For those of us who love scuba diving, few sights captivate us more than the breathtaking spectacle of a coral reef ecosystem — countless coral formations and free-roaming reef fish composing a dazzling natural masterpiece. Although coral reefs cover only 0.2% of the ocean's surface area, they support 25% of all marine biodiversity, making their ecological importance self-evident. Yet right now, a species of destruction — something almost alien in nature — has arrived. Its visitation caused more than 50% of corals on Australia's Great Barrier Reef to die off in the 1960s. Its name: the Crown-of-Thorns Starfish.
The magnificent coral reef ecosystem is now under threat from a Crown-of-Thorns Starfish outbreak.
Crown of Thorns — Who Is the Crown-of-Thorns Starfish?
Before getting into the damage caused by the Crown-of-Thorns Starfish, let's first take a moment to understand exactly who — or what — it is.
The Crown-of-Thorns Starfish, commonly abbreviated as COTS, has the formal Latin name Acanthaster planci and belongs to the family Acanthasteridae. It has more than 12 arms radiating from its central disc, and the surface of those arms is covered in thorn-like spines loaded with potent venom — a formidable defense against predators.
Unlike other sea stars, COTS feed primarily on hard coral. When feeding, they evert their stomachs and secrete acidic digestive fluids to dissolve coral tissue before absorbing it, leaving behind nothing but the bleached white skeleton.
COTS are distributed across coral reefs throughout mainland Taiwan, though in relatively small numbers. However, large-scale outbreaks have been recorded in the Southern Four Islands of Penghu, the Spratly Islands in the South China Sea, and — in 2024 — at Dongsha Atoll.
The Crown-of-Thorns Starfish feeds on hard coral.
An Alien-Like Existence — COTS Larvae That Can Infiltrate Anywhere
Like many marine organisms, the Crown-of-Thorns Starfish passes through a planktonic larval stage in its youth, drifting with ocean currents to distant locations and maximising the range of its reproduction. After 18–20 days, the larvae complete settlement, attaching to the reef surface and beginning life as juveniles.
Crown-of-Thorns Starfish planktonic larvae. Source: Note 1
At this stage, young COTS are small and their venomous spines are not yet developed, making them vulnerable to a wide range of predators on the reef — including various crustaceans, carnivorous snails such as triton shells and frog shells, pufferfish, wrasse, and emperor fish. The seemingly invincible Crown-of-Thorns Starfish turns out to be remarkably fragile in its early life.
Juvenile Crown-of-Thorns Starfish face many natural predators after settlement. Source: Note 1
An Alien-Like Existence — The Crown-of-Thorns Starfish That Grabbed the Invincibility Star
It takes roughly two years for a COTS to grow from juvenile to young adult. During this time, it must survive the gauntlet of predators lurking on the reef. Once an individual's diameter surpasses about 30 cm, it becomes virtually invincible — as if it had grabbed the Super Mario power-up "Super Star."
By this stage, the COTS gradually develops tentacles armed with venom-gland cells, and nearly every creature on the seafloor gives it a wide berth. Even humans who accidentally brush against one will be left in considerable pain!
One of the adult Crown-of-Thorns Starfish's few natural predators — the Humphead Wrasse. photo credit LT DIVE
At this point, the only underwater predators capable of taking on the "invincibility star"-powered COTS are the Titan Triggerfish (Balistoides viridescens), the endearingly dopey-looking Humphead Wrasse (Cheilinus undulatus), and the seemingly docile yet formidable giant triton (Charonia tritonis).
An Alien-Like Existence — The Crown-of-Thorns Starfish's Shadow Clone Technique
The Crown-of-Thorns Starfish has an extraordinary capacity for regeneration, which makes outbreak management particularly challenging.
The Crown-of-Thorns Starfish's alien-like invasiveness stems not only from its venom, but also from its "shadow clone technique." Like other sea stars, COTS can shed their arms when disturbed, with each detached arm capable of regenerating into a new individual. This is one of the reasons populations multiply so rapidly during an outbreak.
A Crown-of-Thorns Starfish Outbreak — What Does It Have to Do With Us?
The Great Barrier Reef, located off the coast of Queensland in northeastern Australia, is the world's largest coral reef ecosystem and a UNESCO World Heritage Site. It attracts millions of visitors every year and contributes AU$6.4 billion to the Australian economy along with a vast number of related jobs. However, the COTS outbreak of the 1960s caused more than 50% of local hard corals to disappear, and rising sea temperatures in recent years have driven an overall decline in coral cover across the reef. Research data show that reduced coral coverage in the southern Great Barrier Reef has already had serious consequences for local marine biodiversity and economic development.
Reflecting on these international cases in the context of Taiwan, rising ocean temperatures are already exerting an inescapable and profound impact on coral reef ecosystems. If a Crown-of-Thorns Starfish outbreak were added on top of that, it would only accelerate the arrival of a full-scale ecological catastrophe for Taiwan's coral reefs.
In areas with coral cover of 20% to 50%, a healthy ecosystem can sustain 10 to 15 Crown-of-Thorns Starfish per hectare. Generally speaking, if you observe more than 15 COTS per hectare — roughly 2 individuals spotted during a 20-minute dive — the density has already reached outbreak level.
Mingshiou Jeng, Academia Sinica
Why Do Crown-of-Thorns Starfish Outbreaks Happen?
The academic community has yet to reach a definitive consensus on the causes of COTS outbreaks, but drawing on the biological traits described above, we can identify some likely contributing factors.
Rising Sea Temperatures
Research has shown that adult COTS release eggs and sperm when water temperatures exceed 26°C. The longer the sea remains warm, the more opportunities COTS have to reproduce and spawn. As global ocean temperatures remain elevated for longer periods, the potential for large-scale COTS reproduction increases accordingly. (Note 2)
Coastal Development and Human Sewage Pollution
During their planktonic larval stage, COTS feed on microscopic phytoplankton in the water column, so their growth is closely tied to algal concentrations. When an environment is over-developed and domestic wastewater or agricultural fertiliser runoff is discharged into the sea, the resulting eutrophication essentially provides an endless supply of nutrients for COTS larvae during their critical growth phase. (Note 3)
Overfishing and the Decline of Natural Predators
Whether it is the juvenile-stage predators of COTS — reef fish, crustaceans, and the like — or adult-stage predators such as the Humphead Wrasse and the giant triton, all are facing intense pressure from overfishing. As natural predators become increasingly scarce, COTS are left to roam the precious coral reef ecosystems that we humans depend on, with virtually nothing to stop them.
What Can We Do?
Through citizen science participation, divers can help monitor Crown-of-Thorns Starfish density during their dives.
Every living creature has its ecological role in the environment — nothing is inherently evil.
Before wrapping up, the Editor wants to emphasise once more: every species has its ecological role, and nothing is inherently evil. A moderate population of Crown-of-Thorns Starfish actually contributes to the cycling of reef ecosystems, allowing certain corals to be removed and the overall ecosystem to develop more healthily. So when we're underwater, spotting a single COTS is no reason to point the finger and condemn it outright.
Before outbreaks occur, establishing long-term ecological monitoring is critical. Beyond relying on government researchers and academics, BlueTrend will also be launching a citizen science programme on Crown-of-Thorns Starfish in the future, sharing how divers can use quantitative and qualitative methods to help monitor COTS density. We should always strive to understand our ocean — only then will we have the opportunity to help when she faces challenges. If we remain completely ignorant of the ocean's condition, we won't even notice when she is being threatened or gradually declining. Most importantly: let the data speak.
Of course, once an outbreak is underway, human intervention becomes inevitable. If the opportunity arises, BlueTrend will also share with everyone the correct methods for removing Crown-of-Thorns Starfish!
Through citizen science participation, marine biodiversity can be preserved.
https://www.youtube.com/watch?v=914Vd6e5sjQ
Further References
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Note 1: Wolfe, K., Graba-Landry, A., Dworjanyn, S. A., & Byrne, M. (2017). Superstars: assessing nutrient thresholds for enhanced larval success of Acanthaster planci, a review of the evidence. Marine Pollution Bulletin, 116(1-2), 307-314.
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Note 2: Yasuda, N. (2018). Distribution expansion and historical population outbreak patterns of crown-of-thorns starfish, Acanthaster planci sensu lato, in Japan from 1912 to 2015. Coral reef studies of Japan, 125-148.
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Note 3: Wolfe, K., Graba-Landry, A., Dworjanyn, S. A., & Byrne, M. (2017). Superstars: assessing nutrient thresholds for enhanced larval success of Acanthaster planci, a review of the evidence. Marine Pollution Bulletin, 116(1-2), 307-314.
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Rivera-Posada, J., Caballes, C. F., & Pratchett, M. S. (2013). Lethal doses of oxbile, peptones and thiosulfate-citrate-bile-sucrose agar (TCBS) for Acanthaster planci; exploring alternative population control options. Marine pollution bulletin, 75(1-2), 133-139.
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Yamamoto, T., & Otsuka, T. (2013). Experimental validation of dilute acetic acid solution injection to control crown-of-thorns starfish (Acanthaster planci).
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Rivera-Posada, J. A., Pratchett, M., Cano-Gómez, A., Arango-Gómez, J. D., & Owens, L. (2011). Injection of Acanthaster planci with thiosulfate-citrate-bile-sucrose agar (TCBS). I. Disease induction. Diseases of Aquatic Organisms, 97(2), 85-94.
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Brodie, J., Fabricius, K., De'ath, G., & Okaji, K. (2005). Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish? An appraisal of the evidence. Marine pollution bulletin, 51(1-4), 266-278.
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Brodie, J. E. (1992). Enhancement of larval and juvenile survival and recruitment in Acanthatser planci from the effects of terrestrial runoff: a review. Marine and Freshwater Research, 43(3), 539-553.
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Ludevese-Pascual, G., Napala, J. J. O., Tomol, R. P., Oyong, L. J. L., Alfante, J. M. P., & Lumain, J. P. L. M. (2022). Crown-of-Thorns starfish (COTS) outbreak in Sogod Bay, Southern Leyte, Philippines. Governance, 4(1), 24-30.
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Cover photo: Photo by David Clode on Unsplash
Further Reading
