Thorn In My Side

Up until about the late 1950's, Acanthaster planci--the Crown-of-Thorns seastar, a natural coral predator--were a rarity off the coast of Australia. By the early 1960's, their population had blossomed, with some scientists debating the cause but linking it at one time or another to familiar troublemakers, including global warming, overfishing, pesticide use, and atomic testing. Some reputable scientists at the time even dismissed the Crown-of-Thorns outbreaks as a hoax, claiming that most coral reefs were in no danger. This was clearly not the case. This once-obscure seastar destroyed enormous swaths of coral reefs all over the Pacific. In What is Natural, a fascinating historical account of the early years of this predator outbreak, author Jan Sapp writes, Facts, theories, values, and politics were so entangled in the controversy that it was often as difficult for us to separate them as it was for scientists to separate anthropogenic from natural change.

The spiny, poisonous Crown-of-Thorns mystified biologists, injured tourists (I've only had a tiny spine tip in my hand and it was excruciating), and destroyed coral at an alarming rate. By 1970, Crown-of-Thorns (or COTs as they are unaffectionately called by those of us in the "biz") infestations were laying waste to reefs from Australia to Guam. Thirty-eight years later, debates still rage over whether, and to what degree, human activity causes the starfish booms. Independent of all the speculation, Crown-of-Thorns outbreaks continue. I've received calls for help from seastar-stricken stakeholders in every Indo-Pacific project site in which I work. Certainly, controls such as seastar removal campaigns can be very effective but by the time an outbreak is detected, a lot of damage has already occurred. In areas of heavy infestation, coral mortality can be as high as 95%. Some of the most likely smoking guns as catalyst for Crown-of-Thorns outbreaks include predator imbalances (over-harvesting of seastar predators such as Napoleon Wrasse and Triton Trumpet conch) though increased sea surface temperatures as trigger to spawning events has also been tossed around.

A report this week indicates that a new outbreak of Crown-of-Thorns is occurring in Indonesia, directly in the heart of the Coral Triangle--the area between Indonesia, Malaysia, the Philippines, Papua New Guinea and the Solomon Islands which contains the highest coral diversity and density on the planet. Researchers from the Wildlife Conservation Society and ARC Centre of Excellence for Coral Reef Studies fear the outbreak is caused by poor water quality and could be an early warning of widespread reef decline.

Young Crown-of-Thorns seastars eat encrusting (coralline) algae which is common on the reef. At about six months of age, they start to eat hard coral and begin to grow more rapidly. Over the next two years, the seastar can grow from about 1 cm to about 25 cm in diameter. When there are few Crown-of-Thorns, they tend to be hidden under corals during the day and move into the open to feed at night. They feed mainly on specific coral species that they prefer, and may not eat the entire coral colony. As a result, a reef can recover quite rapidly from low levels of coral feeding. Some reefs seem to support small populations of Crown-of-Thorns seastars for many years, with only a small reduction in coral cover.

But when seastars are in large numbers, there is intense competition for food and most corals will be eaten, including less-favored species such as soft corals, algae, gorgonians and other encrusting organisms. The seastars aggregate during outbreaks and stay in the open, feeding night and day. During a severe outbreak, there can be several Crown-of-Thorns per square meter and they can kill most of the living coral in an area of reef, reducing coral cover from the usual 25 - 40% of the reef surface to less than 1%. Such a reef can take 10 years or more to recover its coral cover.

Like most seastars, the Crown-of-Thorns is called an extraoral feeder since in order to feed it forces its stomach through its mouth. This membranous structure is positioned around the irregularities of the coral with the help of the seastar's tube feet. The stomach then secretes an enzyme which breaks down the coral tissue, unlocking the major energy reserves of the coral. This material is then transported by cilia to the stomach lining where it is absorbed. The feeding process may take from 4-6 hours. Once digestion is completed the stomach is retracted and the seastar moves along leaving behind the empty, white coral structure.