The Caitlin Seaview Survey's ROV in action, via
The global outlook for coral reefs hasn't been positive for some time. Reefs in China, the Caribbean, as well as the Great Barrier Reef in Australia, are all feeling the pain of human activity. But just because we're wrecking them doesn't mean we know everything there is to know, as evidenced by the recent discovery of Great Barrier Reef coral found far deeper than ever before.
A new survey of deep water reef systems by the University of Queensland's Seaview Survey found light-dependent Acropora coral, which normally lives in shallow waters, as far as 410 feet down. That's far beyond what scuba divers can normally go, and the discovery came via the group's remote submersible. The deepest the team had ever seen Acropora beforehand was around 200 feet, which led expedition leader Pim Bongaerts to call the new discovery "mind-blowing."
What's surprising is the type of coral found. Deep-water corals, which aren't dependent on light, have been found thousands of feet deep, while Puerto Rico is home to the deepest light-dependent reefs yet found, which have been observed as deep as 500 feet.
Acropora polyps rely on the energy produced by photosynthetic zooxanthellae that live inside their cells. To find them so deep, where barely any lights penetrates, is rather incredible.
"What's really cool is that these corals still have photosynthetic symbionts that supposedly still harvest the light," said Ove Hoegh-Guldberg, chief scientist on the project. "It's interesting to know how they can handle such low light conditions – it's very deep dusk, you can barely make out much at the bottom."
It suggests that Acropora are perhaps more flexible, and at the very least more complex, than once thought. Of interest is how the corals reproduce at that depth; Acropora in shallow water reproduce via mass spawning events, which may or may not be possible at such depths when spawn have more vertical space to contend with. Hoegh-Guldberg did note that deeper corals have been shown to be more resilient to destructive storm action, and they also obviously don't have to deal with the physical damage from boats, divers, and tourists that shallower coral do.
That's a positive sign. As we saw with research earlier this year, pollution and agricultural runoff helped caused a major shift in the makeup of coral in the Great Barrier Reef, in which Acropora, which is fast-growing and can grow very large, gave way to coral of the Pavona family, which grows more slowly and doesn't get so big. That's detrimental to the overall health of the reef, as Pavona isn't as capable of regrowing quickly following destructive events, natural or otherwise.
While it's an initial discovery, and any measure of its range and density will require more surveying, to see that Acropora has held on in the depths of the Great Barrier Reef is a positive sign. When the reef outlook is usually negative, it's nice to see a study that doesn't spell total doom.