I9 I 5-] Fauna of the Chilka Lake : Introduction. 15 



which settles on a flat surface is liable to be smothered by the deposition of fine silt 

 in calm weather. 



4. The majority of the animals that can be classed under the heading of weed 

 fauna are associated either with Potamogeton or with Halophila. Young fish of many 

 species take shelter amongst the dense thickets of the former plant , to which the 

 insects of the lake are, at any rate in the salt-water season, almost entirely con- 

 fined. Several species of Decapod Crustacea and at least one very abundant Iyamelli 

 branch mollusc are also characteristic of these thickets. The comparatively large 

 leaves of Halophila act as a base for several small sponges, coelenterates and polyzoa. 

 On the whole the scantiness of the fauna associated with weeds is a little surprising. 



5. Under the heading of free-swimming organisms we must include the 

 majority of the fish, as well as a few medusae and at least one Ctenophore, also 

 several Decapod Crustacea and at least three species of Mysidacea. As a rule the 

 animals falling under it are perhaps the least interesting with which we have to deal, 

 and it has been impossible, except in a very few instances, to add materially to our 

 knowledge of their biology or distribution. 



6. We are hardly in a position as yet to say much about the plancton beyond 

 stating that in the main area of the lake it is never abundant and almost disappears 

 for a time in the earlier part of the rainy season, while in the outer channel it 

 becomes, in the salt-water season, both more abundant and more varied than it ever 

 is inside Mugger-Mukh. One point may be noted, however, viz., that in most of our 

 samples from the main area Copepods and larval molluscs greatly predominate. 



We have not included among the headings tabulated above that of c amphibious 

 fauna,' as perhaps we might have done. There are of coarse a certain number of Crus- 

 tacea, insects and other animals that would naturally fall into this category ; but 

 the amphibious fauna fades so gradually into the terrestrial one, with which we do not 

 propose to deal, that it has seemed best to consider separately the status of each spe- 

 cies that lives only partially in water. 



Regarded as a whole, the fauna of the lake may be described as mainly of marine 

 origin. A few freshwater forms have, however, established themselves, while there 

 is also a marked faunistic element that appears to have originated actually in 

 estuaries or backwaters subject to great changes of salinity and temperature. This 

 element is also well represented in the Cangetic delta and in lagoons on both coasts 

 of Peninsular India. A fourth element consists of species that immigrate at appro- 

 priate seasons either from the sea or from neighbouring streams, ponds and rice- 

 fields, while a fifth — of little importance — is composed of mere casual visitors that 

 drift, swim or crawl into the lake and exist there for a period without establishing 

 their species among its permanent inhabitants. 



The abundance of individuals and poverty of species noticed under the heading 

 of mud fauna is to a very large extent characteristic of the fauna generally and in 

 particular of that of the main area. 



Perhaps the most striking feature of the biology of the permanent residents in 

 the lake is the extraordinary power of individual adaptability to physical changes 



