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Fish scales are bony plates embedded in the skin that vary extensively in shape across taxa. Despite a plethora of hypotheses regarding form–function relationships in scales, we know little about the ecological selective factors that shape their diversity. Here we examine evolutionary patterns of scale morphology using novel three-dimensional topography from the surfaces of 59 species of damselfishes, a prominent radiation of coral reef fishes. We find evidence that scale morphology changes with different flow environments, such that species that spend more time in open-water habitats have smoother scales. We also show that other aspects of ecology lead to highly derived scales. For example, anemonefishes show an evolutionary transition to smaller scales and smaller ctenii (scale spines). Moreover, changes in body shape, which may reflect ecological differentiation, are related to scale shape but not surface properties. We also demonstrate weak evolutionary integration among multiple aspects of scale morphology; however, scale size and shape are related, and scale morphology is correlated between different body regions. Finally, we also identify a relationship between aspects of lateral line pore morphology, such that the number of lateral line pores per scale and the size of those pores are inversely related. Overall, our study provides insights into the multidimensionality of scale evolution and improves our understanding of some of the factors that can give rise to the diversity of scales seen across fishes.

 

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.

 

Hyperspectral data encode information from electromagnetic radiation (i.e., color) of any object in the form of a spectral signature; these data can then be used to distinguish among materials or even map whole landscapes. Although hyperspectral data have been mostly used to study landscape ecology, floral diversity and many other applications in the natural sciences, we propose that spectral signatures can be used for rapid assessment of faunal biodiversity, akin to DNA barcoding and metabarcoding. We demonstrate that spectral signatures of individual, live fish specimens can accurately capture species and clade-level differences in fish coloration, specifically among piranhas and pacus (Family Serrasalmidae), fishes with a long history of taxonomic confusion. We analyzed 47 serrasalmid species and could distinguish spectra among different species and clades, with the method sensitive enough to document changes in fish coloration over ontogeny. Herbivorous pacu spectra were more like one another than they were to piranhas; however, our method also documented interspecific variation in pacus that corresponds to cryptic lineages. While spectra do not serve as an alternative to the collection of curated specimens, hyperspectral data of fishes in the field should help clarify which specimens might be unique or undescribed, complementing existing molecular and morphological techniques.

 

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.

 

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. 

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.