COMMUNITIES IN OTHER INLAND WATERS 361 



ber of species is small. 12 In Samoa, too, the plankton is described as 

 being poor and dwarfed. 13 It is not surprising that cyclic morphologi- 

 cal changes (cf. p. 339) do not occur in the region of uniform tempera- 

 ture; even the cyclic appearance of males among cladocerans is lacking. 

 Decapod crustaceans on the other hand, are much more plentiful in 

 the fresh waters of the tropics than at our latitude ; there are 12 long- 

 tailed and 5 short-tailed forms in Lake Tanganyika; 112 species have 

 been reported for the Indian Archipelago, while Europe has a total of 



only 7. 11 



Parthenogenetic development in Entomostraca is more and more 

 limited toward the equator, though scarcity of males in tropical Cla- 

 docera has been reported. 14 This is not true of the phyllopods and the 

 ostracods. Although there are species of euphyllopods in the colder 

 regions among which males have never been found (Limnadia lenti- 

 cularis) or are very rare, males are in the majority in the collections 

 thus far made of African species {Limnadia, Lepidurus, Apus, etc.) . 

 Males are regularly present among the ostracods of tropical and sub- 

 tropical Africa, in species which are identical with or very closely 

 related to those which in colder zones do not produce males; for in- 

 stance, Cypridopsis newtoni, with both sexes present in Algeria and 

 Tunisia, is known only from females in Germany, and Cypris pube- 

 roides with both sexes in East Africa is represented in Germany by 

 females only. 15 



Polar fresh-water communities. — The low temperatures of the 

 polar regions considerably influence the fauna of their fresh waters. 

 The rivers are covered with an ice sheet in winter. The number of 

 species of fishes inhabiting these rivers is low, and a considerable num- 

 ber of them are migratory forms. Only 45 species are known in so 

 large a river as the Obi (a river basin of 3,000,000 km.), 4 and only 62 

 in all of Siberia excepting the Amur and Lake Baikal. 16 Of over a 

 dozen different river fishes in the Boganida (Taymyrland, 71° N. lati- 

 tude), with the exception of the eelpout, pike, stickleback, and perch, 

 there were only salmonids, of which only 2, Thymallus and Salmo 

 coregonoides, were permanent residents while the rest came up the 

 river to spawn. Similarly, polar rivers of North America do not harbor 

 many species of fishes; 14 are reported from the Yukon and 22 from 

 the Saskatchewan-Nelson River; the Mackenzie contains 23 species of 

 which 11 are Salmonidae. 17 It is the salmon which penetrate farthest 

 to the north; even in Grinnell Land in 82° 34' N. latitude the salmon 

 Salmo arcturus and Salmo naresi have been caught. How resistant such 

 polar fishes can be is shown by the Alaskan blackfish {Dallia pec- 

 tor alis) , a relative of the pike, which abounds in the rivers and 



