362 ANIMALS IN INLAND WATERS 



tundra ponds of Alaska and eastern Siberia. It can lie frozen for 

 weeks, in baskets, and revive again on being thawed out; a frozen 

 fish swallowed by a dog was thawed out in the stomach and regur- 

 gitated alive! 18 



The standing waters of the polar regions are covered by ice to a 

 greater extent than the rivers, and are open for only a brief period. 

 Thus the ice-free period in Lake Myvatn in Iceland (69° 33' N. lati- 

 tude) lasts about 5 months; in Spitzbergen (76°-80° N. latitude), in 

 small water basins with favorable locations, 2*4 months, in larger 

 waters, 2 months, in unfavorable locations only l*/* months. The lakes 

 of the Antarctic Ross Island (64° S. latitude) are ice-free for only a 

 few weeks, and some indeed do not thaw at all during the year unless 

 the temperature is unusually high. But as soon as these lakes are free 

 from ice, their temperature rises, thanks to the constantly shining sun, 

 and remains fairly uniform. The fauna of such waters is poor. Only 

 15 species of Entomostraca (1 phyllopod, 4 cladocerans, 4 ostracods, 

 and 6 copepods) are known in the fresh waters of Spitzbergen, 10 in 

 Bear Island, and only 2 in Franz Joseph Land. 19 Few species are 

 capable of enduring such extreme conditions. There is a subarctic 

 community of plankton crustaceans,* There is also a similar group of 

 rotifers which recurs again and again in the open water. The wide 

 distribution of red color among northern plankton copepods is very 

 noticeable. It is generally true that in the plankton of the northern 

 lakes the floral component of the biota is much reduced in comparison 

 with the faunal; 20 the plankton depends principally on detritus. 



The fact that the time of development is shortened among north- 

 ern copepods is noteworthy. For Cyclops scutifer in northern Sweden, 

 the time from the hatching of the nauplius larva to the appearance of 

 eggs in egg sacs is a month, as a maximum; in middle Europe the 

 same development requires more than 4 months; 21 similar examples 

 are plentiful and represent a biological adaptation to cold rather than 

 a simple physical response to temperature, which, in fact, would give 

 exactly the opposite result. This adaptation may depend upon the 

 direct influence of the extended lighting, but selection may also play 

 a part. Corresponding with the short period of life, only one genera- 

 tion of males appears among many Daphnia; they are thus mono- 

 cyclic. The copepod, Diaptomus minutus, has only a single annual 

 generation in Iceland. The typical cold-water forms of Euphyllopoda 

 appear in Arctic waters, especially those of the genus Lepidurus. They 



* Holopedium gibberum, Daphnia hyalina, Bosmina obtusirostris, Bythotrcphcs 

 longimanus, Diaptomus laciniatus, and certain other species of copepods and 

 perhaps Heterocopc. M 



