350 THE VIEWPOINT OF A PHYSIOLOGIST 



stress examples of phenotypic differences, then reported races, 

 and finally species adaptations. 



Water and Ions. The transitions between sea and fresh 

 water and between regions of low and high humidity on land 

 permit much physiological variation in water economy. Closely 

 related to this is ion balance, although specific elements have 

 scarcely been considered as limiting factors in the distribution 

 of animals. Numerous examples of what appear to be phenotypic 

 osmotic differences between populations are known. North Sea 

 starfish ( Asterias rubens ) are much more tolerant of dilution than 

 populations from the Kiel Canal (Schlieper, 1929). Gills from 

 Mytilus from the North Sea (salinity 30°/oo) consumed 80 ml. 

 O2 per gram per hour, whereas after four weeks at a salinity of 

 15°/oo the corresponding value was 144 ml. O2 per gram per hour; 

 initial O2 consumption by gills from Mytilus living in the Baltic 

 at 15°/oo was 141 and after four weeks at 30°/oo it was 84 ml. 

 O2 per gram per hour ( Schlieper, 1953, 1955 ) . Blue crabs ( Calli- 

 nectes sapidus) collected in the upper parts of an estuary not 

 only tolerate lower salinities but regulate their internal osmo- 

 concentrations down to lower environmental limits than do crabs 

 from the mouth of the river, but after a week in normal sea water, 

 the crabs from the upper river approached in tolerance and regu- 

 lation those from the river mouth (Anderson and Prosser, 1953). 



Several examples of possible osmotic races may be cited. Ne- 

 reis diversicolor in Denmark penetrate into nearly fresh water 

 and tolerate sudden transfer to fresh water better than do N. 

 diversicolor from England. These differences have been inter- 

 preted as indicating two races. However, tests of the regulation 

 of internal chloride as a function of environmental chloride show 

 similar limits of regulation for populations from Denmark, Scot- 

 land, and Finland (Smith, 1955a,b). The limiting salinity may 

 not be the summer value, which has been mostly studied, but 

 rather the spring dilution when the embryos are developing. 

 Perhaps sonic measure other than chloride regulation would re- 

 veal racial differences, but present evidence suggests only non- 

 genetic variation. Two brackish water gammarids, formerly 

 considered one species, are now recognized as two species, Gam- 



