The Star That Devours Reefs

In Mayotte, the quiet battle against crown-of-thorns starfish has begun

In late April, agents from the Parc naturel marin de Mayotte carried out a series of interventions across several lagoon sites to limit the spread of one of the Indo-Pacific’s most feared coral predators: the Acanthaster planci.
Around Choizil, reefs already weakened by the 2024 coral bleaching event and the passage of Cyclone Chido showed fresh feeding scars and unusually high concentrations of adult individuals. Another layer of pressure on coral ecosystems already struggling to recover.
During the first intervention on April 29, park agents treated 86 crown-of-thorns starfish. A second mission on May 5 confirmed the effectiveness of the protocol, with numerous dead individuals found on site. Additional treatments were also conducted around Choizil, the northern white sand islet and Plage du Préfet.

Behind these discreet underwater operations lies one of the most complex ecological phenomena affecting coral reefs across the Indo-Pacific.

A predator hidden beneath the reef

The crown-of-thorns starfish is not an invasive species in Mayotte. It naturally occurs throughout the Indian Ocean and has always been part of reef ecosystems.Yet biologically, it is unlike almost any other starfish.

With more than twenty arms and a diameter sometimes exceeding 70 centimeters, adult individuals are covered in long venomous spines capable of inflicting extremely painful wounds. Beneath its almost prehistoric appearance hides a highly efficient coral predator.

Its feeding strategy is both fascinating and unsettling. The animal extrudes its stomach directly onto living coral colonies, digesting the tissue externally before absorbing it. What remains behind is a stark white coral skeleton — a ghostly scar across the reef.

At low densities, this predation is part of the natural balance of coral ecosystems. By feeding on dominant coral species, crown-of-thorns starfish can even contribute to reef diversity and renewal.

The problem begins when populations suddenly explode.

A biological machine built for proliferation
The crown-of-thorns starfish possesses an extraordinary reproductive capacity.

A single female can release tens of millions of eggs during a spawning season. The larvae then drift within the plankton before eventually settling onto reefs.

Under normal ecological conditions, the vast majority of these larvae never survive to adulthood. Many are consumed by fish and invertebrates or dispersed by currents.

But when environmental conditions shift, survival rates can rise dramatically.

Scientists believe nutrient-rich waters may play a major role. Runoff, erosion, organic pollution and increased nutrient input can stimulate phytoplankton growth — the primary food source for crown-of-thorns larvae.

More plankton can mean more larval survival.

The result can be devastating: within only a few years, entire reef systems may become overwhelmed by thousands of coral-eating starfish.

Why don’t predators stop them?

It is one of the great paradoxes of the crown-of-thorns outbreak phenomenon.How can such a destructive species proliferate within ecosystems filled with predators?

Part of the answer lies in the animal’s own defenses.

Juveniles remain vulnerable, but adults quickly become a living fortress. Their long venomous spines deter most predators, making mature individuals extremely difficult — and dangerous — to consume.

One of the few known natural predators is the Charonia tritonis, a large carnivorous sea snail capable of feeding on starfish despite their defenses.
Yet giant tritons have themselves become increasingly rare across many Indo-Pacific reefs, partly because of shell collecting.
Several fish species may also prey on juveniles or small individuals, including the Balistoides viridescens, the Pseudobalistes flavimarginatus and possibly the iconic Cheilinus undulatus.
But even healthy predator populations often struggle to control outbreaks once numbers begin to rise.
When predator pressure decreases while environmental conditions favor larval survival, crown-of-thorns populations can rapidly spiral out of control.

Reefs already under pressure

For years, coral reefs across the Indian Ocean have been increasingly affected by marine heatwaves and mass bleaching events linked to rising ocean temperatures.

In 2024, large sections of Mayotte’s lagoon suffered severe bleaching. Then came Cyclone Chido, adding physical destruction to already weakened reef systems.

Within this context, crown-of-thorns outbreaks become more than a simple ecological imbalance. They represent an additional stress imposed on ecosystems already pushed close to their limits.

Park agents observed recently consumed coral colonies on several sites, confirming that active predation is ongoing.

The goal of the interventions is therefore not to eradicate the species, but to locally reduce outbreak intensity and give damaged reefs a chance to recover.

A delicate underwater operation

To control the outbreaks, divers inject vinegar directly into the starfish. The method is relatively inexpensive, easy to deploy and considered to have limited environmental impact compared to other chemical protocols. Death usually occurs within a few hours.Yet the operation remains delicate.

​Divers must handle highly venomous animals while working above fragile coral structures, often in difficult underwater conditions. Monitoring also requires repeated visits to assess mortality and track the evolution of local populations.
These interventions are less about eliminating a species than attempting to buy time for the reef itself.

Predator of coral… or symptom of a reef under stress?
The crown-of-thorns starfish is often portrayed as the villain of coral reefs. But reality is more complicated.
This species did not suddenly appear in Mayotte. It has always been part of the lagoon’s ecosystem. What is changing today are the ecological balances surrounding it.
Reefs weakened by marine heatwaves. Cyclones growing more intense. Declining water quality. Fewer natural predators. Ecosystems struggling to recover after repeated disturbances.
In that sense, the crown-of-thorns starfish may be less the root cause of reef decline than the visible symptom of reefs already under severe pressure.
Beneath the surface, marine park agents are trying to contain the immediate damage. But behind every starfish injected with vinegar lies a far larger question: how long can coral reefs continue resisting the combined pressures of climate change and human impact?