Sea Urchins Munch Through Enough Algae to Save a Coral Reef

The introduction of nearly 100,000 urchins over the last two years in the Kāne’ohe Bay, on the Hawaiian island of Oahu, helped save the coral reef by grazing through an invasive non-Hawaiian species of algae introduced in the 1970s.

“With manual removal, you’re spending a lot of time and human hours and effort to take algae off the reef, and there’s no guarantee it won’t come right back,” says Kostantinos Stamoulis, a biologist at the University of Hawaii, Mānoa. “And in many cases that’s exactly what happens. With biocontrol, you’re letting nature itself take care of the problem.”

Prior to dropping four native urchins on every square meter of the reef as part of a self-sustaining system, divers trimmed the algae infestations with an underwater vacuum system. The spiny animals can munch through several times their body weight each day, so they were able to clear the remaining algae and re-exposing the coral beneath. In the two years of the experiment, the invasive algae cover was reduced by about 85%. The reefs were then able to rebound and revitalize. “The great lesson, from this whole natural experiment in Kāne’ohe Bay,” says Iria Fernández Silva, a biologist at the California Academy of Sciences, is “let’s think twice before we bring things from outside.”