Snail slime in real time: Challenges in predicting the relationship between environmental DNA and apple snail biomass

As environmental DNA (eDNA) becomes a fixture in the invasive species management toolbox, expectations of its utility extend beyond presence/absence to analyses that provide more detailed information about target populations. Studies with fish and other vertebrates have reported moderately reliable, positive relationships between eDNA concentrations and biomass. However, few studies have considered this relationship in invertebrates. To address this gap, we investigated whether increasing biomass of apple snails (Pomacea maculata) resulted in a similar predictive relationship with eDNA concentration, and we did so under cold conditions that make apple snails less conspicuous and more difficult to detect with traditional methods. Placing snails in either distilled or stream water, we used a species-specific quantitative PCR assay to measure eDNA concentrations after 24 hr over an apple snail biomass gradient (0, 2, 4, or 6 snails; 143 to 624 g total biomass). Detection success of eDNA derived from apple snails kept in a small volume (i.e., 13 L) of cold water (13 degrees C) averaged 66% overall. Successful detection in distilled water (75%) exceeded the overall average. Lower detection efficiency occurred in stream water (58%). Despite the cold conditions, we observed snail activity in 90% of our replicates, but net eDNA accumulation failed to reflect patterns commonly observed with vertebrates. Censored regression modeling efforts, which account for a disproportionate number of zeros (i.e., non-detections), identified a significant predictive relationship between snail biomass and eDNA concentration, but only starting at a high amount of biomass (similar to 422 g). Future management strategies to monitor apple snails will likely include eDNA, but its utility in ascertaining biomass remains unclear. Considering the ecology of eDNA of invasive invertebrates will help bolster managers' ability to understand the utility and limitations of this valuable tool.