Nestled within the new Pacific Remote Islands Marine Monument lies Palmyra Atoll, one of the last pristine coral reefs left on the planet some 960 nautical miles south of Hawaii. Or near pristine. In 1991 a 100-foot longline fishing ship—the "Hui Feng No. 1"—foundered on the reef under mysterious circumstances. In the wake of that shipwreck, a destructive species of corallimorpharian—a kind of half-coral, half-anemone sea creature—began to take over the reef.

In the intervening years, the species has spread to cover nearly two square kilometers of the reef, though its density declines the further from the shipwreck you go, ultimately disappearing entirely in more undisturbed areas. Near the shipwreck, however, this Rhodactis howesii has overgrown the underlying reef, killing its coral cousins. It also crops up near buoys in the area. "They are very aggressive…and use specialized anatomic structures called 'sweeper tentacles' that have stinging cells," says wildlife veterinarian Thierry Work of the U.S. Geological Survey, who has studied the problem. "These tentacles are used to kill adjacent organisms, like corals, so as to capture real estate."

It remains unclear what exactly is enabling the corallimorpharian takeover, though Work and other scientists who discovered the phenomenon speculate it could be that increased iron dissolving from the man-made structures is boosting R. howesii's growth. "However, this phenomenon is not seen in wrecks on all Pacific islands," Work notes.

A solution? Tow the wreck off the reef and into the deep sea, though estimates for the cost of that range as high as $3 million due to the remoteness of the atoll. And there are no guarantees that it will work. There may already be a sufficient number of R. howesii in place to ensure their spread at the reef, regardless of any intervention. "That said," Work notes, "inaction is most certainly far worse than doing something."

Image: Thierry Work / U.S. Geological Survey