Controlling repeated outbreaks of the coral-eating Crown-of-Thorns Starfish (COTS), Acanthaster planci, is one of the greatest challenges facing resource managers of the Great Barrier Reef (GBR).
COTS, together with cyclones, are responsible for more than half of all coral cover loss on the GBR. As one of several lines of investigation into COTS population management, our researchers are taking a novel approach to the issue by examining the intriguing potential of a snail, the Pacific triton, a natural enemy of COTS.
The Pacific triton (Charonia tritonis, also known as the “giant triton”) is a large marine snail that inhabits coral reefs throughout the Indo-Pacific region. They are rare; historical evidence of harvesting levels is scant, however scientists speculate that overharvesting of the snails for the meat and shell has led to threatened status throughout their range. As such, tritons have been protected on the GBR for decades.
Their diet consists primarily of starfish, but will also prey on other echinoderms such as sea cucumbers. Like other predatory marine snails, tritons have a very well developed sense of smell and can hunt their starfish prey by scent alone. Although tritons will prey on several species of starfish, they are particularly fond of COTS. Tritons are one of the few predators of adult COTS as they are seemingly unaffected by their ‘crown-of-thorns’ - hundreds of sharp spines and toxic saponin coating which otherwise acts as a very powerful deterrent against potential predators.
However, while tritons prefer the often-abundant COTS, they only eat a few per week. With COTS populations on the GBR estimated to be in the many millions, the combination of this low predation rate and the rarity of triton individuals means that they have limited impact on reducing COTS numbers through predation alone.
Despite the COTS sharp spines and having a highly toxic coating (Saponin), tritons are highly effective COTS hunters. However, their low consumption rate is not enough to control COTS numbers.
Sniffing out smart biological control technologies
While we cannot look to this marine snail’s appetite to control outbreaks, scientific findings are emerging that suggest we may be able to harness other triton qualities to influence the starfish populations.
Dr Mike Hall and his team at AIMS, assisted with funding from the federal government through its ‘Caring for Country’ initiative, are investigating the triton versus COTS, predator-prey relationship to disrupt the life cycle of COTS. One focus of the research is on the use of the scent of a triton to alarm and disperse COTS. This line of thinking is based on the team’s observations that when COTS ‘smell’ the scent of a triton they ‘flee’ (as fast as a starfish can), and stampede in an agitated, fearful manner.
“The chemical cues from the triton snail are highly influential when it comes to COTS behavior,” explains Dr Hall. “We only need to introduce water that a triton has been sitting in to disperse a group of the starfish – they don’t even need to see it.”
It is this behavior, sparked by the snail’s chemical cue, that could potentially assist in managing COTS populations on reefs.
Deter, divide and conquer
One avenue by which COTS outbreaks could possibly be diminished is by using their scent as a deterrent. This would operate in the same way a mosquito coil is used to keep mosquitos at bay- the scent of the triton on a reef could alarm COTS to such a level that they avoid these areas. Dr Hall has identified two approaches to this method:
“Reefs could be seeded with tritons – not to eat them, but to repel them. Alternatively, with time and effort, we could identify, isolate, harness and deploy the chemicals responsible for the COTS behavior, without using the snail itself.”
Secondly, tritons may be used to reduce fertilisation success in COTS through their ability to disperse large spawning aggregations. COTS have an incredible capacity to reproduce. One female can release around 150 million eggs in a single spawning event.
“When males and females gather in large aggregations, essentially clumping together almost on top of one another, fertilisation success is incredibly high. If these chemicals can keep the COTS moving around and prevent them from forming these successful breeding aggregations, we could potentially disrupt this outbreak cycle.”
The normally sedentary COTS will aggregate during breeding season, drastically increasing their fertilisation success. Chemical cues from the Pacific triton could disperse these breeding aggregations, reducing fertilisation success.
The team is working at both ends of this chemosensory relationship. As well as work on identifying and isolating the chemical cues emitted from the triton, they are focusing on chemical sensory pathways in COTS. This COTS aspect is part of a broad international collaboration, developing a COTS reference genome, which, among other valuable information, will help find other COTS vulnerabilities that could be exploited.
A key challenge to advancing this research is the rarity of triton subjects. As such, very little is known about their ecology, physiology or reproductive habits, and only a few individuals are available for observation. If triton numbers could be sufficiently bolstered and secured through breeding, this line of research could progress more rapidly and perhaps provide a valuable stock for future reef protection.
AIMS is a leader in COTS research
This innovative line of research postulates an exciting new approach for a decades-old problem, and is still in its infancy. Driven by strong external collaborations, AIMS leads the way in not only advancing knowledge of COTS control measures, but is also pioneering research into the causes and drivers of outbreaks, and management and investment strategies.
To learn more about COTS, please see:
Australian Institute of Marine Science
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