Microstructural morphology of dermal and oral denticles of the sharpnose sevengill shark Heptranchias perlo (Elasmobranchii: Hexanchidae), a deep‐water species

Abstract

The dermal denticles are among the unique morphological adaptations of sharks, which have been acquired throughout their long evolutionary process of more than 400 million years. Species‐specific morphological characteristics of these structures has been applied specially as tools for functional and taxonomic (family‐level) studies. Nevertheless, few studies have explored the diversity of denticle structure in different around the body and oral cavity. In the present study, we described the morphological differences observed in skin and oral cavity of sharpnose sevengill shark Heptranchias perlo, using scanning electron microscopy. Our findings demonstrate substantial variation in morphological structure of the denticles of the body and oral cavity. Overall, the dermal denticles observed across body surface were overlapped, tricuspid, with the central cuspid being more pronounced, pointed, and triangular in shape compared with lateral ones. Unlike, the denticles on the tip of the nose had a smooth crown, with rounded edges, being compact, and overlapped. The oral denticles were found in the ventral and dorsal region of the oral cavity. They also were tricuspid, but with differences in arrangement and ridges. These results suggest a strict functional relationship with the morphological characteristics observed. Such morphological diversity body‐region‐dependent highlights the need for comparative studies that include oral denticles, since this structure has an important functional role in sharks and can be found in fossil and recent records.

Morphological characterization of dermal denticles of the Broadnose Sevengill Shark Notorynchus cepedianus (Elasmobranchii: Hexanchidae)

“…Additionally, it was observed that the width of the central cusp is thinner and apparently as the individual grows it can be observed that it becomes wider, it becomes more noticeable in those of the ventral region (Figure 3 and Figure 4). Overall, the presence of a crown with three cusps and a central ridge that runs the entire length of the crown in N. cepedianus dermal denticles, seems to be a morphological character shared with other species of Hexanchidae family as Heptranchias perlo (Castro, 2011;Dillon et al, 2017), Hexanchus griseus (Castro, 2011;Rangel et al, 2019), and Hexanchus nakamurai (Castro, 2011;Ebert et al, 2013). There are several types of denticles that can fulfil different functions, which are related to hydrodynamics, resistance to rock abrasion, defence, luminescence, silent movements and even food acquisition (Raschi and Elsom, 1986;Raschi and Tabit, 1992;Smith and Heemstra, 1986;Dillon et al, 2017).…”

“…In particular, it has been studied as shark species identification tool (Tanaka et al, 2002;Valenzuela et al, 2008;Marshall, 2011), community's shark reconstruction, especially based on fossil records (Kriwet and Benton, 2004;Kriwet et al, 2008;Dillon et al, 2017;Ferrón et al, 2019;Rangel et al, 2019) and recognition of morphological patterns associated with their ecology (Reif and Dinkelacker, 1982;Reif, 1982;Muñoz-Chápuli, 1985;Reif, 1985a;Ferrón et al, 2014;Ferón and Botella, 2017;Ferrón et al, 2018;Dillon et al, 2020).…”

Ecomorphological, space, and mineral relations of dermal denticles in angular angel shark (Squatina guggenheim)

“…The denticles found in S . guggenheim presented variation in the crown shape according to the region, suggesting that their morphology is related to ecological aspects (e.g., Dillon et al, 2017; Rangel et al, 2019). We found two predominant functional morphotypes, following the denticle's classification proposed by Dillon et al (2017).…”

“…Due to their superb preservation during the fossilization process, the denticles are commonly used as a tool in taxonomic identifications (Allentoft et al, 2012; Mello et al, 2013), and recently used in paleoecological studies to identify and reconstruct shark communities (Dillon et al, 2017, 2020). Although recent studies have assessed the morphology, taxonomy, and function of dermal denticles in active pelagic sharks, sedentary demersal species have been little studied (Ankhelyi et al, 2018; Dillon et al, 2017; Rangel, Amorim, Kfoury Jr, & Rici, 2019).…”

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

“…Shark denticles have a tooth‐like structure with an enameloid and dentine outer layer and an inner pulp cavity. Denticles are complexly shaped with a flattened outer crown area that is connected to narrower neck that extends toward the skin and an expanded base embedded in the dermis (Ankhelyi, Wainwright, & Lauder, 2018; Castro, 2011; Díez, Soto, & Blanco, 2015; Motta, Habegger, Lang, Hueter, & Davis, 2012; Oeffner & Lauder, 2012; Rangel, Amorim, Kfoury, & Rici, 2019; Reif, 1982).…”