For those of you inclined towards a bit of wild swimming, it’s a slightly unsettling thought: floating around in rivers, lakes and the sea are billions of fragments of skin cells, sperm, eggs, mucus, blood, faeces, urine, hair and saliva. Yet this is entirely natural and happened long before humans arrived on the scene. Far from being a cause for alarm, these minuscule traces are a boon for scientists who want to learn more about the creatures living in and around our waterways.
During the summer I met Bogna Griffin, a freshwater biologist from the Marine Institute, on the willow tree-lined banks of the river Maigue in the small village of Bruree, about 35km south of Limerick city. The Maigue begins in the Ballyhoura Mountains in Cork and flows northwest through Limerick, supplemented by tributaries such as the Morning Star, Camoge and Loobagh rivers, before finally emptying into the salty waters of the Shannon Estuary.
Along the 60km-long stretch of the Maigue, Griffin’s attention is on one of its tiniest crustacean inhabitants: the freshwater white-clawed crayfish. Known in Irish as gliomach fionnuisce - literally “freshwater lobster” - it looks like a mini cousin of the lobster, though just the length of a paperclip.
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In the lime-rich waters of the Maigue, the crayfish spends its time tucked away in crevices beneath stones and rocks - trying hard to avoid the attention of hungry otters and eels - while feeding on insects and grazing on water plants. Ireland is regarded as the last stronghold of this species. Because of the work of the distinguished biologist, Dr Julian Reynolds, who undertook the first national survey of crayfish, scientists have had reliable data from across Ireland since the 1980s.
In the past, studying crayfish meant wading into rivers and turning stones in search of the tiny organisms. Researchers such as Griffin don’t need to get their hands wet today. Instead of finding the animal, she needs only a water sample. During the year crayfish shed their hard outer shell - the exoskeleton - which leaves behind fragments that carry DNA traces in the water. From these microscopic slivers, scientists can identify the life in a river - crayfish, salmon, trout, eels, and even the presence of cattle, dogs and humans. This “environmental DNA” has transformed the work of researchers. Rivers become living archives, holding stories and information, waiting to be decoded in the lab.
On the riverbank, Griffin showed me the simplicity of the process. The set-up was straightforward: a water pump, a filter and a collector sat on the bank, with a long, narrow tube trailing into the waters of the Maigue. For about 10 minutes, the pump sucked up water from the river, channelling it through the filter, where any genetic fragments of DNA were trapped. For such a simple operation, it has powerful results. Like a forensic investigator dusting for fingerprints at a crime scene, Griffin needs only the tiniest genetic fragments to confirm the crayfish’s presence.
Environmental DNA - known as eDNA - is revolutionising how scientists study biodiversity. Inaccessible and unexplored places that were once out of reach can now be explored and investigated. One example lies high in the tropical rainforest canopy, where the treetops are full of life: ants and beetles, tree frogs, orchids and mosses, birds such as cranes and toucans, monkeys like the tufted capuchin, roosting bats and arboreal snakes. For researchers the canopy has long been a biologically rich but frustratingly impossible place to access and study. With eDNA, its hidden stories can now be revealed without ladders, ropes, cranes and, most importantly, without the disturbance that human presence inevitably brings.
Last month French scientists published groundbreaking findings from the tropical old-growth rainforest of Sinnamary in French Guiana. With little more than a series of upside-down umbrellas attached to plastic bottles fitted with filters, they devised a new way to explore life in the ancient forest canopy. As the rainwater cascaded from the treetops, down through the forest to the floor, the umbrellas captured it - and with it came fragments of eDNA from monkeys and mossies washed down from above. The experiment proved to be precise, noninvasive and remarkably inexpensive, opening a new window into one of the least-studied realms of the rainforest.
Since 2023 Griffin has been using eDNA to track the fate of the white-clawed crayfish in the Maigue. Her work takes on a new urgency in light of the discovery, a few years previously, of the highly contagious crayfish plague in the river near Adare. This plague, which is caused by a water mould that destroys the internal tissues, will kill a crayfish within weeks. It has since spread throughout the catchment. The impact is already being felt. Locals have told Griffin that they have found hundreds of dead crayfish in the river. One farmer told her that he now sees an otter - once a regular crayfish predator, turning stones in search of them - switching to juvenile fish for food instead.
There is a bit of hope. In other rivers, Griffin explained, populations of crayfish have managed to persist despite the plague. The key lies in restoring the Maigue to what it always was - a clean, healthy and unpolluted place. If we can rid it of pollution, the life that depends on it will stand a far better chance of resisting future infection.
