Spionid polychaete worm infestations of the Pacific oyster Crassostrea gigas in New Zealand

Abstract

Shell blistering induced by spionid polychaetes reduced the market quality of 10-40% of intertidal oysters in the Mahurangi Harbour, North Island in 1991, and, 22% of subtidally grown oysters in Admiralty Bay, South Island in1992-95. Polydora websteri induced the majority of shell blisters in the intertidal oysters. The larvae of P. websteri entered oysters through their mantle cavities. On experimental racks, the recruitment of P. websteri and blistering decreased with aerial exposure but was largely unaffected by sedimentation. Intertidal infestations are best managed by growing oysters at ELWN and at least 0.5m above the substratum. In Admiralty Bay, Boccardia knoxi bored the external shell of subtidal oysters inducing blistering after a lag period. Shell blistering was consistent with a hypothetical spring dispersive phase of planktotrophic B. konxi larvae. This hypothesis was confirmed later when these planktotrophic larvae were located and described during September 1996. In efforts to minimise infestations, experiments were conducted to increase the growth rates of oysters during the time of year B. knoxi larvae were absent. Oyster cultivators have long observed that the growth rates of oyster spat kept in the intertidal are stunted, and these oysters when transplanted to the subtidal, exhibit relatively rapid growth rates. Experiments in this study showed that the rate of growth was closely correlated with the duration of stunting period. Fouling and spionid infestations can be avoided by utilising an annual crop-rotation and using stunted spat to optimise growth and condition of subtidal oysters. Field and tank experiments showed an optimum stunting period of 8/9 months produced the fastest growth rates with minimal blistering. Energy-compensatory and energy-conserving mechanisms increased with the intertidal stunting period. The effect of stunting oysters with a limited diet in a tank indicated that food limitation rather than discontinuous feeding patterns caused the stunting effect. The effect of shell blistering on oyster condition was slight in subtidal oysters. Loss of condition was detected by three out of four condition indices - the dry weight gave the greatest power. Although loss of condition was considered biologically insignificant, by definition the measurable impact effectively rendered B. knoxi a parasite.