One Bucket of River Water Can Reveal Every Species Living In It, Here's How eDNA Works

A fish does not need to be caught to be identified. As it swims, it sheds scales, mucus, urine, and faeces, each carrying fragments of its DNA into the surrounding water. The same is true for every aquatic creature: otters, frogs, turtles, even microscopic zooplankton. A cup of river water, properly collected and preserved, holds a floating library of every species that has passed through in the preceding days.

This is the biological fact that makes environmental DNA, or eDNA, work. Organisms continuously release genetic material into their surroundings. In water, those fragments persist for anywhere from a few hours to several weeks depending on temperature, UV exposure, and water chemistry. Scientists collect a water sample, filter it to trap the DNA particles, then sequence what they find against reference databases of known species. The result is a species list, often more complete than anything a traditional survey could produce.

The equipment fits in a backpack. A field researcher collects one to two litres of water, typically filtered through a fine membrane that catches particles as small as 0.45 micrometres. The filter is sealed, kept cold, and sent to a laboratory. There, the DNA is extracted and run through a technique called metabarcoding, high-throughput sequencing that reads thousands of short genetic fragments simultaneously and matches each one to a species in the database.

The entire field portion takes under an hour. One trained technician can sample multiple sites along a river in a single day, covering distances that would take weeks of traditional netting, trapping, and visual survey. A 2021 study published in Molecular Ecology Resources found that eDNA metabarcoding detected significantly more freshwater fish species per sampling event than conventional electrofishing across 40 European river sites, and did so with no physical disturbance to the animals or habitat.

Rare and nocturnal species are chronically undercounted in conventional wildlife surveys. An animal that passes through a stretch of river at 2 a.m. leaves no trace a daytime observer would catch, but its DNA remains. Invasive species are another strong suit. The Asian carp, which has disrupted North American river systems, was detected in the Great Lakes using eDNA before any fish were physically caught there, giving managers an early warning that conventional monitoring would have missed by months.

eDNA is also the only practical method for surveying species that are dangerous to handle, extremely shy, or present in such low numbers that netting would be both ineffective and harmful. For the gharial, India's critically endangered fish-eating crocodilian, now confined to a handful of river systems including stretches of the Chambal and the Girwa, eDNA sampling offers a way to confirm presence or absence across long river corridors without disturbing nesting sites.

India has over 400 river systems and some of the world's highest freshwater biodiversity, yet systematic aquatic species monitoring remains thin outside protected areas. The Wildlife Institute of India and several state forest departments have begun piloting eDNA-based surveys in rivers across the Western Ghats and the Gangetic plain, targeting species such as the Ganges river dolphin, smooth-coated otter, and several endemic hill-stream fish that have no reliable population estimates.

The Ganga river dolphin, India's national aquatic animal, is notoriously difficult to count. It is fast, surfaces briefly, and its preferred deep-channel habitat is often murky. eDNA sampling from water collected at multiple points along the Ganga can confirm whether dolphins are using a particular stretch, track seasonal movement, and detect population presence in areas where visual surveys are impossible. For a species listed as endangered on the IUCN Red List, that kind of data has direct conservation consequences.

The method has real limits. It confirms presence, not abundance. Finding a species' DNA in a water sample tells you the animal was there; it does not tell you how many individuals are present or whether they are breeding. DNA degrades at different rates in different conditions, a warm, sunlit, fast-flowing river breaks down genetic material faster than a cold, shaded pool, which means absence of eDNA is not always proof of absence of the species.

Reference databases are also incomplete. A species can only be identified from an eDNA sample if its genetic sequence is already catalogued. For many invertebrates, fungi, and lesser-known fish species in India's rivers, those reference sequences simply do not exist yet. Researchers are building them, but the database gap means some of what a water sample contains will, for now, return no match.

The picture that emerges from a bucket of river water is detailed but not total. It is a partial record of a living system, accurate within the limits of what science has already named and sequenced. The species it cannot yet identify are a reminder that the inventory of life in India's rivers is still very much in progress, and that the tool doing the counting is only as good as the library it reads against.