The evaluation of two treatments to reduce mud worm ( Polydora hoplura Claparède) infestation in commercially reared oysters ( Crassostrea gigas Thunberg)

Abstract

The quality of commercially reared oysters is markedly lowered by infestation with the burrowing polychaete Polydora. Two methods to eradicate Polydora hoplura in the Pacific oyster Crassostrea gigas were tested in the laboratory and followed by field trials. Oysters infested with Polydora, obtained from a commercial oyster farm (Algoa Bay, South Africa), were (1) exposed to fresh water for 12 h and, (2) exposed to heated (70 °C) sea water for 40 s. After treatment the oysters were placed in plastic crates and returned to the sea for a recovery period of 2 months. The crates were suspended from long lines following the practice of the commercial oyster farm. Fresh water and heat treatments did not significantly increase oyster mortality. The Polydora infestation per oyster was significantly reduced by both treatments ( F 0.05(1),2,18 = 27.663, P < 0.05). Heat treatment yielded the lowest average infestation (1.13 polydorids per oyster) when compared with the average infestation of the fresh water treatment (1.59 polydorids per oyster) and the untreated control (3.05 polydorids per oyster). Treatment of commercially reared Crassostrea gigas with fresh water or heated sea water seems to be a practical means to reduce Polydora infestation.