The swimbladder as a hydrostatic organ in the Northern Pike, Esox lucius L

Abstract

The aim of this study was to investigate the ways in which the northern pike, Esox Lucius L., adjusts its buoyancy by manipulation of the gas content of the swimbladder. The pike, a physostome, was shown to employ two methods of increasing the volume of gas in the swimbladder. Firstly, by air-gulping in shallow water, and with free access to the surface. Secondly, by gas secretion, made possible because of the existence of a well-developed secretory apparatus at the anterior end of the swimbladder. Pike demonstrated the ability to secrete a gas, rich in oxygen, at rates similar to those demonstrated by some physoclists. Secretion occurred even when air-gulping was possible. Gas release was shown to occur when deep-adapted pike were subjected to a reduction in ambient pressure. Swimbladder gas was also forcibly ejected via the pneumatic duct when pike were exposed to a water current, and under conditions of stress. In addition, swimbladder gas was lost as a result of simple diffusion across the swimbladder wall. In still water, pike adapted to neutral of slight positive buoyancy, while those held in a current were negatively buoyant. Still-water adapted pike demonstrated substantial excess internal gas pressures, while in current-adapted fish, only slight excess internal pressures were developed. Small transient changes in buoyancy, probably arising as a result of the action of the body wall muscles on the swimbladder contents were observed. The physiological and ecological significance of these experimental results and observations are discussed.