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Go to Brilliant.org/SciShow to learn more. {♫Intro♫}. The Congo River is one of the largest rivers in the world, stretching thousands of kilometers across Africa.

And the last three hundred kilometer stretch of the river, referred to as the lower Congo. River, empties into the Atlantic ocean in an incredibly dramatic way -- turbulent and treacherous rapids. And it’s deep, too.

But inhospitable as it may seem, over three hundred species of fish are thriving in the lower Congo River, and many are found nowhere else on Earth, including some that are... frankly bizarre. For these fish, in a sense, the lower Congo isn’t one river. Its a bunch of isolated habitats.

Incredibly powerful rapids separated by stretches of calm water have kept fish populations apart long enough that they can’t breed with one another. This isolation causes new species to evolve side by side, a process called allopatric speciation. Normally it’s caused by much larger barriers, like, a mountain range, but in the lower Congo, some of these populations are only separated by a kilometer or so.

And this isolation is happening in 3D. The Congo is the deepest river in the world, measured at well over two hundred meters in places. That’s roughly equivalent to a sixty-story building.

And researchers have discovered one fish species that is seemingly so isolated from the surface, in deep underwater canyons, that it’s led to some extreme adaptations. There’s a species of cichlid that unlike its colorful aquarium cousins, shares characteristics with fish found in caves. For one thing, they have no pigment.

Delightfully, the local fisherman refer to these weird pale fish as mondelli bureau, which translates to “white man in an office”. And their eyes are basically nonfunctional -- underdeveloped and covered in skin. And when they come to the surface, these cichlids rapidly die of decompression sickness.

But it’s not just cichlids that are blind, white, and weird looking in the lower Congo. This river is also a perfect place to study convergent evolution. That refers to the way species that aren’t closely related can evolve very similar adaptations because of the conditions they’re in.

Pale, blind species of distantly related fish have been found in the river, including the cichlid, along with elephantfish, members of different catfish families, and spiny eels. We’re talking so distantly related, they’re as genetically distinct as armadillos are from blue whales. These characteristics developing in caves make a lot of sense.

They are pretty dark, after all. But aside from the cichlid, these other cave-like fish are found alive, which means they don’t have the same deep water adaptations, and are likely living somewhere a lot shallower. Discovering these species tells us that there are likely other low-light micro habitats in the river -- and that they’re stable and isolated enough to drive the evolution of low light-adapted species.

But while the strange fish that live in the lower Congo River can tell us a lot about how the river’s conditions are driving evolution, it’s such a dangerous place for humans that there’s still a lot we don’t know. Like, where exactly are these other fish living? We’re really just starting to piece together a bigger picture of what’s going on there.

Hydrologists, geologists, and ichthyologists are working together to better understand the unique biodiversity, geology and hydrology of the river, and to make sure local Congolese researchers are driving future efforts. And there are still tons of questions to answer. Like, what genes are driving the evolution of these cave fish characteristics?

It could be the same ones each time -- or totally different ones producing similar outcomes. So while we don’t know yet, these researchers are working hard to find out. This research is super collaborative -- the product of lots of researchers from multiple institutions working together.

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