68 DISCOVERY REPORTS 



Compared with the swimbladder of the 32-5-mm. fish, those of the other two fishes are consider- 

 ably regressed, although the main features can be clearly distinguished. It is probably significant that 

 the dimensions of the fat body surrounding the swimbladder of the 33-5-0101. fish are much the 

 same as those of the swimbladder of the 32-5-011x1. individual. It would seem that as the swimbladder 

 regresses, the space left between the peritoneum and tunica externa is filled with fat. But the marked 

 difference between the swimbladders of these two equal-sized fishes suggests there must be wide 

 variability in the stage at which this regression begins to occur. 



A hydrostatic organ consisting entirely of fat is superior to one containing gas in that fats are 

 relatively incompressible. However, fats are not much lighter than sea water, having a density of 

 about 0-9. In a marine fish the volume of gas-filled swimbladder need only be 5 per cent of the body 

 volume to make the fish weightless in water (Jones and Marshall, 1953). But a marine fish without 

 such an internal float requires about 30 per cent of fat by weight for neutral buoyancy. Clearly the 

 replacement of gas by fat is an inadequate substitution, so far as buoyancy is concerned. In Gono- 

 stoma elongatum, Denton and Marshall (1958) found relatively little fat, the more significant fact being 

 that this species almost achieves neutral buoyancy by having reduced muscular and skeletal systems. 

 But in Cyclothone, as will be shown later (p. 103), there is, in addition to the fat investing the swim- 

 bladder and deposited in the mesenteries, a well-developed system of fat sinuses under the skin. In 

 a well-fed Cyclothone these stores of fat may occupy up to 15 per cent of the body volume (about 

 13 per cent of the body-weight assuming the fish to be neutrally buoyant). Again, the more significant 

 feature, particularly in a poorly nourished fish, is the reduction of the heavy muscular and skeletal 

 tissues. 



The following conclusion seems apt: In a number of bathypelagic fishes the swimbladder regresses 

 after metamorphosis and becomes a convenient site for the deposition of fat, but this plays a relatively 

 small part on the ' credit ' side of the ' buoyancy balance sheet '. 



THE SWIMBLADDER AS A HYDROSTATIC ORGAN 

 The teleost swimbladder acts as a hydrostatic organ by making the fish weightless in water. Whatever 

 movements the fish may make, whether up or down, the nervous control of the swimbladder is such 

 that it continues to function towards this 'desirable' end. In terms of cybernetics, the 'feed-back' 

 is arranged to steer this system towards this ' goal ', the weightless condition. In other terms, here is 

 what the fishes told the diver in one of Isak Dinesen's (1958) 'simple' stories: 'We fish rest quietly, 

 to all sides supported, within an element which all the time accurately and unfailingly evens itself out. 

 An element which may be said to have taken over our personal existence, in as much as, regardless of 

 individual shape and of whether we be flatfish or roundfish, our weight and body is calculated 

 according to that quantity of our surroundings which we displace.' 



Knowing the density of the tissues (about 1-076), it can be shown that the volume of the swim- 

 bladder in a marine teleost must be about 5 per cent of the body- volume if the fish is to be in hydro- 

 static equilibrium with the sea (Taylor, 1921). Measurements of this percentage volume in shallow 

 water species closely agree with this theoretical figure (Jones and Marshall, 1953). Furthermore, 

 Kanwisher and Ebeling (1957) have found a similar agreement in bathypelagic teleosts, their measure- 

 ments of the swimbladder volume in various stomiatoids, myctophids and melamphaids ranging from 

 3-2 to 6-5 per cent of the body-volume. If deprived of their swimbladders, these deep-water fishes 

 would need to sustain a downward force equivalent to 3-2-6-5 per cent of their weight in air in order 

 to maintain themselves at a constant depth. Calculations made by Denton and Shaw show that the 

 energy 'saved' can be quite appreciable (Denton and Marshall, 1958). Yet some energy is required 

 to keep the swimbladder inflated at the appropriate volume and the amount is directly related to the 



