Seasonal and spatial patterns of mudblister worm Polydora websteri infestation of farmed oysters in the northern Gulf of Mexico

B Dzwonkowski,Sm Cole,J Coogan,W Walton,Km Dorgan

doi:10.3354/aei00365

Abstract

Mudblister worms Polydora websteri bore holes into oyster shells, and oysters respond by secreting extra layers of shell, creating a mudblister. When shucked, mudblisters can burst and release anoxic mud. Thus, infestation devalues oysters, particularly on the half-shell market. This study quantified oyster condition index and worm abundances over 2 full growing seasons at commercial oyster farms on the US Gulf of Mexico coast, and our results indicate that oyster growth rate, manipulated through ploidy and stocking densities, had little effect on worm infestation. Larval spionid worms were found year-round. Larval abundances were slightly higher within than away from farms, and larval size distributions were skewed toward smaller larvae within the farms, suggesting that farms may be a source of larvae. Triploid oysters had higher or comparable condition index values to diploids, but during summer months, when worm infestation was high, worm infestation was not correlated with condition index. Previously infested shells deployed at farms became more infested than uninfested shells at moderate infestation levels, but re-infestation was influenced more by farm location than by previous infestation condition. Higher infestation at a farm with more variable salinity as well as higher infestation in 2017 when salinity was lower suggest that salinity may be a potential driver of mudblister worm infestation. Results indicate that oyster farmers on this coast should use desiccation to treat oysters for mudblister worms frequently during the summer, but that manipulating stocking density or ploidy is unlikely to be effective in preventing mudblister worm infestation.

Highlights

Infestation by spionid worms occurs world-wide in a variety of shellfish species including oysters, mussels, scallops, and abalone (Lunz 1940, Haigler 1969, Wargo & Ford 1993, Caceres-Martinez et al 1998, Read 2010)
We tested the hypothesis that previous infestation by P. websteri facilitates re-infestation of oysters. Support for this hypothesis would provide greater incentive for farmers to treat oysters more frequently to prevent established infestations. We addressed these diverse goals through two 1 yr long deployments of oysters, C. virginica (Gmelin, 1791) at farms along the Alabama coast in the US Gulf of Mexico to quantify P. websteri infestation and effects on oyster condition due to differences in stocking density, ploidy, season, and location (Table 1)
We found an overall peak in P. websteri abundance during the late spring and summer, which was more apparent during the short-term 2017 summer sampling (Figs. 3 & S2), but worm abundances varied considerably among oyster farms and between years

Summary

Introduction

Infestation by spionid worms occurs world-wide in a variety of shellfish species including oysters, mussels, scallops, and abalone (Lunz 1940, Haigler 1969, Wargo & Ford 1993, Caceres-Martinez et al 1998, Read 2010). Spionid worms that bore into shellfish hosts primarily belong to the ‘Polydora-complex’ which includes Polydora, Psuedopolydora, and Boccardia spp. Gamble (2016) found that infestation by P. websteri Hartman in Loosanoff & Engle (1943) was significantly lower when floating oyster cages were flipped weekly versus biweekly to expose oysters to air for ~24 h. Cool-air storage of oysters for a prolonged period (at least 10 d) has resulted in 100% mortality of P. websteri with limited mortality of oysters (Brown 2012, Morse et al 2015). While these methods may be effective at limiting infestation of Polydora, they are often time consuming, labor intensive, costly, and can decrease shellfish growth and increase host mortality (Littlewood et al 1992, Nel et al 1996)

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Results