Skeletal labyrinth morphology of four species of living elasmobranchs.

Despite detailed descriptions of cranial anatomy in representatives of most major chondrichthyan groups, the inner ear has been described infrequently and most often from the soft tissue of the membranous labyrinth. However, skeletal labyrinth morphology has been linked with ecology in several groups of vertebrates, and shark skeletal labyrinths bear several specializations for detecting low frequency sounds. Without description of these structures across a broad sample of taxa, future exploration of the ecomorphology of ear shape is not possible. We used high-resolution CT scanning to generate three-dimensional models of the endocranial anatomy in four elasmobranchs: the Nurse Shark (Ginglymostoma cirratum), the Japanese Tope Shark (Hemitriakis japanica), the Horn Shark (Heterodontus francisci), and the Zebra Shark (Stegostoma tigrinum). Major differences are apparent between the skeletal labyrinths of these taxa, which might be ascribed to either phylogenetic history or lifestyle. In particular, the size of the skeletal labyrinth relative to the cranium dramatically differs among these chondrichthyans, as does the diameter and angle of the semicircular canals and the size of the canals relative to the vestibule. Based on the separation of the anterior and posterior semicircular canals, and the lack thereof in S. tigrinum, the degree of specialization for low frequency sound detection may also vary.