Estimation accuracy and eDNA production source
We found that eDNA-based estimation accuracy of species abundance was significantly lower for crustaceans than fish. A previous study hypothesized that species morphology and/or behavior might affect eDNA production, reporting lower eDNA shedding rates in grass shrimp than in fish and jellyfish (Andruszkiewicz et al., 2021). Fish and jellyfish are likely to constantly produce eDNA as epidermis and/or muco-substances (Merkes et al., 2014; Sassoubre et al., 2016), while crustaceans are characterized by their hard exoskeletons and segmented bodies plans (Hadley, 1986) and are thus unlikely to shed large amounts of eDNA from their body surfaces unless they are molting. Consequently, crustaceans infrequently and irregularly shed eDNA, which may impede sufficient eDNA detection in the field and prevent accurate abundance estimationvia eDNA analysis (Dougherty et al., 2016; Mächler et al., 2016; Johnsen et al., 2020). In contrast, estimation accuracy of species abundance was marginally higher for coral and seastars than fish. These species are sessile or move relatively slower than other marine animals. Accordingly, their eDNA after released into environments might reflect species presence abundance more precisely. In particular, under the recent climate change and anthropogenic disturbances, rapid and sensitive monitoring of tropical coral reefs via eDNA would be helpful for the effective conservation of biodiversity and ecosystem functioning on reefs (Nichols & Marko, 2019).
The mean R2 value between herptile eDNA quantity and abundance was similar to that of fish, which is reasonable given amphibians likely shed eDNA constantly from their epidermis and/or mucus. However, the variation (95 % CI) was much larger for herptiles relative to fish. This point could simply be explained by biases derived from the smaller number of corresponding studies or different experimental conditions. Most of the relationships between herptile eDNA quantity and abundance were studied in natural environments and depended on visual counts for abundance estimation (e.g., Thomsen et al., 2012; Kakuda et al., 2019), which might make the relationships less certain than other capture-based survey. On the other hand, Everts et al. (2021) assessed the relationships between eDNA concentration and abundance of American bullfrog (Lithobates catesbeianus ) tadpoles and juveniles using mesocosm experiments, and reported relatively high R2 values (0.64 to 0.99). Accumulating studies targeting various taxa in laboratory conditions and natural environments could help us understand the effects of ecological characteristics (morphology, physiology, and ethology) on the process of eDNA production, and may provide us with keys for improving eDNA-based abundance estimation.