CRUSTACEA DECAPODA. 



By Stanley Kemp. 



The Crustacea Decapoda form an important part of the fauna of the Chilka 

 I^ake and comprise , including casual visitors, fifty-four species. At least thirty-nine 

 are permanent inhabitants and it is not a little surprising that so large a number of 

 forms should have adapted themselves to the peculiar physical conditions that prevail 

 in the lake- system. 



The results of our observations on the physical environment of the fauna have 

 already been discussed in detail in the Introduction to this volume : but it will be 

 well to recapitulate here one or two of the more salient features. 



The lake, which lies on the coast of Orissa, may be divided into two regions, 

 firstly the main area , a large lagoon fed by rivers from the northern end only and 

 even in the flood-season nowhere more than 15 ft. in depth, and secondly the outer 

 channel, some twelve miles in length and but little deeper than the main area, which 

 forms a communication with the sea. The bottom of the main area is mud, or mud 

 with a small admixture of sand ; in the outer channel it is composed of muddy sand 

 in the inner portion and of clean sand nearer the mouth. It is, however, the great 

 seasonal changes in salinity that constitute the most noteworthy feature of the 

 physical conditions. In August, towards the close of the rainy season, the levé" jï 

 the lake is considerably higher than at other times of the year, owing to the great 

 volume of fresh water which is poured into it from the north. The fresh water 

 expels most of the salt water from the lake, with the result that during August and 

 the two succeeding months the northern portion of the main area and the whole of 

 the outer channel up to the sea-mouth are completely filled with water that shows 

 no trace of salinity. The southern end of the lake always remains slightly brackish, 

 though during these months it is far less saline than at other times of the year. By 

 the end of October the floods begin to subside and later, under certain conditions 

 of wind and tide, salt water from the Bay of Bengal enters the outer channel and 

 the main area. For a considerable part of the year the outer channel remains filled 

 with water of the same density as that of the Bay of Bengal in the vicinity of the 

 sea-mouth (sp. gr. 1-0265). In the main area it is improbable that so high a density 

 is ever reached, for a certain amount of fresh water is constantly entering the lake 

 from the north. According to our observations the specific gravity does not rise 

 above 1-0150. 



It is evident, then, that species that permanently inhabit the outer channel can 

 withstand seasonal alterations of salinity varying from that of fresh water to that of 

 water as salt as the sea in the neighbourhood of the lake, while those that exist in 



