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The dead coral reef looked like a graveyard.
“It was pretty bleak,” says Hannah Nelson, a student at California State University, Northridge. “Live coral is extremely colorful. This dead coral was covered in algae and kind of monotone.”
Nelson and her colleagues at the Smithsonian Tropical Research Institute were in Panama’s Almirante Bay in 2013, where they were investigating the recent deaths of the Agaricia coral reefs. The massive bleaching event, attributed to sedimentation and runoff from river dredging and other activities, left some reefs almost completely destroyed. In some areas, less than five percent of the coral survived.
But as the researchers swam toward the dead reefs, they saw something unexpected: life. “There were tons of urchins and other critters in the dead coral,” Nelson says. “It wasn’t as dead as we thought. We wanted to quantify that.”
The researchers returned to the reef in July 2014, and began cataloging the invertebrate diversity still living among the skeletons. Their searching uncovered more than 2,500 living invertebrates from 40 different taxonomic groups, including reef urchins, brittle stars, hermit crabs, and snails. “It’s like when you find dead trees and they are covered in beetles and all of these interesting creatures,” Nelson says.
Both corals and trees have something in common, Nelson says: they’re foundation species that create three-dimensional habitats for other species. “They continue in that role even where they’re dead.” In the case of the Agaricia coral reefs, their highly complex skeletal structures provide habitat for dozens of species even after the coral itself had expired.
Not a lot of research exists on what the invertebrate communities in Almirante Bay were like prior to the bleaching event, so the question remains just how much has changed. “Just because they’re there doesn’t mean they’re as healthy, or that we have the same assemblage, as before,” Nelson says. “There still could be massive losses of biodiversity.”
There are also questions about what could happen to these invertebrate communities in the future.
“Dying coral reefs may still harbor fairly high diversity, but it almost certainly diminishes over time,” says Steve Gittings, science coordinator for the US National Oceanic and Atmospheric Administration’s Office of National Marine Sanctuaries, who is not affiliated with the research. “The retreat to a simple, relatively low-diversity system is the usual outcome.”
Some damaged coral reefs, especially those in deep waters or with complex structures, can and have bounced back in the past, according to research published last year, so it is possible that this slow march to the grave could be averted. Either the original Agaricia coral could return, or other coral species could fill the gap. But barring that, Gittings says species such as sponges will slowly erode dead coral skeletons, making the remnant reef less complex and therefore less able to support great quantities of life. When that happens, other species such as shrimp and crabs die off, to be replaced by more sponges and small crustaceans. That, in turn, leads to cascading effects.
“As these species change, so do [the] animals that depend on them for food, such as the fish assemblage,” Gittings says. “Before you know it, species are disappearing all over the reef without obvious reasons.”
Nelson says the most important lesson from her research is that even dead coral reefs need to be preserved in order to protect the biodiversity that remains. She also suggests that this understanding of foundational structure might inform efforts to create artificial reefs. Those types of artificial reefs probably won’t play the same ecosystem function as live coral, she says. “But could it be helpful in the short term? Maybe.”
Nelson considers this research a sign to be optimistic for the ocean, since it indicates that life can go on. But it also highlights the doom and gloom present in oceanic research.
“The early ecologists studied succession of how living communities mature,” she says. “Increasingly, we’re in the science of studying how communities die and the stages of death. This is one of those stages. But it’s exciting to see that death is not immediate for all the members of the community.”