[xiaoqiang75]

This is the story of one sick little fish. It’s completely colorless. And blind. You won’t see it swimming along with the other fishies, nope. This fish is only seen dead, on the banks of the Lower Congo River. Pathetic, right?

But our story starts with someone way classier, a New York scientist. Melanie Stiassny’s curator of fishes at the American Museum of Natural History. *Dr. Stiassny’s got this thing for the Congo River.

“Oh it’s just the most incredible system,” she gushes. “Particularly for an ichthyologist-”

A what?

“An icthyologist, someone who studies fish. The Congo is just like, oh my goodness, it’s just like a magical place. It’s very rich in species, but it’s extremely poorly known.”

Right. Most of Dr. Stiassny knowledge about the Lower Congo River came from just one expedition done decades ago. It was carried out by two American ichthyologists in the early 70′s, Roberts and Stewart.

“They found some very extraordinary things in the Lower Congo River,” she explains.

Basically, Roberts and Stewart found a crazy number of new fish species there. Which was cool, but weird. Because the Lower Congo River has a few nice areas of rapids, but for the most part it looks like a normal river. At that point, nobody could really explain what was causing it to be such an evolutionary hotspot. Roberts and Stewart just didn’t really have a good explanation for all the new species there.

Their trip also turned up a second mystery, which involves that weird little fish I mentioned earlier. That one with no eyes, and no pigmentation. It’s about the size of you little finger and shaped like a torpedo. They’d really only seen two of them. And that’s if you count the second one, which they found stuck to the bottom of one of their shoes.

It turns out Dr. Stiassny’s kind of a sucker for weird fish stories. She made her own trip out to the Lower Congo River in 2007, and she made a big point of seeing if these fish really existed. Surprisingly, the local fisherman knew exactly what fish she was talking about.

“But the more extraordinary thing,” says Dr. Stiassny, “was that they said they found it dead. And that didn’t make any sense. We asked, ‘Where does it come from?’ They said they don’t know, they don’t know where it comes from , they just find it dead occasionally.”

“So no fishermen ever found it in their nets alive, or anything?” I asked her.

“Nope, no. They would just find it as they were kind of going up and down the river, along the river bank in their wooden canoes.”

At that point Dr. Stiassny was still kind of skeptical. Like “Really, this fish can only be found dead?” But sure enough the locals pointed out the odd fish bodies on the shore. And Dr. Stiassny searched and searched the waters but couldn’t find any of these fish alive.

“After another couple of days,” she recalls, “one of the fisherman actually came with a specimen that was just alive, and he handed it to me, and actually as I examined this poor little thing it died in my hands, which was sad but it was also an extraordinary moment because having this very very moribund animal in my hand I could see that its skin was full of air bubbles. There were huge bubbles of air all along its back, all in its gills. This animal looked very much as if it had died from decompression, it had de-gassed.”

She explained that would be the equivalent of a fish having the bends, like a scuba diver who’s come up too fast from the bottom of the ocean.

“And the only way that we could think that could have happened,” she continues, “was that it was living at depth, and it could have just suddenly risen to the surface, and with such a rapid change in pressure it had degassed. Now that was a very kind of eureka moment. Because that made us think suddenly, well could this river be deep? Could there be depth involved here?”

“Had you even considered that before?” I asked.

“We’d never, we’d never considered that before. We’d never thought for one moment that this was an unusual river besides having the rapids.”

So now it was looking like she didn’t just have a weird fish, she probably had a pretty crazy river out there. Dr. Stiassny got a project going to check out its depth and currents.

“What we were doing,” she says, “was really getting a fish’s eye view of the river. And it was so illuminating. I mean we’d always looked at the river the way a human does, I mean you just look at it and you think oh my, it’s just water, if you’re a fish you can swim around in it no problem.”

No problem? Turns out the Lower Congo River is the deepest river in the whole world. And deep down there, there’s ridiculously strong currents of water flowing in all different directions, even straight up.

So this pretty much solved the mystery of that weird little fish. Who needs pigment and eyes when you’re living too deep underwater for light to reach you? And the fish probably kept turning up dead because it’d get caught in a crazy upward current. Which would shoot it up to the surface way faster than it’s body could handle.

So, so far Dr. Stiassny’s got two pretty cool realizations. She’s discovered the lower Congo River is the deepest river in the world. And she’s figured out what’s going on with the dying fish. But that still leaves the mystery of why so many fish evolved there. Let’s go back to those super strong currents she discovered.

“So you imagine you’re a fish living on one side of the river,” says Dr. Stiassny. “You swim out, into water and then immediately you hit a column of water flowing incredibly fast in the opposite direction. It’s like a wall, you can’t swim across that river if you’re a small fish. So that immediately told us, hey, this is what’s happening. The river itself is clearly presenting a series of really intensive barriers. And that seems to be what’s driving speciation.”

When it comes down to it, this is really one of the oldest ideas behind evolutionary theory. Speciation through isolation—two different groups of a species get separated by some kind of barrier, and the populations on either side of the barrier start evolving different adaptations. Eventually, they become separate species.

Dr. Stiassny realized that a current running through the middle of the river could separate species just like a mountain range running through the middle of a continent. This was the first time anyone had suggested that that kind of evolution-driving-barrier could exist within the water itself.

“I think people haven’t tended to look at rivers in this way,” she explains. “We’ve tended to look at it from a very kind of land-centric way. We’ve assumed that rivers are pretty homogenous, it’s just a bunch of river, under water and fish could swim around. But we’ve never really considered that a river itself could be driving speciation in this way. So it’s kind of cool and very exciting. And it all comes down to that very odd, blind, de-pigmented fish.”

So maybe those little dying fish aren’t so pathetic, after all.