mangrove communities are considered as important elements in coastal ecostems in the tropics beacause they are known to be highly productive systems which function as nursery grounds to many commercially important marine species . In most countries of the tropics, crabs and mollluscs dominant the mangrove forests. Smith et al. have shown that the sesarmid crabs occupy a keytone position in australian mangrove forest. The potential role of crabs in cycling of meterial in mangroves communities and their ability in conserving nutrients whith in the forest are well recognized. Smithet al (1991 showed that reductions in crabs abundance had led to increased sulphide and ammonium concentration in the soil and to reduced productivity and reproductive output by the mangroves. Bertness showed that the reduction of fiddler crabs density for a single growing season decreased above ground production by 47% and burrowing by the crabs increased soil drainage, soil oxidation-reduction potential and theinsitu decomposition of below ground plant debris.
Diversity of fauna in mangrove forest of Sri lanka is so immense but is a relatively poorly dealt subject. The studies dealt with mangrove crabs is Sri lanka are restricted to the study carried out by De Silva 1985 on fauna in five mangrove forest of Sri lanka and the study done by pinto 1984 on the crabs of negombo estuary. It was the felt that it important to recognize the diversity of crabs in mangrove forest of Sri Lanka before initiating studies on their role in the mangrove forest . The present study was carried out to investigate the diversity and environmental interactions of mangrove crabs in the Kadolkele mangrove ecosystem, in the Negonbo estuary. The distribution of crabs in different zone of mangrove was also investigated.