iç)i5-] Fauna of the Chilka Lake : Crustacea Decapoda. 203 



The species classed as seasonal immigrants are particularly interesting. Of the 

 only crab included in this category, Gelasimus annulipes, a well-established colony 

 was found in the outer channel in the salt-water season. The colony inhabited one 

 of the islands in this region and was composed of small and half-grown individuals, 

 some of which were probably hatched in the lake. In the freshwater season, when 

 the floods had raised the water-level of the channel by several feet, making a con- 

 siderable reduction in the size of the island, we were unable to find any trace of the 

 species. It is evident that with the arrival of the fresh water the colony was either 

 exterminated or was compelled to change its quarters. Unfortunately we were 

 unable to determine whether the appearance of the species in the outer channel is an 

 annual event. 



Concerning two other species that are classed as seasonal immigrants, Palaemon 

 malcolmsoni and P. rudis, we are able to offer evidence of a more satisfactory nature. 

 Towards the close of the monsoon these species are common in the main area and are 

 trapped in large numbers by the Uriya fishermen. Their appearance in the lake is 

 unquestionably an annual event coinciding with the freshwater season. During this 

 period egg-bearing females of P. malcolmsoni alone were seen, whereas in the case of 

 P. rudis adult males were commonly found together with ovigerous females. Except 

 for a series of small ovigerous females obtained at Satpara in salt water, which I 

 attribute with very considerable doubt to P. malcolmsoni, no adult Palaemon were 

 found in the lake during the salt-water season and the species are so large and speci- 

 mens are so numerous during the freshwater period, that it is scarcely possible that 

 we can be mistaken on this point. The adults must therefore migrate to the lake 

 annually from the ponds and streams of fresh water in its vicinity, and it is signifi- 

 cant that this migration coincides with the period when the females bear eggs and 

 that in one of the two species it is undertaken only by this sex. The only possible 

 explanation is that the females resort to the lake to hatch out their young in its 

 waters, an explanation which is corroborated in the case of P. rudis by the fact that 

 we obtained at different localities, and at all times of the year, young forms that 

 could definitely be associated with this species. 



Palaemon lamarrei is also classed as a seasonal immigrant. As in the case of 

 P. malcolmsoni only ovigerous females were found ; but these were obtained exclu- 

 sively in the salt-water season, in water of specific gravity varying from roo8 to 

 roil. The species apparently enters the lake for the same purpose as the other 

 two, but apparently prefers brackish water. 



The migrations of the Palaemonidae in the Chilka Lake are most remarkable, for 

 all the species are known to occur in fresh water in localities from which access to 

 tidal creeks and lagoons is clearly impossible. 



Eleven species we regard as casual visitors to the lake, ten being immigrants 

 from the sea and one from fresh water. In the outer channel in March, when the 

 water was as salt as that of the Bay of Bengal in the vicinity of the lake, Ponto- 

 philus hendersoni occurred near the sea-mouth, and on the sand-banks in the same 



