A twilight zone beneath the waves

Between 30 and 150 meters below the surface lies the mesophotic zone, a dimly lit ecosystem that sits between sunlit reefs and total darkness. Scientists call these areas deep reefs, and almost nothing is known about them.

“Anything between 30 meters and 150 meters is really, really unknown.”

Why deep reefs remain a mystery

Diving to 100 meters requires risky technical diving with strict time limits. Divers can only spend 5 to 7 minutes at those depths before they must ascend slowly for hours to avoid decompression sickness.

“We have a very, very short time. And by short time, I mean five minutes. Six, seven minutes.”

The big questions scientists want answered

Researchers want to understand what lives in deep reefs, what energy sources sustain them, how food chains work, and whether these ecosystems might shelter species from warming shallow reefs.

How deep reefs differ from shallow ones

Shallow coral reefs rely on photosynthesizing algae to feed corals, but deeper down, fading light makes photosynthesis impossible. Deep reef animals must survive in dimmer, colder conditions.

A clever solution: prefab reef housing

Scientists deploy autonomous reef monitoring structures, or ARMS, which are stacked PVC plates that sit on the seafloor. Over years, they become colonized by larvae and creatures, creating a mini reef packed with biodiversity.

“It's prefab housing, and then you just look to see who moves in.”

Why ARMS work better than diving

ARMS let researchers collect thousands of creatures at once in a standardized way, making it easy to compare life across different reefs. They also avoid the time and safety limits of deep diving.

Deploying ARMS across the Pacific

From 2016 to 2018, Luiz Rocha and colleagues placed ARMS at various depths around Palau, Guam, the Marshall Islands, and French Polynesia. Some were placed at 10 meters, others at 50 and 100 meters, to compare ecosystems.

The long wait for retrieval

Rocha had to wait longer than planned to retrieve the ARMS because retrieval is more expensive than deployment. Eventually, private funders agreed to underwrite the costly retrieval mission.

Opening the scientific puzzle box

When ARMS were retrieved from Palau's deep reefs, some were sparse while others were fuzzy with colorful organisms. Scientists brought them to the Palau International Coral Reef Center to examine the mysterious ecosystem up close.

Disassembling and cataloging life

Scientists carefully took apart the ARMS plates and photographed them as a time capsule of the deep. They then picked through debris hunting for tiny crabs, worms, mollusks, sponges, and other creatures.

A treasure trove of organisms

The ARMS collected sea squirts, sponges with bristles like caterpillars, soft corals called octocorals, and bryozoans that created swirling honeycomb patterns. Each discovery revealed the hidden diversity of the deep.

Identifying what's new to science

Researchers looked for every unique organism, called a morphospecies, and collected tissue samples for DNA sequencing. A nudibranch expert identified several potential new species on the first day alone.

“We're looking for every unique thing, which we call a morphospecies. And we're trying to get some representative tissue samples from all of these things so that we can sequence the DNA.”

The slow work of identification

Of nearly 3,000 specimens collected, only 3 percent had been identified to species level by July. Most were only identified at broader taxonomic levels, with genetic testing still underway.

Relationships matter more than species counts

Scientists were equally excited about discovering relationships between animals, like pompom crabs holding anemones or skeleton shrimp blending in on hydroids. These connections reveal how the ecosystem actually works.

Comparing life across different depths

Researchers are already using ARMS data from Guam to analyze how much overlap exists between species at different depths in the deep reefs. This work will help answer fundamental questions about ecosystem structure.

Still in the early stages of understanding

This research is comparable to mountain ecology work from the mid-20th century, showing how far behind deep reef science still is. Researchers are just beginning to build the foundation of knowledge.

A puzzle for the next generation

Luiz Rocha acknowledges that scientists won't solve the deep reef puzzle in their lifetimes. But they're generating good pieces of the puzzle that future researchers will use to assemble the complete picture.

“We will not assemble the puzzle, no, but we'll generate some good pieces for the next generation to assemble.”