How Hungry Baby Urchins Are Saving Hawaii’s Reefs

They helped eat through invasive algae that was suffocating corals in Kāne’ohe Bay


For decades, Hawaii’s Kāne‘ohe Bay was plagued by an invasive species of algae that covered the bay’s reefs in knotted clumps and starved its corals of light. But as Alejandra Borunda reports for National Geographic, a team of researchers may have found the ideal solution to Kāne’ohe Bay’s algal overgrowth: voracious baby urchins.

The tiny critters could bring an end to a problem that began in the 1974, when a university researcher introduced a foreign red algae of the genera Kappaphycus and Eucheuma to the bay. According to Joseph Bennington-Castro of the National Oceanic and Atmospheric Administration, scientists were very interested in aquaculture in the 1970s, and these types of red algae are particularly intriguing because they can be harvested for carrageenan, a substance that is used in the medical, agricultural and food industries.

In their native habitats, predators that don’t exist in Kāne‘ohe Bay keep the invasive algae in check. So when they were introduced to the bay, the algae went haywire, growing quickly over the reefs and suffocating the coral. Fittingly, Kappaphycus and Eucheuma are sometimes referred to as “smothering seaweed.”

Brian Nielson, an aquatic biologist at the Hawaii Department of Land and Natural Resources in Honolulu, tells Borunda that experts initially cut the algae back in an attempt to tame it so the coral can get the sunlight that it requires to survive. But this often felt like a Sisyphian task.

“It's exhausting work,” he says. “We used to spend six to eight hours a day in the water pulling algae. When you go to sleep at night, you see algae in your dreams.”

Researchers from the Department of Land and Natural Resources, the Nature Conservancy, the University of Hawaii and the NOAA banned together to find a better way to free the corals from their algal prison. First, they developed an underwater vacuum that they dubbed the Super Sucker: a 40-horsepower pump and large hose that sucks up 800 pound of algae per hour and shoots it onto a barge. That worked well, but the researchers wanted to ensure that the algae would stay in check.

Their solution? Tens of thousands of baby collector urchins.

Collector urchins, which like to feast on seaweed, can eat up to several times their body weight each day, according to Borunda. They exist in bodies of water throughout Hawaii, but their numbers are low in Kāne‘ohe Bay for reasons that are not entirely clear, writes Bennington-Castro of NOAA. The researchers hoped that if they put more urchins into the Kāne‘ohe Bay reefs, the ravenous creatures would chomp their way through the invasive algae. So they developed a hatchery to breed baby urchins, which, due to their small size, would be able to get at the algae in the crevices of the reef.

Over the course of two years, according to the Honolulu Star-Advertiser, the team deposited 99,000 juvenile sea urchins in the reefs. And according to a new report published in Peer J, the experiment was a success: “Over the study period, removal and biocontrol reduced invasive macroalgae cover by 85% at treatment reefs,” the authors write.

There are a few caveats to the experiment. The urchins did not seem to have any negative impact on the reefs, but the study authors note there is a small risk that stocking the bay with urchins will facilitate excessive population growths. It also isn’t clear if this method of algae control would be successful in reefs larger than the ones in Kāne’ohe, where urchin movement was “naturally confined by 10–15 meters deep sandy habitats surrounding patch reefs,” in the bay, according to the researchers.

But one of the authors of the new study, Chris Wall, a marine biologist at the Hawaii Institute of Marine Biology in Kāne‘ohe, notes in a statement that the team is optimistic about its results.

“Our research shows promise as an effective mean to reduce invasive macroalgae with minimal environmental impact, while also incorporating a native herbivore to regulate a noxious invasive species,” he explains.

“The surprise was just how effective this approach was at reducing invasive macroalgae over the two-year period.”

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