402 



THE BIOLOGY OF MARINE ANIMALS 



swims downwards gas is added to the swim-bladder to compensate for the 

 decrease in volume, and when it swims upwards gas is resorbed to reduce 

 the volume of the fish. Gas is secreted into the bladder by a gas gland 

 (Fig. 9.21) provided with retia mirabilia, and is resorbed in a special 

 vascular area known as the oval. The gas-gland is usually situated in the 

 anterior region of the swim-bladder. It consists of a highly glandular 

 epithelium supplied by a rich network of large arteries and veins, which 

 give rise to one or two sets of long capillaries. The oval, where absorption 

 of gas takes place, is a modified region of the posterior wall of the bladder. 

 Here the wall is thin and highly vascular in structure. The oval is sur- 

 rounded by sphincter and radial muscles, which close it off from the 



Fig. 9.21. Air-bladder of Midshipman {Porichthys notatus) 



a, ventral view of air-bladder, b, cross-section of bladder, showing diaphragm with 

 its central opening, and numerous plates of the gas-gland on the ventral floor, c, cross- 

 section of the anterior chamber showing cavities of the anterior horns and the air-bladder 

 muscles, a, air-bladder; d, diaphragm; g, gas-gland; m, air-bladder muscle; s, septum; 

 v, vagus nerve. (After Greene, 1924.) 



bladder lumen or expose it, respectively. Most resorption of gas takes 

 place across the oval. Since gases in the bladder are at a higher tension 

 than in the blood, they tend to diffuse across the oval along the pressure 

 gradient. By constriction of the sphincter muscles about the oval, absorp- 

 tion of gases from the oval can be retarded or halted. 



Gas is passed into the swim-bladder against a pressure gradient by some 

 active secretory process. In fish from shallow water the swim-bladder gas 

 contains 12-22% 2 , but in fish from deep water there is a much greater 

 proportion of 2 in the bladder gas. Thus, the bladder gas of an eel 

 Synaphobranchus pinnatus, taken at 1,380 m (pressure 138 atm), had the 

 following composition (in percentages): 2 , 84-6; N 2 , 11-8; C0 2 , 3-6. 

 At this depth the partial pressures in the swim-bladder would be 117 atm 



