Air-breathing in Fishes 



Because of the greater volume of oxygen in unit volume, it might 

 be expected that the conditions for the respiration of all animals 

 would be more favourable in air than in water. This is not 

 true for fish, however, because their very fine adaptations to an 

 aquatic existence make it difficult for them to take advantage of 

 the more favourable O2 supply. Most fish die out of water 

 because the gills collapse under the forces of gravity and surface 

 tension which bring the secondary lamellae together as the water 

 between them is lost and consequently the effective respiratory 

 surface is reduced. There are some fish in which special support- 

 ing tissue is present which decreases the rate of water loss and 

 restricts such a collapse e.g., some gobies, including the mud- 

 skipper (Periophthalmus), which live on shores where they are 

 liable to find themselves out of water. Other fish, notably eels, 

 make migrations across land through moist grass and under 

 these conditions they rely almost entirely on gaseous exchange 

 through their skins. Cutaneous respiration is also used by these 

 fish when in water as well as by a number of other aquatic verte- 

 brates which usually have smooth and slimy skins. 



When an ordinary marine or fresh- water bony fish (in addition 

 to mud-skippers etc.) is taken out of water, similar physiological 

 changes are found to those which occur when a mammal dives 

 (pp. 62-63). For instance the heart beat of a carp slows from 60/ 

 minute to 5/minute and frequently the respiratory movements 

 are inhibited. Another similarity is that a great deal of lactic acid 

 enters the blood when the fish is returned to its normal environ- 

 ment. This suggests that when it is out of water anaerobic respi- 

 ration proceeds in the tissues but its products are prevented 

 from circulating to other parts of the animal. 



