"Ecology" of eDNA: shedding, decay, and transport

Environmental DNA has huge potential to revolutionize how we census aquatic animals. However, it is fundamentally a different kind of "net" by identifying traces of animals, not visually identifying or capturing animals themselves. Therefore, it is important to consider several processes (biological, physical, technical) and implications on interpreting eDNA signals. We are still conducting research that is fundamental to connecting the dots between eDNA detections and inferences about animals.

How much eDNA do animals shed?

All organisms shed eDNA into the surrounding environment. The amount, or rate, of eDNA shed by an animal is important to understand when trying to link eDNA concentration back to animal abundance or biomass. Especially when looking for eDNA from many different species, it is helpful to understand whether a high eDNA concentration is because there are many individuals shedding eDNA, or if that particular species sheds eDNA at a higher rate than the others. Especially because eDNA comes in all different shapes, sizes, and forms, the same animal might shed more or less eDNA at certain times on varying time scales. We are continuing to learn what impacts eDNA shedding rates and how to account for variations in eDNA shedding rates.

E. Andruszkiewicz Allan, W. G. Zhang, A. C. Lavery, A. F. Govindarajan. 2020. Environmental DNA shedding and decay rates from diverse animal forms and thermal regimes. Environmental DNA, 3:141. Link.

How long does eDNA persist in water?

After it is shed from an animal, eDNA will persist in the water while mechanisms like enzymes, microbial grazing, and sunlight break down the eDNA until it is no longer detectable. Many different factors can affect how long eDNA can persist in water (fresh or marine), but generally persistence times are on the order of days. This is important as it means that eDNA found in a water sample might reflect an animal that was present days ago, rather than at the time the water sample was collected. Incorporating information about eDNA decay rates is critical when interpreting results.

E. A. Andruszkiewicz, L. M. Sassoubre, A. B. Boehm. 2017. Persistence of fish environmental DNA and the influence of sunlight. PLOS ONE, 12(9): e0185043. Link.

E. Andruszkiewicz Allan, W. G. Zhang, A. C. Lavery, A. F. Govindarajan. 2020. Environmental DNA shedding and decay rates from diverse animal forms and thermal regimes. Environmental DNA, 3:141. Link.

How far can eDNA be transported?

Because we are finding traces of the animal, not the animal itself, it is important to understand if the eDNA detected was transported from where the animal shed it. We can use the information we have about how long eDNA lasts in the water column in conjunction with ocean models that provide water velocity and direction to map the most likely locations of where animals were located based on where eDNA was found. Pairing eDNA with existing oceanographic knowledge helps use the tool to its highest potential.

E. A. Andruszkiewicz, J. R. Koseff, O. B. Fringer, N. T. Ouellette, A. B. Lowe, C. A. Edwards, A. B. Boehm. 2019. Modeling environmental DNA transport in the coastal ocean using Lagrangian particle tracking. Frontiers in Marine Science, 6:477. Link.