Shallow reef species may not find refuge in deeper water habitats

Deep water reefs are unlikely to be safe harbors for many fish and coral species from shallow reefs threatened by climate change and human activity. Shallow water creatures may have trouble adapting to conditions in the deep, scientists report in the July 20 Science. Plus, deep reefs are facing the same threats that are putting shallower ones at risk.

The study deals a blow to the “deep reef refugia” hypothesis. That’s the idea that species from troubled shallow reefs could simply move to reefs at depths of 30 to 150 meters, called mesophotic reefs because they exist at the limits of where sunlight reaches. Even though individuals of a typical shallow water species may be spotted at a wide range of depths, it doesn’t mean the majority of that species could survive living in deeper waters, says study coauthor Luiz Rocha, a zoologist at the California Academy of Sciences in San Francisco.   

“When you start looking into details,” Rocha says, “a lot of these species don’t actually live in these depths.” But there was scant data, partly because seeing these species requires scientists to undergo technical diving training.

Rocha and his team wanted to figure out which, if any, shallow water species can also thrive in deep water reefs. And the only way to do that was to get wet.  

The scientists made dozens of dives to depths of up to 150 meters in waters off the Philippines in Asia, the Caribbean island of Curaçao, the mid-Atlantic island of Bermuda and the western Pacific island nation of Micronesia. While cataloging 687 coral species and 1,761 fish species, the team found that deep water ecosystems were strikingly different from surface reefs and had many distinct species evolved to live in darker and colder waters.

While the researchers didn’t quantify the overlap, in some places, it looked like about half of all species were found in both deep and shallow reefs, says coauthor Richard Pyle, a zoologist at the Bishop Museum in Honolulu. The rest were found in only deep water or shallow water. 

The team also noted that none of the deep water reefs were pristine. The researchers saw tangled fishing line, broken bottles and coral bleaching, a problem that can be caused by consistently higher water temperatures (SN: 2/3/18, p. 16). These problems were particularly severe in the Philippines reefs.

“One of the things that people assume is that these deep ecosystems are less impacted,” says oceanographer Kimberly Puglise at the National Oceanic and Atmospheric Administration in Silver Spring Md., who was not part of the study. But deep reefs are also susceptible to damage, and yet usually don’t fall within protected areas as shallow reefs do.

The study shows that deep water reef ecosystems are “not just important because they may be able to protect shallow reefs,” Puglise says. “They’re important in their own right.”

This research doesn’t rule out the possibility of species migration to deeper waters. A meta-analysis published in the June 11 Environmental Evidence found that some species do well in both shallow and deep waters, though that work relied on reported depth ranges rather than in-person research.

Coral reef ecologist Tyler Smith at the University of the Virgin Islands in St. Thomas, who was a not a part of either study, says there could be other regions of the world with more depth overlap between reef species. “It might depend on where you’re looking.”