DNA is a special molecule contained in the cells of all living things and contains the unique information or “code” necessary to build and maintain an individual organism. DNA determines the characteristics of each living thing and how it looks or functions. It also determines the variation between individuals within a species. In humans, for example, our DNA is 99.9% the same as that of any other human, with a tiny proportion unique to each and every one of us.
Environmental DNA (eDNA) is DNA released from an organism into the environment and provides trace evidence of their current or past presence. This includes the faeces, urine, gametes and mucus of living organisms or their decaying remains after their death.
Once released into the environment, eDNA only remains useful for a relatively short period. Other biological organisms, including bacteria, fungi and plankton, can feed on the remaining material containing eDNA, while physical forces, such as ultra-violet light and temperature further break it down.
In general, cold, dry conditions slow down the breakdown of eDNA. In permafrost, for example, eDNA can be preserved for hundreds of thousands of years. In soils or aquatic sediments, the time may vary from months to even thousands of years but this depends on environmental conditions. In aquatic environments, such as rivers and lakes, DNA released by an organism can only be detected for a few days.
Environmental DNA is useful for fisheries scientists because it can be collected and analysed to establish the presence of a species in a particular area. This analysis involves collecting a small volume of water or sediment from a location and analysing it for the presence of DNA. We then check the DNA that we have found against an ever-growing library of organisms, to see what we have found.
Conventional DNA methods require samples of tissue, blood or other biological material. Environmental DNA is a non-destructive and non-invasive method for surveying and has little or no negative effect on the organism of interest. It requires less resources and expertise to collect and is especially useful for finding evidence of endangered or rare species, without putting them at risk.