Photograph courtesy of Olin Feuerbacher, U.S. Fish and Wildlife Service
Read Caption

Devils Hole pupfish are likely the world's rarest fish, and their population dropped to 35 in 2013. Researchers have since made a breakthrough in their captive breeding.

Photograph courtesy of Olin Feuerbacher, U.S. Fish and Wildlife Service

Brutal beetles kept world’s rarest fish from breeding—until now

Researchers have made a breakthrough in their effort to save the Devils Hole pupfish, which produced more eggs in 2018 than ever before.

Olin Feuerbacher couldn’t believe his eyes. It was December of 2017, and he’d been reviewing film captured the night prior, from a captive population of Devils Hole pupfish—the rarest fish on the planet.

These inch-long, electric blue fish have been stranded in a submerged limestone cavern in the Nevadan desert since the last Ice Age. The fish only live upwards of a year, and as such the population is prone to large swings, but lately it’s looked grim: In 2013, the entire population dropped to just 35 fish.

In the years since, the U.S. Fish and Wildlife Service opened the Ash Meadows Fish Conservation Facility, which houses a gigantic, 100,000 gallon replica of Devil’s Hole that can be controlled and protected. The goal was to create a “lifeboat population” of pupfish that could supplement or replace the ones in the wild if they should ever go extinct.

As Feuerbacher watched the infrared footage, which can visualize objects in the dark, a tiny pupfish larva smaller than a peppercorn flitted into the camera’s frame. This was big news. When a population gets as low as that of the pupfish, every animal—wild or captive, larva or adult—is critical to the species’ survival.

“I was pretty excited to see there was reproduction going on in the tank, and I just watched it for a little bit,” says Feuerbacher, a fish biologist with the U.S. Fish and Wildlife Service. “Then I saw a beetle swim past.”

It began circling the fish, and closing in.

“Then it just dove in and basically tore the fish in half right while I was watching,” says Feuerbacher.

From horror to hope

Scientists have known for a few decades that diving beetles share the waterlogged limestone cavern known as Devil’s Hole with the pupfish. With over 4,300 species known to science from every continent except Antarctica, there aren’t many inland bodies of water diving beetles haven’t colonized. In fact, Feuerbacher says that when he and other scientists descend into the hole to do pupfish counts, they can often feel the aquatic insects biting at their legs.

And when they built the Ash Meadows facility, the scientists tried to create a mirror image of Devil’s Hole, which meant bringing in water, substrate, and algae from the natural environment. So the fact that some diving beetles had hitchhiked along didn’t seem like a bad thing at first.

View Images

Diving beetles have been found to eat the eggs and larvae of the critically endangered Devils Hole pupfish.

As time went on, the scientists started to suspect that the diving beetles might be nibbling on pupfish eggs here and there. But for one to take down a larva twice its size? That was surprising.

But it was also a relief. Captive breeding efforts since the facility opened hadn’t been going well and no one could figure out why. Now, they knew what they had to do.

Beetle battle

In March of 2018, the scientists started actively removing diving beetles from the refuge tank using traps that catch the insects as they surface to breathe.

During the first beetle collection, facility manager Jennifer Gumm says they caught 500 beetles in three hours. And on the very next pupfish egg collection, which is done by leaving out pieces of carpet that the fish like to lay their eggs on, the team retrieved close to 40 pupfish eggs.

Before this, they had been lucky to find four or five pupfish eggs during a refuge collection. Usually, it was zero.

The researchers have concluded that the beetles were eating the eggs, and with more of the insects removed, more offspring can now survive. Unfortunately, we’re talking about a beetle the size of poppy seed in a 100,000 gallon tank, which means total eradication may be impossible.

The team isn’t currently removing beetles from Devil’s Hole itself for a few reasons. For one, the researchers don’t know how that might impact other aspects of the ecosystem, Gumm says. Second, beetle predation on eggs hasn’t yet been observed in the wild. Third, there appear to be fewer beetles in Devil’s Hole than in the refuge, and it may be that in the artificial environment they have less food and are more apt to eat eggs.

Since last spring, the team has been able to take eggs produced in the refuge and raise them to adulthood in a separate lab, thanks also to a new antimicrobial treatment that prevents the eggs from developing harmful fungi.

“That’s something people have been trying to do for better than 40 years,” says Feuerbacher.

There are currently around 50 fish in the captive refuge, says Gumm, and another ten to twenty maturing in the propagation laboratory, some of which have also started producing eggs. This means that the scientists are now collecting eggs from three sources. What’s more, the team counted 187 pupfish in the wild last fall—a fifteen year high.

“2018 was a really great year for the Devil’s Hole pupfish,” she says.

Blind, Cave-Dwelling Fish Found in Otherworldly Cave Chinese and French scientists found a rare, blind fish during a recent expedition.

The 10,000th beetle

While the recent breakthroughs are exciting news for a species on the brink, they may also yield important scientific discoveries.

When fish evolve in isolation, they can develop weird adaptations, says Prosanta Chakrabarty, an ichthyologist at Louisiana State University.

Devil’s Hole pupfish (Cyprinodon diabolis) have no pectoral fins, for instance, and they’ve evolved to survive in a habitat that’s warmer and lower in oxygen than most fish can handle.

“How do they live in a space that is smaller than many people’s offices?” Chakrabarty says. “Most animals wouldn’t have the genetic diversity to hang around in such a small space at such low numbers, so figuring out how they survive is important to evolutionary biologists.”

The problem is you can’t do experimental studies on a species that’s just struggling to survive. But if things keep going as they are with the refuge population, Feuerbacher and Gumm say we might one day be able to do the controlled studies necessary to figure out not just how pupfish have persisted all these years, but how we can better help them in the future.

In the meantime, the beetle battle will continue. Since last March, the team has removed thousands of pupfish-snacking insects from the captive breeding refuge.

“Our technician is keeping track,” says Gumm. “I’m going to get him a cake when he gets his 10,000th beetle.”