Ocean Watch

Monday, April 5, 1999

Crowned hunter is
part of reef life

Last week, an acquaintance told me how great the diving is at Johnston Atoll, a U.S. military base and wildlife refuge about 500 miles southwest of Honolulu. "The reef is practically untouched," he said. "I know some guys there who go diving every chance they get. They look for crown-of-thorns starfish and kill them."

"Are the starfish causing a problem?" I asked.

"Well, they eat coral. And the coral there is fantastic."

"But is a bloom going on? Are the starfish out of control?"

He didn't know, and we dropped the subject. The next day, I came across a local newspaper clipping reporting that crown-of-thorns starfish had been seen on a pristine reef off Kahoolawe. The headline read, "Island researcher spots threat to reef."

But when I read the article, I learned that the federal researcher, who saw only one starfish, said that the creatures don't appear to be a threat at this time. And when I called the U.S. Fish and Wildlife manager at Johnston Island, I got a similar response. The crown-of-thorns starfish are not currently causing problems there, and the manager will see that the killing of these animals stops.

Oh, the poor, maligned crown-of-thorns. Just because they evolved to eat coral, undergo occasional population booms and have pokey spines, people hate them.

This is silly. Crown-of-thorns starfish have a place on the reef as important as any other plant or animal that lives there, including coral.

Crown-of-thorns starfish grow to about 20 inches across and are covered with sharp, mildly venomous spines. These nocturnal animals hide during the day, then venture out at night to find coral by following its scent. Crown-of-thorns starfish prefer branching and plate corals rather than massive or encrusting types.

Once a starfish reaches its food source, it everts its stomach, oozes some enzymes and digests the soft coral polyp right out of its skeleton. In one day, a crown-of-thorns starfish can eat an area about as big around as its disc-shaped body.

Since it's the polyps that carry color in corals, the starfish leaves behind a bleached white skeleton.

This is normal. Predator starfish help keep the balance between stony corals and the countless other organisms competing for space on the reef.

Usually, crown-of-thorns starfish are spaced pretty far apart on reefs and cause no problems. However, in the 1960s and 1970s, some Pacific reefs saw a starfish population explosion. In some places, as many as 15 crown-of-thorns were packed into a square yard of reef.

The corals, of course, took a huge hit, and people got worried. On Australia's Great Barrier Reef, researchers tried killing the starfish by injecting formalin into them, but nothing seemed to do much good until, eventually, the starfish disappeared on their own.

No one knows what caused the starfish bloom, but researchers favor two theories. One is that heavy rains after a long dry spell caused runoff from agricultural land to fertilize the ocean and thus feed an unusual number of crown-of-thorns larvae. (Each adult produces about 65 million eggs in one spawning season.)

Another theory, based on cores taken from Australian reefs, suggests that fluctuations in crown-of-thorns populations have occurred for thousands of years. Thus, the bloom was part of the reef's normal life cycle.

If you find a crown-of-thorns starfish, consider it your lucky day. Admire the creature's lovely colors, rich skin texture and big tube feet, then leave it in peace.