whale (Kogia breviceps), which was found to have additional core layer fiber bundles running along the span of the fluke blade. These additional fibers may increase stiffness of the structure by resisting tension along their long spanwise axis.

A biorobotic adhesive disc for underwater hitchhiking inspired by the remora suckerfish.

Remoras of the ray-finned fish family Echeneidae have the remarkable ability to attach to diverse marine animals using a highly modified dorsal fin that forms an adhesive disc, which enables hitchhiking on fast-swimming hosts despite high magnitudes of fluid shear. We present the design of a biologically analogous, multimaterial biomimetic remora disc based on detailed morphological and kinematic investigations of the slender sharksucker (Echeneis naucrates). We used multimaterial three-dimensional printing techniques to fabricate the main disc structure whose stiffness spans three orders of magnitude. To incorporate structures that mimic the functionality of the remora lamellae, we fabricated carbon fiber spinules (270 µm base diameter) using laser machining techniques and attached them to soft actuator-controlled lamellae. Our biomimetic prototype can attach to different surfaces and generate considerable pull-off force-up to 340 times the weight of the disc prototype. The rigid spinules and soft material overlaying the lamellae engage with the surface when rotated, just like the discs of live remoras. The biomimetic kinematics result in significantly enhanced frictional forces across the disc on substrates of different roughness. Using our prototype, we have designed an underwater robot capable of strong adhesion and hitchhiking on a variety of surfaces (including smooth, rough, and compliant surfaces, as well as shark skin). Our results demonstrate that there is promise for the development of high-performance bioinspired robotic systems that may be used in a number of applications based on an understanding of the adhesive mechanisms used by remoras.

Three-dimensional analysis of scale morphology in bluegill sunfish, Lepomis macrochirus.

Fish scales are morphologically diverse among species, within species, and on individuals. Scales of bony fishes are often categorized into three main types: cycloid scales have smooth edges; spinoid scales have spines protruding from the body of the scale; ctenoid scales have interdigitating spines protruding from the posterior margin of the scale. For this study, we used two- and three-dimensional (2D and 3D) visualization techniques to investigate scale morphology of bluegill sunfish (Lepomis macrochirus) on different regions of the body. Micro-CT scanning was used to visualize individual scales taken from different regions, and a new technique called GelSight was used to rapidly measure the 3D surface structure and elevation profiles of in situ scale patches from different regions. We used these data to compare the surface morphology of scales from different regions, using morphological measurements and surface metrology metrics to develop a set of shape variables. We performed a discriminant function analysis to show that bluegill scales differ across the body - scales are cycloid on the opercle but ctenoid on the rest of the body, and the proportion of ctenii coverage increases ventrally on the fish. Scales on the opercle and just below the anterior spinous dorsal fin were smaller in height, length, and thickness than scales elsewhere on the body. Surface roughness did not appear to differ over the body of the fish, although scales at the start of the caudal peduncle had higher skew values than other scales, indicating they have a surface that contains more peaks than valleys. Scale shape also differs along the body, with scales near the base of the tail having a more elongated shape. This study adds to our knowledge of scale structure and diversity in fishes, and the 3D measurement of scale surface structure provides the basis for future testing of functional hypotheses relating scale morphology to locomotor performance.