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

Abstract

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.

Teeth outside the mouth: the evolution and development of shark denticles

“…1C-E, H) (Cooper et al, 2018). They exhibit dramatic variation in their morphology (Gabler-Smith et al, 2021), ranging from the petal-like denticles of the rostrum (Fig. 1C) to the sharp, protruding denticles of the dorsal head surface (Fig.…”

“…The denticles of adult sharks facilitate numerous functions, including hydrodynamic drag reduction during locomotion, the provision of defensive armour, and communication via the binding of luminescent photophores (Oeffner and Lauder, 2012, Wen et al, 2015, Reif, 1985). Consequently, the dermal denticles of elasmobranchs have evolved to exhibit various shapes and sizes, both within and across species (Motta et al, 2012, Gabler-Smith et al, 2021). Patterns of morphological variation in shark denticles are also observable across deep time (Sibert and Rubin, 2021).…”

Ridges and riblets: Shark skin surfaces versus biomimetic models

“…Biological data were collected from 17 species of sharks that were either preserved specimens from the Museum of Comparative Zoology Ichthyology Collection or freshly frozen material. As shark denticles are hard tooth-like structures, we anticipate no differences in surface topography between tissue that was preserved in alcohol or freshly frozen: the effects of freezing or preservation on different skin regions within species would also not be a concern (see Wainwright et al, 2017;Gabler-Smith et al, 2021). There were two regions of interest: (1) skin from the midbody under the first dorsal fin, and (2) skin from the midcaudal fin region in the center of the tail surface (not from the leading or trailing edges).…”

“…Sharks have been a particularly important group stimulating the manufacture of biomimetic materials, specifically due to the unique structures on their skin: dermal scales or denticles. Shark denticles are tooth-like structures, cover the surface of all shark species, and vary in their shape, size, and surface characteristics including the presence/absence of ridges, ridge height, and ridge spacing (Rief, 1985;Castro, 2011;Motta et al, 2012;Ankhelyi et al, 2018;Popp et al, 2020;Gabler-Smith et al, 2021). Although dermal denticles likely have various functions (e.g., abrasion reduction, parasite deterrence, protection from predators), the ability of these microscopic structures to reduce drag and enhance thrust during aquatic locomotion has been of considerable interest in the biomimetic and bioinspiration community for years (Bechert et al, 1985;Garcıá-Mayoral and Jimeńez, 2011;Wen et al, 2014;Domel et al, 2018a;Suprapaneni et al, 2022).…”

Placoid scales in bioluminescent sharks: Scaling their evolution using morphology and elemental composition

“…The morphology and arrangement of placoid scales can be species-specific and is often used in species identification (Reif, 1985b;Raschi and Tabit, 1992;Deynat, 2000;Gravendeel et al, 2002;Claes and Mallefet, 2009;Dillon et al, 2017;Ferroń and Botella, 2017;Ankhelyi et al, 2018;Duchatelet et al, 2020a). Intraspecific variation of placoid scale morphology is observed at different body regions (Duchatelet et al, 2020a;Feichtinger et al, 2021;Gabler-Smith et al, 2021;Naylor et al, 2021) but, within a dedicated body region, the intraspecific variation is low (authors' pers. obs.).…”