Freshwater ecosystems are rich but fragile. Of the new fish species discovered, many are critically endangered. Among the standout species is Yang’s plateau loach, a cavefish from China’s Yunnan Province whose expanded swim bladder chambers protrude visibly from its body, giving the impression of a built-in lifejacket.
A wave of scientific discovery is rewriting what is known about life in rivers, lakes and wetlands, while exposing how quickly it could disappear.
Freshwater ecosystems cover less than 1% of the Earth’s surface, yet scientists described 309 new species of freshwater fishes last year alone, according to a new report released this week.
New Species 2025, published by the conservation initiative Shoal, highlights the remarkable diversity being uncovered beneath the surface — and the urgent conservation questions that follow.
By spotlighting newly described freshwater fishes each year, Shoal hopes to accelerate awareness, encourage collaboration and support the journey from freshwater species discovery to conservation action.
Many of the 309 newly described species are found nowhere else on the planet. From cave-dwelling fish in China to seasonal killifish in East Africa and minnows from Anatolian streams, the discoveries reflect years of fieldwork, taxonomic expertise and international collaboration.
Across continents and climates, taxonomists identified species that had lived unseen in caves, lingered unrecognised in museum collections, persisted in seasonal pools that dry to cracked mud or flowed quietly through rivers believed to be well studied.
Mike Baltzer, the executive director of Shoal, described 2025 as a “bumper year for freshwater fish descriptions with 309 new names added to the ledger of freshwater fish diversity”. It was a staggering number, reflecting nearly one “new” species discovered every day throughout 2025.
From the seasonal wetlands of the Democratic Republic of the Congo came four new Nothobranchius killifishes.
“It is the most in one year since 2017 and the third-highest number since records began way back in 1758. From Asian karst caves and peat swamps to Amazonian rapids, African seasonal wetlands to Appalachian rivers, it is a reminder that freshwater biodiversity is still unfolding before us,” he wrote in the report’s foreword.
Baltzer noted that many of this year’s new species were known from single drainages, individual tributaries, isolated wetlands or solitary cave systems. “Freshwater ecosystems fragment landscapes naturally. Over evolutionary time, a ridge or subtle shift in drainage is enough to isolate and diversify.”
Taxonomy, he said, was “patient work”, requiring comparisons across collections, careful measurement, genetic sequencing and many years of accumulated field knowledge. Specimens sometimes sat on laboratory shelves for decades before they were formally described.
“Without a formal scientific description, a species cannot be assessed for the International Union for Conservation of Nature Red List. It cannot be properly regulated in trade or embedded within legislation or management plans. It cannot be counted accurately in biodiversity assessments. A species without a name exists biologically but remains invisible institutionally.”
Taxonomy therefore remained one of conservation’s quiet foundations. “And it begs questions: How many species remain undocumented? And how many may disappear before anybody has had the opportunity to recognise them?”
The report, Baltzer said, offered a snapshot of a moving frontier. “It reflects where taxonomic attention is focused, where biodiversity remains under-surveyed and where evolutionary processes continue to surprise us. Above all, it underscores a simple truth: freshwater life is richer than we fully understand.”
Among the standout species is Yang’s plateau loach, a cavefish from China’s Yunnan Province whose expanded swim bladder chambers protrude visibly from its body, giving the impression of a built-in lifejacket.
“Adapted to the perpetual darkness of karst rivers, it appears to hover motionless in the water column — an elegant solution to life where energy is scarce.”
In nearby Sichuan, another subterranean surprise emerged: the Sichuan mountain cave loach, the first obligate cave-dwelling member of its genus. Pale, reduced-eyed and ghostlike, it extended the ecological boundaries of its lineage into a realm previously unrecorded, the report said.
From the seasonal wetlands of the Democratic Republic of the Congo came four new Nothobranchius killifishes. The annual species hatch, mature and reproduce in temporary rain pools that might exist for only weeks. When the water disappears, the adults perish but their drought-resistant embryos endure in the mud, waiting for the next rains. Each shallow depression can hold a species found nowhere else on the planet.
In Brazil’s Atlantic Forest, the Floripa sabrefin killi survives in rain-fed pools on a single island increasingly transformed by development. Barely reaching 2.6cm in length, it lives in wetlands so shallow and grass-covered they can appear dry to the untrained eye.
North America’s discoveries show that biodiversity can remain hidden even in well-studied regions.
“Two brilliantly coloured darters, the Birmingham darter and Gurley darter, were distinguished from their relatives in Alabama streams. And two large river fishes long recognised but unnamed, the sicklefin redhorse and Apalachicola redhorse, finally received formal descriptions after decades of careful work.
“Elsewhere, the newly described Anatolian minnow from Türkiye demonstrates how detailed revision can reveal that what once appeared widespread is, in fact, a mosaic of distinct local lineages.”
In Africa, two spine killifishes were described from museum specimens that had waited years for careful re-examination — a reminder of the hidden discoveries resting in collections.
But the discoveries come with a warning.
Of the 309 freshwater fish species described last year, only one has been assessed for the IUCN Red List of Threatened Species: Nothobranchius sylvaticus, a forest killifish from Tanzania. It has been classified as critically endangered.
Freshwater fishes are among the most threatened vertebrates on Earth. The IUCN says about one in three assessed freshwater fish species is at risk of extinction. Habitat loss, dams, pollution, invasive species and climate change are driving widespread declines, often less visible than crises affecting terrestrial wildlife.
Yet freshwater ecosystems underpin the lives of billions of people, supporting drinking water supplies, food security, livelihoods and cultural traditions.
The report highlights another important pattern: many newly described fish species have extremely small ranges, sometimes limited to a single spring, cave system or stretch of river.
“In freshwater systems, where fragmentation and degradation are widespread, this makes them especially vulnerable,” Baltzer said.
He noted that the 309 species named in 2025 “expand our knowledge of the living world”.
“What happens next depends on whether that knowledge is translated into stewardship.”