OUR knowledge of the ecology and habits of land crabs of the family Geocarcinidae, especially of the genus Cardisoma is wanting in several respects. In the tropical Indo-Pacific, where two species of this genus are recognised, the available information is restricted to stray observations, mostly pertaining to locality records. 
During a recent visit to the Andamans and Nicobar Islands in February-March 1960, we were interested to see in several places close to the shore in Andamans, characteristic earthen mounds, some of exceptionally large size and each resembling a termitarium. A close scrutiny showed that the mounds were crab castles, and in some fields several of these castles of varying sizes were seen generally ranging in height between 30 and 50 cms. and spread out almost to the shoreline. The largest of such aggregations both in numbers and size we found in places adjacent to Phoenix Bay, Port Blair, and at Mayabundur, where each mound was about 75 cms. high with an almost equal basal diameter. Although we were told that several hundreds of these crabs teem over the colony areas and adjacent fields at night, this was not to be the case, atleast during the period of our visit and only on the last night of our stay at Port Blair were we able to catch one of the crabs from a colony near Marine Drive, Aberdeen Bay which we visited or passed by at different hours of the night as well as during day time when observations were made. The crab is a large female Cardisoma carnifex (Herbst) (Plate I, fig. 2), a species having a fairly wide distribution in the tropical Indo-Pacific. 
 

cardisoma camifex, tropical indi-Pacific.