133 Asian Fisheries Science 16 (2003): 133-140 ISSN: 0116-6514 Asian Fisheries Society, Manila, Philippines 133 Predation of Cultured Mussels, Mytilus galloprovincialis, by Stylochid Flatworms, Imogine mcgrathi, from Twofold Bay, New South Wales, Australia W.A. O’CONNOR 1 and L.J. NEWMAN 2 1 NSW Fisheries Port Stephens Fisheries Centre Taylors Beach, NSW, 2316 Australia 2 School of Resource Science and Management Southern Cross University PO Box 157, Lismore, NSW, 2480 Australia Abstract The Stylochid flatworm, Imogine mcgrathi (Jennings and Newman 1996), was found to be a predator of the mussel Mytilus galloprovincialis. These flatworms consumed mus- sels at a rate of 0.032 individuals·day -1 or 12.6 mg·day -1 in laboratory trials and occurred at densities as great as 386 m -1 of mussel culture rope in Twofold Bay. I. mcgrathi has previ- ously been found to be a predator of the oyster, Pinctada imbricata. In this study, I. mcgrathi collected from mussel ropes restricted their predation solely to mussels when offered oysters and mussels of the same size as food source. When offered only P. imbricata, these same I. mcgrathi appeared incapable of eating the oysters. I. mcgrathi have the potential to pose a significant threat to mussel culture and their abundance in culture warrants careful scrutiny. Introduction Often called the Mediterranean mussel, Mytilus galloprovincialis is far less parochial than the name might suggest. Found in Europe, Asia, southern Africa and Australia (McDonald et al. 1991), M. galloprovincialis forms the basis of several significant mussel culture industries. In Australia, mussel cul- ture is in its infancy, particularly in New South Wales (NSW) where currently approximately 31 tonnes of mussels are cultured annually (ABARE 2000). How- ever, proposals are in place that could see a significant increase in the produc- tion of M. galloprovincialis in the near future. https://doi.org/10.33997/j.afs.2003.16.2.004