436 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



stratified and the lower layers receive no oxygen. That this is the 

 correct explanation is confirmed by the observation that in open pools 

 in the outer parts of the Chaco swamps some oxygen — up to 20 percent 

 saturation — was present in the lower water after unusually cool and 

 rainy days or cold nights. 



The deoxygenation of the water was paralleled by a high content of 

 free carbon dioxide — up to 40 cc, per liter in the water of the outer 

 parts and 70 cc. per liter in the central parts. This was clearly due 

 to its inability to escape to the air by diffusion. 



For animals breathing aquatically by gills these waters are therefore 

 a very difficult environment, and this of itself is enough to explain 

 the poverty of the zooplankton, even though, as was shown to be the 

 case, the plankton is adapted to live at a low concentration of oxygen 

 (5 percent saturation). All the fauna shows adaptation to life in a 

 deoxygenated habitat. These swamps are a well-known habitat of 

 the air-breathing lungfish Lepidosiren^ and many of the teleost 

 fishes have evolved accessory air-breathing organs. Some of the 

 smaller fishes, however, do not breathe air. They succeed in main- 

 taining their aquatic respiration by living near the surface and using 

 the thin oxygenated surface film, nibbling at it but not breaking the 

 surface. The invertebrates also show many adaptations. A small 

 oligochaete {Aulophorus) lives in the surface film of the outer region 

 of the swamp. Being an oligochaete it needs a tube, and this it makes 

 for itself from the spores of the waterferns. It carries this tube 

 about with it. Another oligochaete {Drilocrius) lives in very shallow 

 water at the edge of the swamp making burrows in the mud. From 

 time to time it extends from its burrow to the surface of the water 

 where it captures a bubble of air in a modified part of its tail which 

 is specialized for respiration. With this it retreats into its burrow. 

 The large aquatic snail AmpuIIaria has a lung for air-breathing and 

 lays its eggs in masses on the stalks of plants above the water. Some 

 of the fishes make nests which float at the surface of the water and have 

 below them a foam of air bubbles which the young use for their respi- 

 ration. Others lay their eggs in the mud of the outer part of the 

 swamp, but during the wet season when the lower water may contain 

 some oxygen. Lepidosiren lays its eggs in an L-shaped burrow in 

 the mud guarded by the male, which is said to aerate the nest with air 

 brought from the surface and excreted from the vascular filaments 

 which it bears on its pelvic fins during the breeding season. 



I have worked on similar stagnant waters in two other parts of the 

 Tropics— in the forests of British Guiana and in Uganda [3, 4]. In 

 the Guiana forests the swamps (pi. 2, fig. 2) were shaded and protected 

 from the wind by the trees above them. They were often as completely 

 deoxygenated as the Chaco swamps but in them complete deoxygena- 



