Physico-chemical characterisation of the extrapallial fluid of a common tellinid bivalve Macoma birmanica (Philippi) in the mudflats of Sundarbans mangrove, Bay of Bengal

The chemical composition of the extrapallial fluid of Macoma birmanica was examined in order to understand the process of calcification. Concentration s of inorganic ions Na+, K+, Ca2+, Cl- and SO42- were found in higher propo rtion in the pallial fluid than those in the ambient medium. Calcification from pallial fluid was found to be related to the removal of proton from th e fluid through the excretion of NH3. The concentration of total positive c harge is 14.1% higher than that of negative charge, indicating the presence of non-difusible negative ions. From EPR measurements, apparent complexati on capacity of Mn in the fluid was in the order of 0.37 x 10(-5) M and form s 1:1 chelate with stability constant of 1.1x10(4) per mole. It was found t o be highly undersaturated with respect to the solubility of skeletal arago nite in the pallial fluid and has the capacity to keep high concentration o f calcium in solution through the formation of ion pair with HCO3-. Precipi tation of aragonite was initiated through the formation of Ca chelates.