Trilobites: This Device Helps Fish Make It Out of the Twilight Zone Alive

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A portable decompression chamber helps scientists protect fish from injury when they are brought to the ocean’s surface.

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Divers collect fish with a portable decompression chamber developed to help scientists study species from the ocean’s mesophotic zone.CreditCalifornia Academy of Sciences

In the ocean, sandwiched between sun-filled shallows and ever-darker depths, is a strange place where the noon sun shines only as bright as twilight.

Some 200 to 500 feet below the surface is the mesophotic zone, where ancient reefs once drowned, leaving behind empty bodies now inhabited by new corals and colorful mystery fish.

Much of this zone remains unexplored: It is often skipped by scientists on their way to the ocean bottom. Just as we begin to discover this slice of the sea, climate change, overfishing and other human activities may be threatening its very existence.

Those who study the zone have long needed a safer way to bring its fishy residents to the surface without killing them.

If you’ve ever traveled with snacks from the bottom of a mountain to the top, you may have noticed that an unopened bag of chips will expand as you ascend.

Deep divers carry tiny bubbles of air in their bodies, and fish have swim bladders; both expand as bodies ascend and pressure decreases. And if care isn’t taken to decompress slowly along the way, bubbles expanding in the body can become fatal.

Some scientists poke holes into fish swim bladders with needles to solve the problem. But Bart Shepherd, who directs the Steinhart Aquarium at the California Academy of Sciences, with colleagues there and at the Monterey Bay Aquarium, thought there was a less intrusive way.

They developed transparent collection jars two feet long that fit into chambers, small and simple enough for divers to carry and manipulate.

Between 2014 and 2017, Mr. Shepherd and his colleagues journeyed to the Philippines, Vanuatu, Palau and Pohnpei to test the contraptions, collecting 174 fish from mesophotic depths as part of Hope For Reefs, an initiative to better understand and protect some of world’s least known coral habitats.

As the divers ascend with their catch, they insert the jar into the chamber and blow a bubble into it. This maintains the pressure until it reaches the surface. There, a valve releases air from the bubble over two or three days so the fishes have time to adapt to surface pressure.

Take a deeper dive into interesting ocean research

After decompression, the fishes temporarily occupied makeshift kiddie-pool aquariums in hotel rooms before they were packed and shipped by air to California. Most survived the journey.

“That chamber becomes the world’s deepest aquarium,” said Mr. Shepherd. “Now in our exhibit we have eight undescribed species of fish on the public floor that people can see that they’d have no opportunity to see anywhere else in the world.”

With this technology, Mr. Shepherd hopes that more people will gain a better understanding of these unusual and understudied aquatic environments that are often not included in marine protected areas or sanctuaries.

“Most people have an image of a coral reef in their head from “Finding Nemo,” personal experience, or “Blue Planet,” but if you ask them what does a mesophotic ecosystem look like, people have no idea,” he said.

“I want to shine a light into the twilight zone and show people that these ecosystems exist, are under threat, and there’s something we can do about it.”

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