Polychaetes infestations of flat oyster (Ostrea edulis, LINNEO 1758) in coastal area of the Po delta (n.e. Italy).

Suspended culture system is a new method to produce flat oysters (Ostrea edulis Linneo 1758) into the open sea in Italian coast. 150 long-line cultured flat oysters were tested respectively in July and December 1997 for the presence of Bonamia ostreae. The oyster farming is located 5 miles offshore in the east of Porto Levante (North-East Italy) in 25 metres of water. The lanterns are suspended 12-15 meters up from the bottom. The stocking density is 30 specimens per tray (300 specimens per lantern). The total stock is 100.000 molluscs for a two year cycle of production. Oysters mortality during 1996 has been relatively low (10%). In 1997 the mortality reached 60% due to an algal bloom. During sampling 84% of the oysters examined presented wide burrows repleted with muddy debris on the inner surface of both valves. Polychaetes (Anellida) were detected inside the burrows belonging to five families: Terebellidae, Serpulidae, Syllidae, Nereidae, Eunicidae. In particular the following Taxa were identified: Syllis sp., Nereis sp., Nereis caudata and Eunice pennata. Examination with a binocular microscope revealed the holes of Anellida's penetration on the shell's outer surface. The impacts of these worms on molluscs are variable but the most damaging form of infestation results from boring action and the subsequent formation of mudblisters. Oyster shells containing mudblisters are unsuitable for the shellfish market given their have an undesirable appearance.