Crown-of-thorns Starfish Populations

A. planci, commonly known as the crown-the-of- starfish, is large sea star that lives in tropical zones of the Red Sea, the Indian Ocean, and the Pacific Ocean. Covered with long. sharp spines that contain a neurotoxin that causes a stinging sensation in its prey, A. planci feeds on coral polyps, the animals that build reefs by secreting calcium carbonate to create hard skeletons that eventually become a reef. Until 1962 when large numbers were first seen off the coast of northeastern Australia, A. planci was considered rare. Soon after 1962, however, large numbers were observed on reefs throughout the Pacific and Indian oceans. With every population explosion, coral reefs were devastated. The starfish numbers declined in the mid-1970s, but a second, even worse population explosion occurred between 1979 and 1991, followed by a third that lasted through the 1990s and began waning only in 2008.

Scientists are uncertain why these outbreaks are occurring. On the one hand there is a possibility that this may be a natural part of reef ecology. Humans, after all, have been exploring marine environments only since the invention of diving gear during the Second World War. For all that today’s marine biologists know, crown-of-thorns outbreaks may have occurred on reefs ever since reets first appeared. Indeed, an analysis of bits of fossilized reef buried in the ocean floor indicates that the species has been devouring Great Barrier Reef coral for at least 8,000 years. More controversially, some scientists think this same study provides evidence for ancient outbreaks.This interpretation, if correct, would support the idea that periodic outbreaks-perhaps driven by the El Nino or fluctuations in temperature salinity and plankton levels-are normal. Furthermore, ecologists generally think that populations of species able to produce large numbers of offspring in short time are inherently unstable. The crown-of-thorns starfish is just such an animal. When conditions are right a single female can produce up to 60 million eggs each year.

But another study, in which researchers examined feeding scars on long-lived corals, provides evidence that outbreaks have become increasingly frequent in recent decades This supports the view that such outbreaks are not natural, but rather a side effect of the environmental changes induced during the past century by humans. It has been proposed, for example, that overfishing and overharvesting of reef resources such as the giant triton snail and its20-inch(50 centimeter) trumpet-shaped- shell-the traditional trumpet of choice among South Sea islanders-has resulted in reduction of predators that would formerly have kept the crown-of-thorns under check by preying on juveniles. This theory lacks direct evidence, and there is little proof that triton snails or any other predator eats huge numbers of starfish. But recent surveys have revealed that among the Australian reef areas most favored by A. planci. those protected by anti-harvesting laws are nearly four times less likely to be hit during an outbreak. Some researchers now believe that overfishing of top predators can have a dramatic cascading effect on the food chain. In this case, commercially harvested fish such as coral trout normally eat the smaller fish that in turn feed on various invertebrates whose diet would include juvenile starfish. Fewer trout would mean an increase in smaller fish, which would result in fewer starfish predators and thus more starfish.

Finally, there is pollution. It has been observed that many outbreaks around the Pacific have begun two or three years-the time it takes for starfish to develop to a point where they can be easily detected-after heavy rains, particularly heavy rains that followed a prolonged dry spell. They also occur frequently near human settlements. This has led some researchers to propose that agricultural and urban runoff temporarily alters the near-shore ocean environment in a way that increases the survival rates of young starfish.It has been suggested that pesticides flushed into the ocean by rain may have less impact on planci than on its predators or competitors. Alternatively, nitrogen and other nutrients may be boosting the plankton upon which starfish larvae feed. But this theory also has its weak points. Why, for instance, is the crown-of-thorns starfish the only reef resident experiencing such dramatic outbreaks??

1.A. planci, commonly known as the crown-the-of- starfish, is large sea star that lives in tropical zones of the Red Sea, the Indian Ocean, and the Pacific Ocean. Covered with long. sharp spines that contain a neurotoxin that causes a stinging sensation in its prey, A. planci feeds on coral polyps, the animals that build reefs by secreting calcium carbonate to create hard skeletons that eventually become a reef. Until 1962 when large numbers were first seen off the coast of northeastern Australia, A. planci was considered rare. Soon after 1962, however, large numbers were observed on reefs throughout the Pacific and Indian oceans. With every population explosion, coral reefs were devastated. The starfish numbers declined in the mid-1970s, but a second, even worse population explosion occurred between 1979 and 1991, followed by a third that lasted through the 1990s and began waning only in 2008.