Assessing the presence, settlement and growth of the invasive Mediterranean fanworm, Sabella spallanzanii, on mussel farms

Biofouling invasive species cause economic harm to shellfish aquaculture and require effective management strategies. Therefore, a better understanding of the life cycle and spread of key pest species for more efficient surveillance approaches is needed. The main purpose of this study was to assess effectiveness of detecting the invasive Mediterranean fanworm, Sabella spallanzanii, at mussel farms using environmental DNA and RNA (eDNA/ eRNA) approaches and compare the results to visual surveys that were undertaken to detect presence, settlement seasonality and growth of the fanworm on different settlement substrates. Monthly biofilm and seawater samples were taken around three mussel farm sites and tested for S. spallanzanii eDNA and eRNA signals using a species-specific droplet digital PCR (ddPCR) assay. Occupancy modelling applied on the eDNA-based detections indicated the fanworm's probability of detection at 90% of all sampled locations regardless of month or depth. Environmental DNA ddPCR copy numbers were highly variable but were detected in every monthly sample. Specifically, month and sampling matrix (seawater or biofilm) contributed significantly to predicting the eDNA signal strength (p < 0.001). Unfortunately, eRNA detections were very low and only ten out of 48 samples yielded a positive eRNA detection. Observations from settlement substrates showed that it took around 8 months for the fanworms to grow to ~5 cm in length and they preferred to settle on more complex rather than smooth surfaces. Our results suggest that a species-specific eDNA/RNA ddPCR assay at a mussel farm, followed by further visual or settlement monitoring could aid increased vigilance for detecting the arrival of newly invasive species at an early settlement stage.