It’s hard to make a starfish frightening, but Acanthaster planci manages it. The crown-of-thorns is the size of a dinner plate, covered in venomous spines up to 5 centimetres long, and it eats coral. Not nibbles – eats. A single adult can consume up to 10 square metres of living coral per year. During an outbreak, when populations reach hundreds of thousands of animals across a reef system, the damage is visible from space.
I’ve seen them at Osprey Reef, at Flynn Reef, at sites along the outer barrier where the current brings the nutrients they need. Up close, they’re genuinely impressive – alien-looking, deliberate, moving with a slow purpose that makes their destructive capacity feel almost intentional. It isn’t, of course. They’re just eating. The problem is the scale.
What Crown-of-Thorns Actually Do
Crown-of-thorns starfish are corallivores – they feed exclusively on coral. Their feeding method is visceral: they evert their stomach out through their mouth, spread it over the coral surface, and digest the living tissue externally, leaving behind the white calcium carbonate skeleton. A feeding scar is immediately recognisable – a white patch with a starfish-shaped outline, the skeleton clean and bare.
They prefer fast-growing Acropora corals, which are also the corals most important for reef structure and recovery. A crown-of-thorns outbreak on a reef that’s already been bleached is a compounding disaster – the corals most capable of rapid recovery are the ones being eaten first.
At natural population densities – roughly one adult per hectare – crown-of-thorns are part of the reef ecosystem. They’ve been present on Indo-Pacific reefs for millions of years, and at low densities they may actually increase coral diversity by preferentially eating the fastest-growing species and giving slower-growing corals space to establish.
What Causes Outbreaks?
This is where it gets complicated, and where the science is still genuinely contested.
The leading hypothesis – supported by the most substantial body of evidence – is the nutrient enrichment model. Crown-of-thorns larvae are planktonic and feed on phytoplankton. In normal conditions, phytoplankton concentrations are low enough that larval survival rates are minimal. When nutrient runoff from agricultural land – primarily nitrogen and phosphorus from fertilisers – reaches the reef, phytoplankton blooms, larval survival rates spike, and the conditions for an outbreak are set.
The timing supports this. Major outbreaks on the GBR have consistently followed periods of high rainfall and runoff from Queensland’s agricultural regions. The 2010-2012 outbreak, one of the most severe on record, followed the exceptional La Nina rainfall of 2010-2011.
A secondary hypothesis involves the removal of natural predators – particularly the giant triton snail, Charonia tritonis, which was heavily collected for the shell trade in the mid-20th century. Tritons eat crown-of-thorns, and their depletion may have removed a population check. The evidence here is less conclusive, but triton conservation is now taken seriously as a precautionary measure.
The GBR’s Outbreak History
The first recorded outbreak on the GBR was in the 1960s, at Green Island near Cairns. Whether this was genuinely the first outbreak or simply the first one noticed is debated – the reef wasn’t being systematically surveyed before then. Subsequent major outbreaks occurred in the 1970s-80s, 1993-2005, and 2010-present, with the current outbreak considered the fourth and most geographically extensive.
AIMS estimates that crown-of-thorns outbreaks have been responsible for approximately 42% of the coral cover decline on the GBR since the 1980s – more than bleaching, more than cyclones. This figure is contested by some researchers, but the scale of the impact is not.
What’s Being Done
The primary management response is manual culling – divers injecting individual starfish with ox bile solution or dry powder, both of which kill the animal within 24 hours. It’s effective at the site level but logistically challenging at scale. The GBRMPA runs a dedicated control program, and citizen science programs allow recreational divers to report and cull starfish on sites they visit.
More recently, researchers have developed an autonomous underwater vehicle – the COTSbot – capable of detecting and injecting crown-of-thorns without human intervention. Trials have been promising, and the technology is being refined for deployment at scale.
I find crown-of-thorns genuinely fascinating, which I realise is an unusual position. They’re a native species doing what native species do. The outbreak problem is a human-created problem – we changed the nutrient balance of the water, and the starfish responded to the conditions we created. Blaming the starfish is like blaming the symptom. The reef’s crown-of-thorns problem is, at its root, a water quality problem. And that’s a problem we actually know how to fix.
