Spionid polychaete infestations of intertidal Pacific oysters Crassostrea gigas (Thunberg), Mahurangi harbour, northern New Zealand

Four species of spionid polychaete infested shells of Pacific oysters Crassostrea gigas, in the Mahurangi Harbour, northern New Zealand: Polydora websteri, P. hoplura, Boccardia (Paraboccardia) acus, and B. (B.) chilensis. Three mudblister types, chiefly attributed to P. websteri, were characterised based on their morphology and position within the shell. Fortnightly washing of oysters for 5 and 11 months of growth had no effect on spionid infestations or oyster condition factors. On experimental intertidal racks P. websteri recruitment correlated more strongly with intertidal exposure levels than sedimentation rates. Increasing exposure significantly decreased the incidence of larval new mudblisters, P. websteri infestations, and shell blisters. Dry weight condition index values, dry meat weights, and dry shell weights were not significantly correlated with increased mudworm infestations. Oysters were more suitable for the half shell market and oyster condition was significantly greater towards the harbour channels and entrance. Spionid infestations can be avoided by growing oysters above extreme low water neap and 0.5 m above the mud substratum. (C) 1997 Elsevier Science B.V.