Fresh finds: new freshwater fish discoveries shine a light on bountiful but imperiled ecosystems

Water may be the life-force for all species on Earth, but when it comes to freshwater ecosystems, we still have much to learn about the biodiversity that calls them home. Shoal, a partnership between Re:wild and Synchronicity Earth, recently compiled a report of the more than 200 freshwater fish species that were first described by scientists in 2021.

“Every newly described species tells us more of the story about the intricate connections among all living things and the Earth that support us, sparking wonder and enriching our lives," says Harmony Patricio, the freshwater fish conservation program manager for Re:wild and Shoal.

From an undiscovered species hiding in plain sight among a lookalike in a science lab to another found at the bottom of a well in one of the most populated cities on the planet, the results also reveal the many ways that scientific research can surprise us.

According to Ralf Britz, head of the ichthyology team at the Senckenberg Natural History Collections in Germany, about half of the fish species described by scientists every year reside in fresh water.
 
“When you look at the entire amount of water on our planet, only about 2-3% of that is fresh water,” he says. “And of those 2-3% it’s only a fraction that is fresh water in rivers, smaller streams or lakes. . .So we have a tiny fraction of the world’s water volume harboring more than 50% of all fish species on this planet. That’s remarkable and can’t be stressed often enough.”
 
Britz’s discovery of a tiny fish called Danionella cerebrum may have only become official last year, but it was based on examining descendants of fish collected in a river tributary in Myanmar years before, thought to be a species called Danionella translucida. The new species was identified based on genetic and morphological evidence, and would have been nearly impossible to distinguish based on external appearance alone.

“They’re all transparent, they don’t have any pigment patterns,” Britz says of the thumbnail-sized fish. “There’s very little externally that you can use to distinguish the different species.”

Britz started doing more research into the species after what was thought to be Danionella translucida was beginning to be used more for research in neuroscience labs. Compared with other common lab fish species such as Zebrafish (Danio rerio), Danionella species are even more useful for this research because of one unique feature: the top of their brain is covered only by a thin layer of skin, instead of a skull roof.

“When you want to study brain function in a vertebrate, you face the issue that the brain is surrounded by a skull. So, to be able to look at the brain in situ in a live animal. . .you often have to make some experiments where you remove part of the bony skull roof.”

Because Danionella has only transparent skin over the top of its brain, it is visually accessible without doing dissections. This trait, combined with the species’ relatively complex brain activity given their brain size, means that Danionella is becoming a model organism for scientists studying the human brain. 

In Brazil, a catfish sample collected in the Xingu River basin in 1990 waited 30 years before being identified as a species new to science — now called the Wolverine Pleco (Hopliancistrus wolverine) — simply because for a long time no one had the time or resources to confirm it. Lucia Rapp Py-Daniel, an ichthyologist with the National Institute of Amazonian Research who first encountered the species on that 1991 survey, was also one of the first scientists to experience its unique defense mechanism: retractable “spikes” (known as odontodes) that emerge from the fish’s gill covers to pierce whatever is bothering it.

“They pinch you so strong that it bleeds,” she says. “We have never seen this behavior before.”

As this species thrives in oxygen-rich waters such as fast-moving rivers and waterfalls, it may be susceptible to one of the biggest threats to freshwater species worldwide: damming, which radically alters their habitat. Although there has been no assessment of how well this particular species is faring in the wild, Rapp Py-Daniel says that her team has observed the dwindling of other species.
 
“Some species we are not finding anymore,” she says. “Probably, eventually they can recover, but we don’t know how long it’s going to take, or how many species are going to be able to recover, so it’s a big question mark.”

In 2019, Praveenraj Jayasimhan, an ichthyologist with India’s Central Island Agricultural Research Institute, Port Blair, got a call from his colleague Tejas Thackery with a report of an unknown species of blind eel that had been found in a well in Mumbai. At first, Jayasimhan didn’t believe him; the original source was a photo that had circulated on Facebook in 2005, and it seemed implausible that a previously undiscovered species could be living in the middle of India’s largest city. Still, Thackery started searching around the city, and eventually found himself looking into a well on the grounds of a school for blind children.

“It took almost three days to pump out the entire water from the well,” Jayasimhan says.

But when a team member was lowered into the well with a rope, several eels emerged from the tiny, subterranean water channels from the well walls. Blood-red despite its dark habitat, the species — now known as the Mumbai Blind Eel (Rakthamichthys mumba) — is thought to have split from its closest living relative more than 1 million years ago. It is the first completely blind subterranean fish species known to exist in India’s Northern Western Ghats region.

“Since this species was found in a highly populated city, the only conservation measures we can take are to avoid using bleaching agents in the well,” Jayasimhan says.

Because these species are recent discoveries, no one knows for sure how well they’re doing in the wild. But for many freshwater species, hydropower is likely the biggest threat, particularly in tropical countries.
 
“Here in Germany if you go to a stream you might have maybe 10 species, and in the entire of Germany you might find something like 35, 40 different fish species,” Britz says. “And in a tiny little stream in Myanmar, maybe 5 meters wide and 10 kilometers long, you can find easily 50 different species, and there’s probably 600 different species in Myanmar. So, damming a river in Europe is very different than damming a river in the tropics.”
 
Britz has seen this in effect in the rivers of Myanmar. “The first time we went to the type locality of Danionella translucida, and also to that of Danionella cerebrum, there were literally hundreds, thousands of fish in there. But the last time we went back, in 2019 … there were just very, very, very few. And the reason for that is probably because they dammed the river that leads to that lower part of the river up in the mountains.”
 
One thing is certain: more species are waiting to be found across the globe, provided their habitats remain intact.
 
“The process of describing new species expands our knowledge of evolution and deepens our understanding of relationships among species and with their surrounding environments,” Patricio says. “Conservationists rely on this knowledge to design effective strategies to save threatened species and sustain biodiversity.”