Polydora infestation on Crassoslrea madrasensis : a study on the infestation rate and eradication methods

Abstract

Polydora ciliata a common polychaete worm, which bores and resides within the oyster shells, was found in the oyster populations of this estuary. The percentage infestation in the oysters occurring in the natural bed and in different age classes of farmed oysters was studied. Infestation was low in the natural bed than in the farmed oysters. About 80% of small oysters (<6 month) were uninfested in the natural bed while in the farmed oysters of the same age group, only 44% were uninfested. The severity of infestation was observed to increase with age in the natural bed as well as in farmed oysters. Experiments were conducted to eradicate these worms by dip treatments in formalin, chlorine and freshwater. Formalin treatments in three different doses, 1000,500 and 250ppm for 30 minutes, I hand 2 h respectively were capable of removing 79.6%, 69.1% and 69.6% worms from oysters with minimum mortality (6.6,1.6 and 0% mortality) to test oysters. Eradication treatment using chlorine at doses 1000, 700 and 500 ppm for 3,5 and 6 h were successful in eliminating 78.3%, 65.1 % and 57.7% worms from shells with test oyster mortality of 15%, 11.6% and 3.3%. Freshwater treatment for 3,6, 9 and 12h and aerial exposure after brushing the oysters with formalin were not effective in eradicating the mudworm.