Monitoring for incipient dreissenid mussel invasions using environmental DNA and RNA: review of current state of knowledge and considerations for implementation

The establishment of populations of zebra and quagga mussel ( Dreissena polymorpha and D. bugensis ) is a continuing challenge for aquatic ecosystems and natural resource managers. Monitoring aquatic systems for the environmental DNA and RNA (eNA, collectively) of Dreissena is an effective means for detecting early-stage invasions by these species. There are several factors that should be considered when designing such a monitoring programme, many of which can be informed by existing literature in the eNA field, including several Dreissena eNA studies. We identified seasonal timing of Dreissena surveys to be a critical consideration, as detection will be maximised during spawning and veliger dispersal, but precision in identifying source locales may be greatest outside of this season. While many sampling decisions are specific to individual study locations, sampling from the surface and, in the case of lotic reaches, from areas of high energy, such as riffles, are likely to increase detection if Dreissena are present. Likewise, consideration of the particulars of the local environment, including seasonal wind and current patterns, will likely allow for more strategic sampling of Dreissena eNA and more informed inferences from survey outcomes. We also identified several future research areas that would better inform Dreissena eNA studies, such as behaviour of the transportation of eNA, specifically for Dreissena , in both lentic and lotic environments and the possibility of using genetic eNA techniques to indicate periods of Dreissena spawning and veliger presence.