The opening and closing mechanism of the spiracle in the spiny dogfish shark, Squalus acanthias (918.20)

Abstract

Benthic sharks ventilate their gills by using a dual pump mechanism. They take water in by sequentially opening their spiracle and mouth, and expel water through their gills by closing their mouth and spiracle and compressing their oropharyngeal cavity (Hughes 1960). However, the mechanism of opening and closing the spiracle and the possible role of the pseudobranch are not fully understood. Therefore, we analyzed the functional morphology of the spiracle in a Spiny Dogfish Shark by micro‐dissection and digital imaging. The immovable edges of the spiracle are held in place by radially arranged subcutaneous collagen fibers. The spiracle is opened and closed by the spiracular flap, which is formed by the underlying spiracular muscle. In the closed spiracle, the spiracular flap and muscle are contracted, wide, and flat. When the chondrocranium is raised to open the mouth, the spiracular flap and muscle are stretched, elongated, and narrowed to open the spiracle and break the watertight seal within the oropharyngeal cavity. The spiracle closes when the mouth is closed by the contraction of the levator palatoquadrati, rostral levator hyomandibulae and spiracular muscles. The pseudobranch serves as a hydraulic cushion that ensures a watertight seal in the closed spiracle.Grant Funding Source: Supported by LSU Foundation to DGH