Crown-of-thorns starfish

There is emerging evidence that poor water quality resulting from floods and extreme weather events in the summers of 2009 to 2011 have created conditions for crown-of-thorns starfish numbers to increase at some locations on the Great Barrier Reef.

To support the resilience and diversity of the Reef, short and long-term strategies that target the immediate problem and the underlying causes are underway.

What is the crown-of-thorns starfish?

The crown-of-thorns starfish (Acanthaster planci) is a coral eating starfish or sea star native to coral reefs in the Indo-Pacific region. They are named after the dense spines radiating from their arms and they belong to the same group as all starfish, sea urchins, sea cucumbers and brittlestars.

Crown-of-thorns starfish are an important functional species on healthy coral reefs. They feed on the fastest growing corals such as staghorns and plate corals, allowing slow growing coral species to form colonies, therefore increasing coral diversity.

What is the problem?

When crown-of-thorn starfish numbers are low, corals can recover from any damage caused by their feeding - maintaining a balanced and healthy environment. At these times, control is not required, as this is a naturally occurring event.

However, when the crown-of-thorns starfish population increases to a point where the coral can no longer recover from the damage caused, there is significant danger to the health of the reef. This is called a crown-of-thorns starfish outbreak.

While population booms of these animals are probably a naturally occurring event, their frequency has increased from once in 80 years or so, to approximately once every 15 years. The decline in water quality from increased nutrients entering waterways and then the Reef is one likely factor leading to the increased frequency of outbreaks.

How do outbreaks occur?

When a dramatic increase in nutrient levels in the water coincides with these animals spawning season (November-January), the larval crown-of-thorns starfish are able to develop, grow and survive at much higher than normal rates.

The increased use of nutrients, pesticides and other pollutants on the land results in more of these entering waterways and the Great Barrier Reef. Current scientific evidence indicates that elevated nutrient levels are linked to outbreaks of crown-of-thorns starfish.

Previous outbreaks have followed large, drought-breaking floods, which release large amounts of nutrients into the Reef ecosystem. The elevated nutrient levels cause an increase in phytoplankton, which is the main food source for crown-of-thorns starfish larvae. This enables a much higher than usual number of larvae to survive and develop into juvenile crown-of-thorns starfish at which stage they start feeding on coral.