Chapter 3 - The Shark

Abstract

This chapter considers the anatomy of the shark. The spiny dogfish shark—Squalus acanthias—belongs to Chondrichthyes, which first appeared in the Silurian Period and includes Elasmobranchii and Holocephalimorpha. Among the specialized features that unite these groups are unique perichondral and endochondral mineralization, distinctive placoid scales, an inner ear that opens externally through the endolymphatic duct, pelvic claspers in males, and a cartilaginous skeleton. Sharks have fusiform bodies with well-developed paired fins and a powerful heterocercal tail. Five to seven pharyngeal slits and usually a spiracle are present. A row of serrated, triangular, and pointed teeth commonly line the upper and lower jaws and are followed by rows of teeth that can rapidly replace broken or worn teeth. The closely related skates and rays are flattened with enormously expanded pectoral fins, and the pharyngeal slits open ventrally and the prominent spiracle opens dorsally. The tail and caudal fin are generally reduced and often whip-like. Locomotion is accomplished through wave-like flapping of the fins rather than lateral undulations of the trunk and tail. Chimaeras possess an operculum that covers the gills. The peculiar globular head with dental plates and long, thin tail have inspired their alternate designation as “ratfish.” Additional features of chimaeras are that a spiracle is present only as a transitory structure during ontogeny, the anterior vertebrae form a synarcual that articulates with an anterior dorsal fin, and the male has, in addition to pelvic claspers, a cephalic clasper.