Sundarbans biosphere reserve of India is an ecologically important zone which shelters diverse forms of animals and plants. Scylla serrata (Arthropoda: Crustacea: Decapoda) or estuarine mudcrab is an important consumer of this mangrove ecosystem. Blood or hemolymph of S. serrata contains hemocytes, capable of performing diverse physiological functions. Hemocytes the circulating blood cells are considered as immunoreactive agents capable of performing phagocytosis, nonself adhesion and aggregation. Arsenic is a major environmental threat in the specific districts of the state of West Bengal of India. Estuarine water of Sundarbans mangrove bears the risk of arsenic contamination due to anthropogenic and geological consequences. Magnitude of hemocyte aggregation was studied in depth under the exposure of 1, 2 and 3 ppm of sodium arsenite for 1,2,3,4,7,15 and 30 days of exposure. Arsenic exposure yields a dose responsive decrease of hemocyte aggregation against a control level of aggregation of 67.77%. Estuarine ecosystem of Sundarbans mangrove is characteristics of severe intra and interspecific struggle. In this dynamic ecosystem, the inhabitants participate in the struggle of niche occupation for survival and existence. Situation often leads to a state of acute predation and fight among animals. As a result, the animals experience physical wounding and loss of body fluid. Aggregation of hemocyte at wound site prevents the loss of blood and entry of microorganism and considered as an immunological response. Magnitude of aggregation was screened under the experimental exposure of sodium citrate, EDTA, heparin, fructose and mannose at different concentrations. Study was aimed to screen the effect of chelating agent, sugars and anticoagulants on aggregation. For all the chemicals screened, a drastic decrease in the occurrence of free cells were reported which is suggestive to role of these agents in the physiological process of hemocyte aggregation. Moreover exposure to arsenic may lead to gradual loss of blood cell hemeostasis of mudcrab distributed in its natural habitat. Continuous exposure to toxic arsenic may lead to a population decline of mud crab and loss of biodiversity in this region. © 2011 Nova Science Publishers, Inc.