When Andrew Altieri visited a reef on the Atlantic coast of Panama that had died five years before, he expected to see a big garden of algae coating the reef’s skeleton. That’s what normally happens to a dead-bleached reef because overfishing has reduced the herbivorous fish that would graze on the algae and keep it from covering the reef. But this time he was surprised. “Here it is a few years later and I’m with my students and we’re like, ‘where’s all the algae?'” says Altieri.
The two dominant grazers that usually handle the algae, parrotfish and sea urchins, weren’t around. Looking more closely, he and fellow researcher Caitlin Kuempel found the answer: creatures so small that they had always been overlooked in the past. A tiny species of parrotfish and sea urchins the size of ping pong balls were grazing on the reef. As Jackson Landers of the Smithsonian Magazine notes, “The ability of these tiny grazers to step in and fill the role of their larger relatives is somewhat analogous to taking bison off of a prairie, only to learn that prairie dogs were able to maintain it.”
Coral is an animal with a plant inside its cells. The plant is a form of algae that gives the coral its color and provides the animal with most of its nutrition. The issue for some reefs, in particular Australia’s Great Barrier Reef, is an increase in the ocean’s temperature. When under heat stress, the coral expels the algae, revealing the white or “bleached” skeleton beneath the living coral, and often killing it. Reefs suffering from these bleaching events can actually heal and begin growing again, if the algae is controlled by grazers. The bleached skeleton becomes the foundation for new baby corals to grow on.
It’s unclear whether the tiny herbivores found on Altieri’s Panama reef can help it return to life. While some reefs are being assisted by coral hatcheries, where corals are grown to size and glued on to the old skeleton, this reef has not been selected for that work because of the expense and time involved. Altieri and other scientists at the Smithsonian Tropical Research Institute are watching to see if the reef re-colonizes naturally. They expect to be able to tell if it’s working by the end of 2017.