Invasive parasites

Infestations of mud worms – parasitic polychaetes in the genus Polydora – are responsible for substantial losses to commercial oyster industries worldwide. These polychaetes burrow into the shells of bivalves and cause unsightly blisters that release detritus, mud, and fecal material, fouling oyster meats. Even when blisters remain intact, they compromise the aesthetic presentation of oyster meats on the shell (Figure 1) and reduce an oyster’s value for canning or smoking.

Figure 1. (a) Deformities of the inner surface of an oyster valve from freshly shucked Crassostrea gigas, collected in Oakland Bay, Puget Sound, WA on 3 October 2017. (b) Close-up of a single blister, with scale bar indicating 2mm. (c) Close-up of a burrow, which is in the process of developing into a blister, with scale bar indicating 2mm. Both (b) and (c) contained worms identified by our team as Polydora websteri.

Oyster industries around the globe have experienced economic losses due to mud worm infection. Oyster farm losses due to Polydora infestation have been recognized since the 1940s along the US east coast. As a non-native nuisance species, widespread expansion of Polydora caused the collapse of oyster aquaculture on Oahu, Hawaii. In New South Wales, introduction of Polydora in translocated oysters was responsible for the historical disappearance of once-extensive subtidal oyster beds in the 1860s. Polydora have been described as “the greatest obstacle to intensive oyster culture” in Australia.

To our knowledge, there have been no reports in the scientific literature of any Polydora species in Pacific oysters (Crassostrea gigas) from Washington. However, growers and government agency aquaculture specialists have begun to observe these blisters over the past years. Recently, our team detected Polydora websteri and other Polydora species from four of seven sampled sites across Washington State (Figure 2; Lopes et al. in prep). At one site, 53% of sampled oysters were infected. Given the fact that these worms have never before been formally reported from the Pacific Northwest, Polydora is likely to be invasive in the region.

Figure 3. Map displaying the seven sampling sites in Washington State where we analyzed oysters for mud worm infection. Oyster farms were sampled at sites 1, 3, 4, 5, and 6, and public oyster beaches were sampled at sites 2, 3, and 7. Pie charts indicate proportion of oysters showing blisters or burrows (green) and oysters with no evidence of infection (blue).

Pacific oysters are the most important cultured shellfish in the Pacific Northwest and mud worm infections represent a serious threat to the sustainability of this industry. To defend against this threat to the region’s oyster industry, our team is working to identify the growing areas that are currently affected, the environmental factors predispose an area to becoming infected, and the treatments are most effective for reducing worm burdens on farms. Answering these questions will arm oyster growers with the tools needed to predict, understand, mitigate, and prevent Polydora-driven production losses.


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