THE EVALUATION OF 2 TREATMENTS TO REDUCE MUD WORM (POLYDORA-HOPLURA CLAPAREDE) INFESTATION IN COMMERCIALLY REARED OYSTERS (CRASSOSTREA-GIGAS THUNBERG)

The quality of commercially reared oysters is markedly lowered by infe station with the burrowing polychaete Polydora, Two methods to eradica te Polydora hoplura in the Pacific oyster Crassostrea gigas were teste d in the laboratory and followed by field trials, Oysters infested wit h Polydora, obtained from a commercial oyster farm (Algoa Bay, South A frica), were (1) exposed to fresh water for 12 h and, (2) exposed to h eated (70 degrees C) sea water for 40 s. After treatment the oysters w ere placed in plastic crates and returned to the sea for a recovery pe riod of 2 months. The crates were suspended from long lines following the practice of the commercial oyster farm. Fresh water and heat treat ments did not significantly increase oyster mortality. The Polydora in festation per oyster was significantly reduced by both treatments (F-0 .05(1,2.18) = 27.663, P < 0.05). Heat treatment yielded the lowest ave rage infestation (1.13 polydorids per oyster) when compared with the a verage infestation of the fresh water treatment (1.59 polydorids per o yster) and the untreated control (3.05 polydorids per oyster). Treatme nt of commercially reared Crassostrea gigas with fresh water or heated sea water seems to be a practical means to reduce Polydora infestatio n.