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Crown of thorns Starfish (alamea)

January 28, 2013

Our reefs have several kinds of starfish (aveau, fetu). Most have five “arms,” like the brilliant blue starfish (Lynkia laevegata), but the crown-of-thorns starfish (alamea, Acanthaster planci) has about 15 arms. They’re a big starfish, with adults commonly over a foot in diameter. They can be a beautiful dark red, or a dark green, often with some red markings. This starfish is one to look at, but not touch. The upper surface is covered with nasty spines. Anything but the very lightest touch, and the spines will stick into you, and you will be very sorry. There are toxic chemicals in the spines, and once you are stuck, it will hurt, and continue to hurt for hours if not days. If you get stuck by several spines, the area may go numb and stay that way for a week or more, and if you get stuck by a lot of spines you could become weak or paralyzed in that part of your body or even have more trouble.

The underside of a starfish has little suckers at the ends of their tube feet. They “walk” on them and hold on to the substrate with these suckers. It is said that if you have been stung by a crown-of-thorns, have the tube feet touch or walk on the area where you have been stung, and it will reduce the pain. A much better idea is not to get stung in the first place.

Crown-of-thorns, like most starfish, have an unusual way of eating. They push their stomach out their mouth to cover what they want to eat, then their stomach digests their food outside their mouth, and then they pull their stomach back inside them. If you think about trying that on your dinner, you’ll realize just how amazing that is. Crown-of-thorns are a bit unusual in what they eat: coral. They eat the outer living part of the coral, leaving the dead white skeleton. Sometimes little patches of live coral tissue are left and the coral survives and can slowly re-grow, other times the coral is completely killed.

Most of the time, crown-of-thorns are pretty rare on coral reefs. But back in the 1950’s huge numbers of them appeared on coral reefs in Japan and ate the corals killing most of the reefs there. Then in the 1960’s the same thing happened in Australia’s Great Barrier Reef. In 1978-1979, huge numbers of them appeared in American Samoa and killed most of the corals here. When such outbreaks happen, people may try to save the reef by
killing as many crown-of-thorns as they can, but cutting them up doesn’t kill them, they just grow the missing parts back. It’s best to spear them and take them out of the water, where they die quickly. One idea about why such huge outbreaks of millions of these starfish occur is that humans have killed the predators that feed on them, like trumpet triton mollusks and humphead wrasse. So it is best to leave those predators on the reef. Another idea that has more evidence to support it is that unusually heavy rainstorms wash nutrients from the
land into the water, which feed the tiny plants and animals that are the food of the young starfish, so more juveniles survive and they grow up into the invading hordes of coral-killers. From this viewpoint, it makes sense to reduce the amount of soil, piggery sewage, and high phosphate laundry detergents that flush down our streams into the ocean. These nutrients might help more crown-of thorn juveniles survive and grow up to eat our corals.

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