Exploring Husbandry and Equipment Solutions to Infestations of Polydora sp. on a Maine Oyster Farm
Polydora websteri is a marine polychaete worm that has had strongly negative effects on oyster farmers in Maine, and the world over. P. websteri builds burrows inside the shell of an oyster, where it deposits its eggs, waste and mud. When the oyster is shucked for the half-shell market, the burrows are frequently cracked and opened, spilling their contents and making the oyster unsuitable for consumption.
Growers and scientists around the world have tried various methods to reduce or eliminate infestation of Polydora on their oyster farms, with only partial success. Regular air drying appears to have a positive effect, as does a combination of dipping in a saturated salt brine solution, followed by air drying.
A field project was undertaken during 2008 and 2009, in partnership with Jesse Leach and Eric Moran, of the Bagaduce River Oyster Company, to evaluate combinations of equipment, air drying and salt brining, in hopes of reducing the incidence of Polydora. Funding from the project came from the USDA's Northeast Sustainable Agriculture, Research and Education program (NE SARE).
We tried several types of growout equipment: Oyster Gro cages, standard floating shellfish bags, Dark Sea trays and bottom cages, combined with salt brining (5 to 15 minutes) and air drying (1 hour) in different combinations. Seed were stocked into the experimental units in 2008, and the experiment concluded in the fall of 2009. Notes were taken on growth and the level of Polydora infestation within each treatment. A treatment was attempted where 6 small green sea urchins (Strongylocentrotus droebachiensis) were placed into some shellfish bags, but the urchins did not survive long in the estuarine environment of the oyster farm, and were not viewed as viable options for that setting.
After two years of growth, the oysters in virtually all the treatments had high levels of Polydora, to the point where most oysters would have been unmarketable. The salt brine did have a limiting effect on Polydora, as did the regular air drying by the bottom cages, but there were still too many burrows for the oysters to be fully acceptable.
The project did lead to some promising areas however. At the Darling Marine Center, and at the Center for Cooperative Aquaculture Research, Bagaduce River oysters were subjected to short duration freezing (-5 ℃, up to 24 hours) or longer duration storage in cool, moist air (~ 3℃, up to 8 weeks). Both short duration freezing and longer duration cool storage did have a limiting effect on worm survival, with minimal mortality of the oysters.
Moist air storage appears to be a fair option for producers who need to break the cycle of infestation, and who have operations where this can be logistically achieved. More information is needed on the life history details of Polydora, including critical limits on temperature and salinity, term of viability for eggs in burrows, distribution of Polydora in coastal waters, and the critical factors that allow some locations to suffer heavy infestations while other areas are not impacted.
For more reading, consult:
Nell, J. 2007. Controlling mudworms in Oysters. Primefact #590, Dept. of Primary Industries, New South Wales, Australia.
Overwintering American Oyster (C. virginica) seed by cold damp air storage. Hidu, H., Chapman, S. 1988. Journal of Shellfish Research, vol. 7, no. 1, pg. 163.
For more information on blister worms in Maine and regionally, contact:
Dr. John Kraeuter
University of New England.
Dana L. Morse
Maine Sea Grant and University of Maine Cooperative Extension
Darling Marine Center
193 Clark's Cove Road
Walpole, ME 04573
Dr. Paul Rawson
University of Maine School of Marine Sciences
220 Murray Hall
Orono, ME 04469