Gas secretion and resorption in the swimbladder of the codGadus morhua
Tóm tắt
The codGadus morhua has a closed, compliant swimbladder which occupies 5% of its volume. Pressure changes caused by vertical movements lead to the expansion and compression of the swimbladder gas, and the fish responds to the accompanying changes in density with compensatory swimming movements and the resorption or secretion of gas. A simple physiological model, based on estimates of cardiac output, the blood supply to the swimbladder and the oxygen-carrying capacity of the blood, is developed to set limits to these processes. An apparatus is described for making observations on the rate of buoyancy adaptation by cod subjected to pressure changes within the range 1.15 to 7.50 ATA at temperatures from 0 to 17°C. One hundred and twenty eight experiments were made with 38 cod ranging in length from 25 to 50 cm and in weight from 138 to 1440 g. The results showed that the rate of gas resorption increased markedly with the pressure to which the fish were adapted (from 0.14 ml kg−1 min−1 at 1.5 ATA to 1.54 ml kg−1 min−1 at 7.0 ATA), but not significantly with the weight of the fish or with temperature. In contrast, the rate of gas secretion increased markedly with temperature (from 0.02 ml kg−1 min−1 at 0°C to 0.11 ml kg−1 min−1 at 17°C), increased slightly with pressure, and decreased with the weight of the fish. The rate of gas resorption was much faster than that of gas secretion, and the difference between the two rates increased with pressure. The difference in performance is discussed in relation to the restriction that the swimbladder might impose to the speed and extent of vertical movements. It is suggested that when a closed swimbladder has a hydrostatic function it may be advantageous if neutral buoyancy is maintained only at the upper limit to the diurnal vertical range.