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 to 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 to trailing edge denticles. Surface skew was also higher in leading edge denticles (P = 0.009), though most values were still negative, indicating more 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.
The vertebrate immune system provides an impressively effective defense against parasites and pathogens. However, these benefits must be balanced against a range of costly side-effects including energy loss and risks of auto-immunity. These costs might include biomechanical impairment of movement, but little is known about the intersection between immunity and biomechanics. Here, we show that a fibrosis immune response in threespine stickleback (Gasterosteus aculeatus) has collateral effects on their locomotion. When freshwater stickleback are infected with the tapeworm parasite Schistocephalus solidus, they face an array of fitness consequences ranging from impaired body condition and fertility to an increased risk of mortality. To fight the infection, some stickleback will initiate a fibrosis immune response in which they produce excess collagenous tissue in their coelom. Although fibrosis is effective at reducing infection, some populations of stickleback actively suppress this immune response, possibly because the costs of fibrosis outweigh the benefits. Here we quantify the locomotor effects of the fibrosis immune response in the absence of parasites to investigate whether there are collateral costs of fibrosis that could help explain why some fish forego this effective defense. To do this, we induce fibrosis in stickleback and then test their C-start escape performance. Additionally, we measure the severity of fibrosis, body stiffness, and body curvature during the escape response. We were able to estimate performance costs of fibrosis by including these variables as intermediates in a structural equation model. This model reveals that among control fish without fibrosis, there is a performance cost associated with increased body stiffness. However, fish with fibrosis did not experience this cost but rather displayed increased performance with higher fibrosis severity. This result demonstrates that the adaptive landscape of immune responses can be complex with the potential for wide reaching and unexpected fitness consequences.
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