The musculature and skeleton of the pelvic fin of the crocodile shark, Pseudocarcharias kamoharai (Matsubara, 1936) (Elasmobranchii: Lamniformes): Morphology and sexual dimorphism

Pseudocarcharias kamoharai (Matsubara, 1936) (Elasmobranchii: Lamniformes) is a small, viviparous, pseudocarchariid shark distributed throughout tropical ocean waters worldwide. Although the appendicular skeleton of elasmobranchs has been explored in the literature, little is known about the clasper skeleton. Additionally, the appendicular musculature in general is not well studied in elasmobranchs, especially that of the pelvic fins and claspers. The same can be said about the siphon sac, a muscular bladder associated with the clasper and related to reproduction in male elasmobranchs. Here, we aim to expand the amount of morphological information related to the secondary sexual characteristics and to describe the pelvic musculature and skeleton, including a description of the clasper of the crocodile shark. The connected siphon sacs have a general inverted "Y" shape, with their anterior margins "M"-shaped and their posterior portions forked. The musculature of the ventral surface of the pelvic fin in both sexes is similar. However, the musculature of the dorsal surface of the male has an extra muscle in comparison to the female, the flexor internus. Moreover, the clasper has a few exclusive muscles, such as the dilatator. In relation to the clasper skeleton, its morphology follows what is seen in other lamniforms. This study marks the first report of sexual dimorphism related exclusively to the pelvic fin musculature in Lamniformes.