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This study was carried out to investigate a paradigm of predation under natural conditions in Bizerta lagoon. The free-living marine polyclad flatworm Imogine mediterranea was found to feed on the commercial mussel Mytilus galloprovincialis. Such predation could represent a real threat to mytiliculture at this site. Our field experiments have shown that the mortality of the commercial mussel as a result of active predation by this polyclad is approximately 52% when one polyclad and one prey were placed together in a same experimental container. We found a statistically significant relationship between the size of the polyclad and the extent to which the mussels were disabled or completely eaten. Moreover, in our field experiments, the times required for predation-caused decreases of populations of the bivalves, that is, LT10, LT25 and LT50, decreased significantly with increases in the size of the polyclad. The time, LT50, required for polyclads to kill half of the bivalve population in field experiments was approximately 4.5 days. A simple disabling (an attack by the polyclad on the adductor muscle of the mussel) was shown to be sufficient to cause irreversible changes leading to mortality. Furthermore, disabling and mortality were found to be linearly correlated. The importance of mucous secretions, pharyngeal type and body musculature in supporting predation by polyclads is discussed. We suggest that measures to control the polyclad pest could incorporate the control of this flatworm's embryonic and postembryonic developmental stages. [ABSTRACT FROM AUTHOR]