A Decade-Long Experiment Is Unlocking the Mysteries of the Ocean’s ‘Deep Reefs’

“As soon as you drop any kind of clean structure on a reef,” Rocha says, “it starts getting colonized by larvae of everything around it—of sponges, gorgonians, corals. And then, because there’s a lot of hiding spots between the plates—so between the floors of the hotel—they start getting colonized by shrimp, by mollusks, by ascidians, by anything you can imagine in the ocean. And it becomes this stable, diverse mini reef, if you will, packed with a lot of biodiversity that would be very hard to collect all at once in any given reef.”

A few years after that initial drop, you come back, collect the ARMS, and then study the mini reef that has grown on them. You can sample and touch and analyze things to your heart’s content.

It’s not only a very effective way to collect lots of creatures, all at once, but also a great way to do so in a standardized way. If you went down—or sent a robot down—to collect an encrusted rock from a reef, you could learn a lot about the life on it, but it would be hard to compare the life on that rock to life on other rocks from other reefs, or even the same reef, because each rock would have its own unique characteristics. Not so for ARMS.

“ Every one’s the same,” Chris Meyer, director of the Smithsonian Global ARMS program, told me. “It’s prefab housing, and then you just look to see who moves in.”

To Rocha, this seemed like a great way to get a really good, long look at the deep reefs instead of the brief glimpses he was getting from diving. It was also, he told me, fairly cheap to deploy them. So from 2016 to 2018, with some private funding, he and his colleagues left ARMS in some of the deep reefs around Palau—as well as in Guam, the Marshall Islands, and French Polynesia. They installed some around 50 meters, and some around 100, and then some around 10 meters, so he could compare the deep reefs to their shallower neighbors.