Sulfide oxidizing activity as a survival strategy in mangrove clam Polymesoda erosa (Solander, 1786)

Abstract

The clam Polymesoda erosa is a large dioceous bivalve mollusk found in the mangroves restricted in high tidal Avicennia sp. zone of the mangrove forest. The present study was aimed at understanding the occurrence of P. erosa in mangroves in relation to its ability to oxidize sulphide. In January 2011, sampling was conducted in 4 different mangrove forests viz. Siolim, Nerul, Chorao and Chapora (locations) in Goa. Sediment bacterial abundance was statistically similar at all the sites with high values at Chapora (2.5x10 sup(9) to 4.5x10 sup(9)) and lower values were at Siolim (1x10 sup(9) to 2x10 sup(9)). The total number of bacteria per gram of fresh tissue of P. erosa was highest (3.8x10 sup(9)) in Chorao station in the size class of 7.0-7.9 cm whereas; bacterial count was lowest in the same size class clams from Nerul (0.5x10 sup(9)). Cytochemical staining for sulfide oxidizing activity with Benzyl viologen resulted in the formation of purple color at the sites of sulfide oxidation in clam tissues from all collection sites. Gill tissue showed the intense purple color compared to foot and mantle. Both highest as well as lowest concentration of sediment sulfide was recorded in Nerul compared to other stations and ranged from 0.1-11.4 mM/g of sediment. Both highest (3.31U/g) as well as lowest (0.7U/g) values of sulfide oxidizing activity of clam tissue were recorded at Chorao in the specimens of 5.0-5.9 and 6.0-6.9 cm size class respectively. P.erosa may be a symbiotic bivalve as it contains endosymbionts in its gill tissue in addition to sulphide oxidizing regions and sulphide oxidase enzyme activity. All these characteristics may enable the clam to use it as an additional source of food or for its survival in the sulphide containing sediments of the mangroves.