ROTATORIA 179 



This lake is a small fresh-water lake in a "kar" at the head of a small tributary of 

 the Chany-chenmo River. The lake has a maximum determined depth of 14 m. When 

 visited 11-13 July, 1932, it was covered by a sheet of ice with a maximum thickness of 

 about 1 111. which was melting around the edges and from below. Most of the water was 

 at about 4.0°C., falling to 1.25 below the surface of the ice. A single specimen of Kcratdla 

 cochlcaris was noted in a vertical haul made from 13.5 to the surface. An extended study 

 of the lake will be given in a later paper. 



Togom Tso, altitude 5334 m. (Figure 7). 



This very small lake, lying between Togarma Tso and Chagra, was visited on the 

 afternoon of 9 July, 1932, when it was found to be almost entirely covered with ice. A 

 small belt of free water at the edge varied in temperature from 0°C. against the ice to 

 9.5°C. at the extreme margin. A few specimens of undeterminable bdellofds, one perhaps 

 Dissotrocha aculcata var. tiiberculata, and of FiUnia longiseta. were obtained in this marginal 

 water. The chloride content of the latter was less than 0.00005 N., the alkali reserve 0.0003 

 N., the pH 7.3. 



Although many lakes in the western part of Tibet apparently lie at about the altitude 

 of these three it is doubtful if any habitats capable of supporting planktonic or other swim- 

 ming rotifers exist much above 5500 m. On the other hand, Heinis (1910) has shown 

 that in the Alps the muscicolous fauna extends to 4000 m. so that it is reasonable to sup- 

 pose that bdelloid rotifers e.xist in the Himalaya at altitudes of over 6000 m. 



In considering the limiting factors determining the existence of organisms at very high 

 altitudes, it is clear that many which apply to terrestrial plants and animals cannot affect 

 aquatic forms. Thus terrestrial organisms may theoretically be limited by low temperature, 

 low oxygen tension, perhaps intense ultra-violet radiation, low CO^ tension in the case of 

 plants and some animals with a complex respiratory mechanism, and in the case of animals 

 deficiency in fuud supply. As will be pointed out in a later paper the oxygen tensions in the 

 high-altitude lakes examined, owing to their coldness, lie within the values frequently found 

 in surface waters in low-lying temperate countries. The penetration of ultra-violet light 

 into water is slight (cf. Carter and Beadle, 1930, and some unpul^lished observations made 

 on this expedition) . It may be of importance in the surface layers of water at high altitudes, 

 but much less so than in the case of terrestrial habitats. The available COo content of natural 

 waters is largely regulated by the quantity of alkali carbonate in the water, while so far as 

 the food supply of animals is concerned, evidence available as to the productivity of Ororotse 

 Tso suggests that this lake compares favourably with lakes at much lower altitudes (Hutchin- 

 son 1933). It seems, therefore, that temperature is the most important limiting factor in 

 the ecology of the high-altitude members of such a group as the Rotatoria. 



Sufficient is known of the high-altitude rotatorian fauna of Europe to justify some 

 comparison between that fauna and the present collection. From the monumental work of 

 Zschokke (1900), the papers of Brehni and Zederbauer (1904) on the Tyrol and Monti 

 (1906) on the Italian Alps and from the catalogue of Swiss Rotatoria by Weber and Montet 

 (1918), it is possil)le to prepare lists of the rotifers of the Central European Alps. Zschokke 

 indeed gives such a list of 65 valid, fully determined species from over 1450 m., and by 

 inclusion of later records this list is raised to 108. The limits chosen by Zschokke, however, 

 include the upper part of the forested zone ; if only the region above the forest line, from 

 1700 ni. upward, be considered, the list is reduced to 89 species. Such a list has indeed 



