The Muscular Basis of the Respiratory Pumps in the Dogfish (Scyliorhinus Canicula)

Abstract

ABSTRACT The mechanism of gill ventilation in the dogfish has been shown to be fundamentally the same as that found in teleost fishes (Hughes, 1960b; Hughes & Shelton, 1962). Water enters the respiratory system through both the mouth and the spiracle during expansion of the oro-branchial cavity (Woskoboinikoff, 1932) and after passing across the gills it enters the parabranchial cavities before being ejected to the outside through the five pairs of gill slits. The flow across the gills is maintained partly as a result of the increased pressure in front of the gill resistances but also because of the suction pump action of the parabranchial cavities. The muscular activities producing the changes in volume of these two cavities and hence the required pressure gradient across the gills have not been established and descriptions of the relationships of muscles and skeleton are not always clear in detail. Woskoboinikoff (1932) and others were of the opinion that the coraco-mandibularis muscle was of importance during the phase of the cycle when the mouth opens and the oro-branchial cavity expands, but this was categorically denied by Balabai (1938) in a footnote to his paper. From observations on dogfish, anaesthetized so that they no longer pumped water across their gills, it was suggested (Hughes, 1960 a) that the main muscular action during the cycle was due to the constrictor muscles. These compress the branchial region and on their relaxation the elastic properties of the skeleton and ligaments are sufficient to account for expansion of the parabranchial cavities. A dogfish in this condition can be made to pump water through its respiratory system by compressing the branchial region by hand. It was also clear in Scyliorhinus that, as had also been observed by Satchell (1959) in Squalus, the respiratory cycle begins with a compression and is followed by relaxation and a relatively long respiratory pause.