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Observations on the Physiology of the Swim Bladder in Cyprinoid Fishes by H

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Observations on the Physiology of the Swim Bladder in Cyprinoid Fishes by H OBSERVATIONS ON THE PHYSIOLOGY OF THE SWIM BLADDER IN CYPRINOID FISHES BY H. M. EVANS AND G. C. C. DAMANT. (Received 2.6th February 1928.) (With Five Text-figures.) THE swim bladder of fishes is primarily a hydrostatic organ and with rare exceptions contains or tends to contain the exact quantity of gas which is necessary to make the specific gravity of the whole fish equal to that of the water in which it is swimming, so that it can rest in mid-water tending neither to rise nor sink: this normal con- dition is called neutral buoyancy. Since gas is compressible and water is not, any increase of external or atmospheric pressure by acting through the non-rigid body walls will reduce the volume of gas in the swim bladder and cause the fish to sink in the water (condition of negative buoyancy). This condition can also be produced by aspirating some of the gas from the swim bladder or by attaching a small weight to the fish. When in a state of negative buoyancy produced by any of these treatments (provided that the interference has not been excessive), the fish will compensate (i.e. restore its neutral buoyancy) by introducing additional gas into its swim bladder. In fish with closed swim bladders (Physoclisti) it has long been known that this additional gas is mainly oxygen and is secreted into the swim bladder by organs known as red bodies or gas glands. Such organs are absent in many fishes whose swim bladders are furnished with ducts communicating with the exterior, and experiments which have been published on the method by which such fish compen- sate are inconclusive. Working with such Cyprinoids as Carp, Roach and Goldfish, which have a long pneumatic duct leading from the oesophagus to the posterior sac of the swim bladder, we find that when a condition of negative buoyancy is induced artificially they combat it by rising to the surface, taking air into the mouth and passing it thence to the swim bladder. If they are prevented from doing this an alternative remains, for we find that in spite of the absence of red bodies they can slowly secrete a gas rich in O2 into their swim bladders. The following experiments demonstrate these facts. When fish are observed in glass sided aquaria it is possible to judge with great exactness whether they are in neutral buoyancy and if not how far compensation has proceeded, provided that incautious movements or changes of illumination are not allowed to scare them into rapid excited swimming. Two small goldfish were placed in a corked glass bottle containing water (Fig. 1). The cork was pierced by a tube at which the experimenter sucked with his Physiology of the Swim Bladder in Cyprinoid Fishes 43 mouth till the reduction of atmospheric, pressure (by about 25 cm. Hg) was seen to cause the escape of a bubble or two of air from the swim bladder via the pneumatic duct and mouth. They were then quickly transferred to a glass aquarium (Fig. 2) where one was imprisoned under a bell-jar completely filled with water and so cut off from access to the surface while the other was left free to promenade and "take the air." The fish were much agitated and showed marked negative buoyancy, dropping "like stones" to the bottom at each temporary cessation of active swimming. After five minutes both were trying to reach the surface; the prisoner J) 1 Fig. 1. could only knock its head against the glass dome, but the free fish came to the sur- face and took gulps of air then dropping back to the bottom where it spat out a few bubbles, repeating the performance at short intervals. Some of the air remained inside for the negative buoyancy of this fish steadily diminished and had disappeared in ii hours. Meanwhile the prisoner retained its negative buoyancy, though after 24 hours it was thought to be somewhat less heavy than at the start; after 48 hours compensation had progressed further but the negative buoyancy was still so great that the fish rested on the bottom and fell there (though less heavily than at first) after each swimming effort had ceased. Air was now blown up under the bell-jar so as to form a large bubble at the top. 44 H. M. EVANS arid G. C, C. DAMANT The prisoner immediately swam up, gulped air in the same way as the free fish had done and, in less than two hours it had fully compensated and was resting com- fortably in mid-water (neutral buoyancy). In another type of experiment negative buoyancy was produced by puncturing the swim bladder and removing a certain quantity of gas. Roach were placed in a large tank in which there was a constant flow of water. A vertical wire netting par- tition divided the tank into two equal compartments one of which was provided with a wire netting cover resting about one inch below the surface of the water. The fish confined in this half are called non-access fish because they cannot reach the surface or gulp air while the fish in the other half are called free-access fish. Fig. 2. The puncture was made in all the experiments at a point 2 scales above the lateral line at a distance of 7 lateral line organs posterior to the head. This corresponds with the centre of the anterior sac. They were punctured with the needle, of a hypodermic syringe and approximately 2 c.c. of gas removed from the swim bladder of each. Eight of them were then placed in the free-access and seven in the non-access com- partment of the tank. All showed marked negative buoyancy. After two hours three of the free-access fish were partially compensated and were observed to be swimming on the surface at frequent intervals with \ inch of the head exposed; they gulped air and spat out froth. After seven hours all the free-access fish were poised freely in the tank and swam without effort in a condition of neutral buoyancy while the non-access fish were observed to be swimming up to the wire netting and then coming down tail firsts still showing marked negative buoyancy and resting hori- zontally on the bottom or at an acute angle with their tails touching it. After 24 hours three non-access and three free-access fish were killed and the Physiology of the Swim Bladder in Cyprinoid Fishes 45 gas from their swim bladders collected over water and carefully measured when it was found that each of the free-access fish contained more gas per gramme of body weight than did any of the non-access fish, the average difference being between 3 and 4 cc. in the case of 100 gm. fish. The subsequent history of the remaining fish was as follows. After 29 hours the four non-access fish were still in negative buoyancy and trying to reach the surface; at one moment all four were swimming upwards in an almost vertical attitude. The fish with free access naturally remained in neutral buoyancy. After 48 hours two of the four non-access fish appeared to be nearly, if not quite, compensated; all four could move more freely and no longer swam obliquely with the head pointing upwards. After 72 hours two were in neutral buoyancy but two still tended to rest on the bottom. After 112 hours all four had neutral buoyancy. Summing up, we see that of four fish without access to the surface two required between two and three days to compensate and two required more than four days, while all eight of those with access to the surface compensated within seven hours. Further experiments showed that the period of compensation in non-access fish is variable and may be much longer; we have cases in which neutral buoyancy was not attained till the 16th, 28th and even the 48th day. The following experiment was devised to study more closely the feeble and variable power of compensation which the foregoing had shown us was possessed by Cyprinoids prevented from gulping air. In it negative buoyancy was produced by a more natural method than aspiration and means were provided for measuring the progress of compensation with some accuracy. Three goldfish were confined in a large aspirating bottle supplied at its base with running water from a tap. After passing through the bottle the water escaped through a discharge pipe which could be raised or lowered so as to vary the hydrostatic pressure within the bottle between nine feet of water as a maximum and six inches which was the depth of the con- tainer. With no access to air {i.e. when the aspirating bottle was completely filled with water (Fig. 3, A) the fish under pressures of a few feet of water showed negative buoyancy and uneasiness with the usual signs of wanting to gulp air. After half an hour the discharge tube was lowered and the pressure reduced to normal, whereupon two of the fish started to float upwards with positive buoyancy showing that they had already partly compensated towards the increased pressure. By similar experiments it is found that compensation goes on slowly and in about six hours fish can compensate to five feet of water pressure, but in two experiments where the fish were left under a head of eight feet of water for 24 and 48 hours respectively they remained in negative buoyancy, though by lowering the pressure it was found that they had compensated to five feet but apparently could carry the process no further in the time.
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