84 discovery reports 



Compressibility of gases 

 The swimbladder gases of deep-sea fishes consist largely of oxygen (Scholander and Van Dam, 1953 ; 

 Kanwisher and Ebeling, 1957) and this gas alone will be considered. 1 At normal temperature and 

 pressure the density of oxygen is 1*429 g./l., which value is about one seven-hundredth of the density 

 of sea-water. In other words, at a depth of 7200 m., the density of oxygen is equal to that of seawater, 

 and the gas would thus have lost its positively buoyant properties. 



A fish having 95 g. of fat-free tissue, with a density of 1-076 (taking Taylor's (1921) estimate) and 

 a 5-ml. swimbladder will have a density of about 1-07, which is well above the value for seawater 

 (1-028) at this depth. Clearly a gas-filled swimbladder would be virtually useless. However, we shall 

 see later that abyssobenthic fishes with well-developed swimbladders may range as deeply as 5000 m. 

 At such a level the density of oxygen is about 0-7. Thus to achieve neutral buoyancy, either the 

 volume of the swimbladder must be considerably increased or the density of the tissues be reduced. 

 There is no evidence of the first desideratum being met, but there is good indication of a general 

 lightening of the tissues (see p. 96). 



Energy requirements of the swimbladder 



The combined partial pressures of the gases dissolved in seawater at any depth total no more than one 

 atmosphere. The oxygen tension is thus about one-fifth of an atmosphere, yet the partial pressure of 

 this gas in the swimbladder may be 200 atmospheres or more (Scholander, 1954). Thus, between the 

 uptake of oxygen by the blood circulating through the gills and its entry, under the appropriate 

 pressure, into the swimbladder, it is clear that considerable energy will be required to concentrate the 

 gas. By using the energy of compression equation, Parr (1937) calculated that, to fill its swimbladder 

 to the requisite buoyant volume, a fish living at a depth of 1000 m. would use 300 times as much 

 energy as the amount it would need at 10 m. While appreciating this, Bruun (1943) pointed out that 

 the absolute amount of energy needed is quite small. He expressed it thus: 'If we take an adult 

 Spirilla of a weight of 10 g. and with a shell containing about 0-5 ml. gas, it would cost about 13 g. 

 calories to fill it at 2000-m. depth, corresponding to only a few mgs of food.' Looking at the problem 

 from much the same aspect, Scholander (1954) has calculated that for each part of oxygen secreted 

 into the swimbladder a minimum of 3 per cent must be diverted to the work of compression. Like the 

 compressibility factor, it would thus seem that the energy problem is not so serious as it might 

 first appear. 



Gas requirements of the swimbladder 



While the proportion of oxygen needed for the energy of compression seems not to be excessive, gas 

 must be available in sufficient quantity, if the swimbladder is to be kept inflated at the volume for 

 weightlessness in water. The greater the depth the more the gas needed for a given unit of buoyancy 

 (see pp. 89-90). But it should be remembered that most species of bathypelagic fishes have swimbladders 

 with a capacity of 0-5 ml. or less. On the other hand, many of these fishes move up and down in the 

 sea each day. As these migrations take place within the upper 1000 m. of the ocean, the effects of 

 pressure on gas-density need not be considered. (At 500 m. the density of oxygen is about 0-07.) But 

 as it swims downwards to regain its daytime station, the main physical stress facing a fish is the pro- 

 vision of enough oxygen to fill the swimbladder. From this general introduction we may turn to the 

 physical and biological aspects of vertical migration. 



1 The swimbladder also contains nitrogen (from about 2 to 15 per cent), argon and carbon-dioxide. If the partial pressure 

 of C0 2 ever reached 50 atmospheres, which from the papers cited, seems unlikely, it would exist as a liquid at temperatures 

 of 1 3 C. and below. 



