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. We recently documented these worms in Washington State Pacific oysters (Crassostrea gigas) for the very first time, and with support from the Western Regional Aquaculture Center (WRAC, NIFA, USDA), we are leading an effort to map the distribution of these worms across the Pacific Northwest and to offer growers options for avoiding and treating infestations on their farms.

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.

Our goal is to help the Pacific Northwest avoid the fate of these other regions. We will accomplish this by:

  • Mapping the current distribution of Polydora spp. at commercial oyster farms across the US Pacific Northwest (Alaska, Washington, Oregon, and northern California).
  • Identifying the environmental factors that predict high infestation rates.
  • Identifying the most effective intervention that growers can use to reduce transmission and mitigate the negative impacts of infection on product value.

To date, our team has detected Polydora websteri and other Polydora species at four of seven sampled sites across Washington State (Figure 2; Lopes et al. 2019). 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 2. 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).

Shellfish aquaculture is a low-input, sustainable means of seafood production that augments the supply of locally produced, nutritious food. We seek to address the needs of the Pacific Northwest’s aquaculture sector by assessing the scale of this threat and exploring options for reducing the barrier it presents to oyster aquaculture development. Through our new industry advisory group (the Healthy Oysters Steering Committee), we will ensure a coordinated and complementary exchange of information and ideas across our public–private partnership, including information on effective treatments for Polydora infection and tools for on-farm identification and quantification of Polydora burden. Given that non-native marine species move easily across space, our discovery of Polydora websteri and other Polydora species in Washington portends similar problems throughout the Pacific Northwest region. Our project will provide oyster farmers with tools to prevent an emerging nuisance species from reducing oyster production and profitability in the Pacific Northwest.


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