Abstract
The dermal layers of several elasmobranch species have been shown to be sexually dimorphic. Generally, when this occurs the females have thicker dermal layers compared to those of males. This sexual dimorphism has been suggested to occur as a response to male biting during mating. Although male biting as a copulatory behaviour in Scyliorhinus canicula has been widely speculated to occur, only relatively recently has this behaviour been observed. Male S. canicula use their mouths to bite the female’s pectoral and caudal fins as part of their pre-copulatory behaviour and to grasp females during copulation. Previous work has shown that female S. canicula have a thicker epidermis compared to that of males. The structure of the dermal denticles in females may also differ from that of males in order to protect against male biting or to provide a greater degree of friction in order to allow the male more purchase. This study reveals that the length, width and density of the dermal denticles of mature male and female S. canicula are sexually dimorphic across the integument in areas where males have been observed to bite and wrap themselves around females (pectoral fin, area posterior to the pectoral fin, caudal fin, and pelvic girdle). No significant differences in the dermal denticle dimensions were found in other body areas examined (head, dorsal skin and caudal peduncle). Sexually dimorphic dermal denticles in mature S. canicula could be a response to male biting/wrapping as part of the copulatory process.
Highlights
IntroductionThe scale arrangement (squamation) of sharks covers the entire integument, including the fins, claspers (males), nictitating membrane (where present), oral cavity, gill bars and the inside of the gill slits [1]
The scale arrangement of sharks covers the entire integument, including the fins, claspers, nictitating membrane, oral cavity, gill bars and the inside of the gill slits [1]
Sex had a significant effect on the length of the dermal denticles present on the pectoral fin (ANCOVA, F=6.53; d.f.=1; P=0.020) posterior to the pectoral fin (ANCOVA, F=7.72; d.f.=1; P=0.013) pelvic girdle (ANCOVA, F=7.41; d.f.=1; P=0.014) and upper caudal lobe (ANCOVA, F=12.08; d.f.=1; P=0.003) regions of mature adults
Summary
The scale arrangement (squamation) of sharks covers the entire integument, including the fins, claspers (males), nictitating membrane (where present), oral cavity, gill bars and the inside of the gill slits [1]. The significance of this coverage on elasmobranch fishes has generated a great deal of literature, a large amount of which focuses on the evolutionary nature of the dermal denticles, examining the divergence of extant species’ dermal denticles from those of extinct species through the utilisation of the fossil record. There is increasing interest in the role that sexual conflict plays in driving the evolution of traits in both males and females. As one sex develops strategies or features to gain reproductive advantage, so the other sex develops counterstrategies to mitigate or oppose these advantages
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