Among the Thais species, the broad consensus has long been that Thais clavigera (Kuster) is the most noxious predator of cultured oysters in Taiwan. Recently, two new Thais species (i.e., T. rufotincta Tan & Sigurdsson 1996 and T. keluo Tan & Liu 2001) have been identified and named in Taiwanese waters yet their impact on oyster culture is unknown. In this study, the overall impact of the three species on the oyster industry was estimated on the basis of their distribution in the field, their feeding rate and their temperature preference based on laboratory tests. The proportion of T. clavigera varied monthly from 24% to 100%, whereas the percentage of T. keluo was negatively correlated with low tide levels (P < 0.01). Thais clavigera occurred widely in the intertidal zone and T. keluo and T. rufotincta near the subtidal, this distribution pattern was consistent with their specific-preferred temperatures in the upper-limits. As shown in the laboratory, T. clavigera, T. rufotincta and T. keluo preferred 36°C, 32C° and 30°C water, respectively. In the field, the respective average feeding rate of T. clavigera, T. rufotincta and T. keluo was significantly different at 0.054, 0.010 and 0.038 oysters snail−1 day−1 (P < 0.05). Based on the abundance of the three Thais species at oyster cultural sites (Liu 2002) and their feeding rates, when oyster predation was made up of T. clavigera, T. rufotincta and T. keluo, predation was respectively 87%, 11% and 2%. The most destructive Thais species in the oyster industry remains T. clavigera, and it accounts for 87% to 100% of all intertidal losses in Taiwan. To the other one-third subtidal culture industry, owing to the use of off-bottom raft or longline method, the distribution of snails extended to subtidal may be limited and the reported major predator is the flatworm of Stylochus orientalis.
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Vol. 25 • No. 2