38 VERTEBRATE RESPIRATION 



bladder to increase in volume and the density of the fish -f swim- 

 bladder will fall. 



This latter condition is very easily demonstrated experiment- 

 ally in a large flask as shown in fig. 11. The decrease in pressure 

 tends to make the fish rise and elicits compensatory movements 

 of the animal. In the case of physostomatous fishes (i.e. with the 

 swimbladder still connected to the gut) the reaction is a 'gas- 

 spitting reflex' whereby air is ejected from the bladder following 

 relaxation of the pneumatic duct sphincter. The loss of gas 

 reduces the density of the fish + swimbladder and it is thus 

 restored to neutral buoyancy at the lower pressure. In physo- 

 clistous fishes (e.g. Perch) such a reaction is not possible because 

 the swimbladder is isolated, and the animal makes compen- 

 satory movements of the pectoral fins which increase in fre- 

 quency as the force tending to raise the fish increases. Further 

 decreases in pressure result in active movements of the tail, at 

 first asymmetric contractions of the lobes of the caudal fin and 

 later strong downward swimming. Over long periods such fish 

 depend entirely on their ability to reabsorb gas at the oval. When 

 the pressure is restored to its original level the compensatory 

 movements stop. In physostomes in which the volume of swim- 

 bladder gas has been decreased, the fish is more dense than 

 water and it tends to sink to the bottom of the tank. It soon 

 swims to the surface and attempts to replenish the gas lost from 

 its swimbladder by gulping air. If this is prevented, however, the 

 fish continues to strive for the surface but eventually neutral 

 buoyancy is restored by gas secretion. 



This experiment is equivalent to the situation which arises 

 when a fish swims rapidly towards the surface of the ocean. 

 Under such conditions the swimbladder will rapidly expand, 

 the density of the fish is reduced and it will tend to rise even more 

 rapidly to the surface because of its lowered density. This shows 

 the basic instability which is intrinsic to this system of main- 

 taining a buoyant condition. This factor eff'ectively limits the 

 rate at which fish can migrate vertically in the sea and those 

 without a swimbladder are at a distinct advantage. It has been 

 calculated that a fish at 20 m. depth may swim rapidly to about 



