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Science

Predator versus predator: Giant sea snails take on reef-eating starfish

Scientists in Australia look to rare mollusc to protect Great Barrier Reef

A giant triton sea snail devours a crown of thorns starfish on the Great Barrier Reef. (Courtesy of Australian Institute of Marine Science)

SYDNEY -- In the warm tropical waters of northeast Australia, a potential battle is looming between two exotic predators of the sea. Its outcome could well determine the environmental health of one of the world's greatest natural wonders -- the Great Barrier Reef.

On one side is a giant sea snail that was hunted almost to the point of extinction 50 years ago because its beautiful swirled shell was so admired by collectors. On the other side, the sea snail's prey is the fast-multiplying crown of thorns starfish, a spiky venomous predator whose voracious appetite for live coral has laid waste to vast swathes of the magnificent coral formations that grow along the Great Barrier Reef's 2,300km length.

Mass outbreaks of the starfish in recent years have lifted their numbers into the many millions and had a devastating impact on the reef, accounting for more than 40% of the coral cover lost in the last 30 years. In terms of destructive potential, that ranks the starfish second only to the tropical cyclones that sweep through the area in the southern hemisphere's cyclone season. Other coral losses come from bleaching caused by sea-warming, coastal runoff and pollution.

The health of the Great Barrier Reef's coral is of fundamental importance to the tourism industry in Queensland state, but it is of international and national significance too, with a social, economic and cultural worth to Australia of about $56 billion, according to a report released in June this year by Deloitte Access Economics for the Great Barrier Reef Foundation.

Crown of thorns starfish congregate on coral formations along the Great Barrier Reef. (Courtesy of AIMS)

The World Heritage-listed reef is home to 1,400 species of fish and 400 types of coral. To help preserve that precious asset, marine scientists are looking to the giant triton sea snail (Charonia tritonis), which reaches up to half a meter in length and may just be the embattled reef's long-term environmental savior.

It is virtually the only natural predator of the crown of thorns starfish, but its own numbers are so low and its appetite so small -- it consumes just one starfish a week -- that it has been unable to play any real role in reducing starfish numbers.

Now scientists at the Australian government's tropical marine research agency, the North Queensland-based Australian Institute of Marine Science, are working on a research and breeding program that could see more of the sea snails bred for release into vulnerable areas of the reef.

Scent of danger

They have found that the mere presence of a giant triton sea snail is enough to trigger a mass exodus by starfish, potentially limiting their breeding and preventing "outbreak" numbers. Dr. Mike Hall, principal research scientist at the Australian Institute of Marine Science in Townsville, noted recently that the crown of thorns starfish had a "particularly well-developed sense of smell, and there is nothing more alarming to it than the scent of a giant triton. Similarly, he said, once a starfish is detected by a giant triton, it will be "purposely hunted down and devoured."

The new project underway at the institute builds on its previous research work, which found that the crown of thorns starfish actively seek to avoid an area where triton sea snails are present. The research showed that exposing the starfish to the scent of the sea snail produced what it called a "consistent alarm response," with the starfish attempting to flee from the perceived threat.

According to the institute, that means potentially the triton sea snails could be deployed to prevent starfish from closely aggregating during their spawning season, thereby helping to limit them reaching outbreak numbers.

On top of this, the sea snail's chemicals potentially could be synthesized to create alarm-inducing "baits" to agitate starfish, it said.

The giant triton sea snail -- named after the Greek god Triton, son of the god of the sea Poseidon -- is native to coral reefs in both the Pacific and Indian Oceans, but until the 1960s was hunted by shell collectors to the point where it is now extremely rare. It is a protected species in Queensland waters.

From giant tritons held at the institute's laboratories in Townsville, scientists have managed to hatch more than 100,000 swimming larvae in the last month. The institute said on Sept. 19 that while the breeding project opened up "exciting" possibilities in using the sea snail as a management tool to combat the starfish, there was still much work to be done. It said the snails' dietary requirements and the cues that triggered their metamorphosis into adult snails were not fully understood.

Prolific adversary

The snails face a formidable adversary in the crown of thorns starfish, which have an incredible capacity to reproduce. One female can release around 150 million eggs in a single spawning event.

As many as 150,000 crown of thorns starfish can congregate in 1 sq. km of the reef during an outbreak. An adult starfish can eat as much as 10 sq. meters of live coral a year.

The institute said control measures against the starfish had changed little in the last 50 years, with the primary method being manual lethal injection.

The triton sea snail breeding project, led by marine chemical ecologist Dr. Cherie Motti at the AIMS research station in Townsville, began in mid-June this year and will run until June 2019.

The Australian Institute of Marine Science at Townsville has begun a giant sea snail breeding project, under the leadership of Dr. Cherie Motti. (Courtesy of AIMS)

But the AIMS scientists are mindful that biological pest control programs have gone awry in the past -- most catastrophically with the South American cane toad, introduced from Hawaii in the mid-1930s to counteract a beetle that was then attacking sugarcane fields in north Queensland.

The toads, which carry a toxin that is lethal to many Australian small animals, lizards and birds, have had a devastating impact on native wildlife, and have thrived in tropical waterways to the point where they are now a national environmental hazard. The toads have spread as far south as New South Wales and across the top of tropical Australia into Western Australia.

The research manager at the institute, Dr. David Souter, said that to avoid any repeat of the cane toad disaster, any eventual release of sea snails onto the Great Barrier Reef would only be done under tight controls. He said little was yet known about the life cycle of the sea snails, and how long they took to grow to maturity.

The last thing the reef needs is more environmental stress, alongside the threats posed by the crown of thorns starfish, storm damage from tropical cyclones, coastal runoff, and bleaching events caused by rising sea temperatures.

Earlier this year, the reef's protector, the Great Barrier Reef Marine Park Authority, estimated that almost 30% of shallow water corals died last year from bleaching. This happens when the reef's waters become too warm, prompting the corals to expel the tiny marine algae that live inside their tissue in a symbiotic relationship that supplies most of the corals' energy.

According to the Deloitte Access Economics report, the reef supports 64,000 jobs and is a critical factor supporting economic activity across Australia, particularly in the tourism sector.

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