Coral-eating starfish have their place in healthy reefs

Published October 14, 2017 in the “Ocean Watch” column, Honolulu Star-Advertiser ©2017 Susan Scott

A crown-of-thorns starfish eats live coral bodies, leaving their white skeleton cups intact. ©2017 Susan Scott

BRAMBLE REEF, AUSTRALIA >> On our sailboat, Honu, no wind means using the boat’s loud, hot motor to go somewhere, something we sailors resist. Last week, though, the lack of breeze and Honu’s diesel engine were a godsend, allowing us to explore sections in the outer area of the Great Barrier Reef Marine Park.

Being out in the Coral Sea, anchored in striking white sand patches surrounded by city-block-size coral heads and boulevard-wide reef flats was a wilderness experience like no other. We snorkeled for hours on end, and each time we rested, the same word came to both Craig and I in describing what we saw: pristine. These reefs are among the few remaining places on Earth that show no sign of human disturbance.

In some areas, though, nature had wielded a heavy hand. Massive coral heads lay on their sides, and car-size table corals stood upside down, tipped over by cyclone-­driven waves.

We also saw crown-of-thorns starfish, the largest of all starfish species, slurping the soft bodies out of corals’ hard cup skeletons. The starfish everts its stomach through its mouth, oozes digestive juices onto the live coral body and sucks it up, leaving only the white calcium carbonate cup.

Crown-of-thorns starfish, COTS for short, have 10 to 20 arms, grow to 20 inches across and are a normal part of healthy coral reefs. Coral deaths open space for juvenile plants and animals to settle, thus giving reefs variety in shapes, surfaces and species that can withstand, and recover from, natural damage, including starfish blooms. Of the reefs we visited, however, in two places COTS were clearly more numerous than a healthy reef can handle.

COTS blooms have been a threat to the Great Barrier Reef since the 1970s. Researchers once thought that human collecting of the giant triton snail, called Triton’s trumpet, was to blame, since the snail eats COTS. The outbreaks, however, aren’t caused by a shortage of starfish predators, but rather by the survival of exceptional numbers of the starfish’s babies.

River runoff causing algae blooms that feed drifting starfish larvae is another suspect. But COTS outbreaks also occur off remote islands where there’s no such runoff.

Of the 100-some COTS research papers published over the decades, the cause of the blooms remains the same: unknown. It may be a normal part of reef evolution.

Just when we started to worry about COTS destroying these pristine reefs, the starfish vanished. The next acres of reef had zero COTS, and the corals, fish and giant clams sparkled with such beauty, I felt happy to be alive.

Sailboats might need the wind to sail, but sitting becalmed on the Great Barrier Reef, sunny day after sunny day, landed us smack in the middle of ocean paradise. Now that’s a godsend.

Crown of Thorns starfish, French Polynesia, 2006. Courtesy Scott R. Davis