Can cleaning invasive zebra mussels off native mussels help them survive?

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Exotic zebra mussels (Dreissena polymorpha) have infested native unionids and caused their deaths in the Great Lakes since 1986. The abstract states that no other such parasitism of native unionids occurs in North America.

Researchers tested the survival of unionids threatened by zebra mussel infestation in a field experiment, suspending cleaned and uncleaned unionids in nearshore waters of western Lake Erie. Between 5 July 1990 and 3 July 1991, they assessed survival and removed newly settled zebra mussels from the cleaned unionids at eight intervals ranging from 21 to 77 days.

After one year, survival was 0% among uncleaned unionids and 42% among cleaned unionids. Of the 10 species examined, only individuals from 3 species—Amblema plicata plicata, Fusconaia flava, and Quadrula quadrula—survived one year. These species have relatively thick shells, which may have contributed to their survival.

Removing newly settled zebra mussels may be important for unionid survival: 98% of the zebra mussels removed after the initial cleaning were small, measuring less than 10 mm long, and could rapidly grow and cover the unionids.

The researchers stated that how zebra mussels cause unionid mortality was still unknown. At the time of the study, removing zebra mussels from unionids was the only known method that successfully reduced unionid mortality in waters colonized by zebra mussels.