32 



Comparative Animal Physiology 



It has frequently been reported that fresh-water animals consume more 

 oxygen per unit weight than do brackish and marine forms. The following 

 figures 1^^ showing the cubic millimeters of oxygen consumed per gram of 

 weight per hour for three species of Gammarus support this: 



Further, the oxygen consumption in unanesthetized G. chevreuxi was 516 

 mm.Vg'm./hr. in 100 per cent sea water and 648 mm.Vgm./hr. in 25 per cent 

 sea water. The oxygen consumption of crayfish is 30 to 40 per cent higher in 

 fresh water than in isotonic sea water.^-'^- -" Part of the extra energy made 

 available in fresh water must provide for the salt and water secretory move- 

 ments against concentration gradients; part may be concerned with maintain- 

 ing low permeability to salt and water. 



Krogh suggests that the capacity to absorb salt actively is of wide occurrence, 

 especially among fresh-water animals. He gives evidence from Fritsche indi- 

 cating that Daphnia absorbs salt. The fairy shrimp, Chirocephaliis diaphanns, 

 loses salt rapidly in distilled water but recovers it in a very dilute salt solu- 

 tion. ^'-^^ Branchipus and Apus (Lepidiinis) are unable to remain alive long 

 in distilled water, fresh water, or dilute Ringer solution without food. Krogh 

 suggests that these animals lack the power of active salt absorption. 



In contrast to those animals which are dependent on salt absorption, Gavi- 

 marus pulex remained healthy in glass-distilled water which was changed 

 twice daily and lost chloride for only the first 2 to 5 days. -^ Also Aselhis and 

 larvae of the mosquito Aedes live well in distilled water. In these animals salt 

 retention is important and survival is possible, at least over periods of days, 

 without salt absorption. However, adjustment to low tonicity of the environ- 

 ment does require some initial loss of chloride. -^ 



Regulation Permitting Life in Salt Lakes : Brine Shrimps. The fresh-water 

 Crustacea have specialized in hypertonicity in dilute media. A few Crustacea 

 have gone to the other extreme and maintain hypotonicity in concentrated 

 brine. Anemia, the brine shrimp, lives in Great Salt Lake in a salt concentra- 

 tion of 22 per cent and in salt lakes of even higher concentration in other parts 

 of the world. Artemia is normally hypotonic but varies its internal concentra- 

 tion with the medium. "^ In a lake of 8.4 per cent salinity the blood con- 

 centration was equivalent to 1.3 per cent sodium chloride. In an external 

 salinity of 17.4 per cent the blood concentration was equivalent to 2.4 per cent 

 NaCl, hence blood concentration did not increase in proportion to concentra- 

 tion of the medium. ^^^ Weight is gained or lost when Artemia is transferred 

 to hypotonic or hypertonic media. The permeability of Arte^)lia to heavy water 

 is low compared with that of Daphnia (Ussing, from Krogh^^'^). Artemia 

 takes up dyes, which enter most Crustacea through the gills, only by 

 mouth. -"^ There must be other mechanisms, besides the low permeability, 

 which enable the brine shrimp to have a blood concentration as low as 10 per 

 cent of that of the medium. There is some evidence for water storage in the 

 intestine in a dilute medium. ^^'^ The gills of Artemia are larger in media of 

 high salinity than in media of low salinity, an effect probably related to lower 



