Non Technical Summary

Commonly called 'wafers' or 'oyster leeches', flatworms are a pest to shellfish culture in many areas of the world, including the Australian east coast. One species in particular, Imogine mcgrathi, has long been known as a pest for Sydney rock oyster culture, but more recently has been found to eat pearl oysters and mussels. With the increased interest in culture of these species, the need arose to assess the potential impact of these flatworms and to develop methods for their control.

Two series studies were done involving flatworms collected directly from the pearl cages and mussel lines, respectively. In the first experiment, I. mcgrathi ate pearl oysters at a rate of 0.035 to 0.057/day. Predation was affected by flatworm size with larger worms able to consume larger oysters and greater dry weights of oyster flesh. Irrespective of flatworm size, predation was generally confined to oysters less than 40 mm in size and only occurred at night. In the second experiment, I. mcgrathi ate mussels at a rate of 0.32/day; but, when I. mcgrathi collected from mussel ropes were offered both mussels and oysters of a similar size, they ate only mussels. When offered only pearl oysters as food, these same I. mcgrathi did not feed at all, indicating that the nutritional history of I. mcgrathi affects their potential impact on a species.

Given the slow rate of predation, the potential impact of flatworms in bivalve culture will largely depend on the numbers of worms present. When culturing juvenile pearl oysters in Port Stephens, flatworms were found in relatively low numbers (3.2 per oyster spat collector bag) and were not considered a threat. In contrast, in Twofold Bay, flatworms occurred at numbers as great as 386/m of mussel culture rope and had the potential to eat the entire crop in less than six weeks.

To develop methods for their control, flatworms were exposed to solutions ranging in salinity from 0 to 250 ppt for periods from 5 min to 3 h. I. mcgrathi survived exposures of 3 h at salinities in the range 7.5 to 60 ppt, inclusive. Beyond this range, the duration of exposure tolerated by flatworms decreased until 0 and 125 ppt, at which, the flatworms no longer survived exposures of 5 min. For ease of use, current protocols to kill I. mcgrathi call for soaking in baths of salinities of less than 2.5 ppt for 30 min.