Interpreting environmental DNA (eDNA) signals: insights from eDNA shedding and decay rates from diverse animal forms

Environmental DNA (eDNA) analysis is a promising new and innovative method of exploring the ocean’s midwater to identify both specific target species and whole communities of organisms. However, the current literature regarding eDNA shedding and decay primarily focuses on fish in surface waters, which are warm and subject to sunlight. Because the ocean’s midwater contains animals of many different forms in cold, dark habitats, a knowledge gap exists in interpreting eDNA results from mesopelagic water samples. This gap limits the ability to link eDNA results to biomass and time since the eDNA was shed from the target organisms. We seek to answer two important questions: (1) how eDNA shedding differs between diverse animal forms and (2) how long eDNA can persist in the water column in conditions representative of the mesopelagic ocean. Here, we quantify the shedding rates of a jellyfish, a fish, and a crustacean to explore the differences in shedding rates between different animal forms. Coastal organisms are used as a proxy for mesopelagic species due to the organism hardiness and the ability to set up mesocosm studies. We also quantify eDNA decay rates under different three temperature scenarios representing mesopelagic conditions. The three decay conditions are 6°C, 15°C, and 23°C and all are conducted in the dark. These decay rates will allow more accurate estimates of persistence in the ocean’s midwater compared to existing estimates in the literature from surface waters. Both the shedding and decay rates in this study will advance the interpretation of genetic results from eDNA methods in mesopelagic waters and will contribute to linking eDNA signal to organism biomass. Future work will include using these new estimates with a hydrodynamic model to identify eDNA sources in space and time given a detection from a water sample.