Australia is home to some of the world's most beautiful reefs.
This includes the lush Great Southern Reef , which wraps around Australia's southern coastline, and the world-renowned Great Barrier Reef .
But the corals of the Great Barrier Reef and the kelp forests of the south are both plagued by prickly problems - voracious starfish and sea urchins.
The coral-eating crown-of-thorns starfish is one of the greatest threats to the Great Barrier Reef. And on the temperate Great Southern Reef, long-spined sea urchins have eaten their way through thousands of hectares of kelp forests. Both species are native but can boom in numbers, and the urchins have spread to new areas due to climate change.
Scientists tend to study these two species as isolated threats. But our new study shows that while they pose high but differing risks, investing in control programs and innovative research could help curb these two prickly problems.
Overabundant species
Crown-of-thorns starfish and long-spined sea urchins threaten reefs by building up large populations that overgraze coral or kelp.
Crown-of-thorns starfish are native to the Great Barrier Reef. Their population growth follows a boom-and-bust cycle. Since data were first available in the 1950s, the number of crown-of-thorns starfish has surged by more than 1,500 per square kilometre roughly every 15 years . But once they eat all the available coral - the foundation of their diet - the population crashes . When the starfish eat too much coral, this represents another threat to reefs already stressed by climate change .
Long-spined sea urchins are native to the seas off mainland Australia. Tasmanian waters were once too cold. But as the oceans have warmed due to climate change, the sea urchin has been able to spread south .
Their spread into Tasmania in recent decades, in their millions, has caused extensive damage to kelp forests.
Our neighbours in New Zealand are also battling an influx of these urchins.
The long-spined sea urchins can build up to densities hundreds of times higher than peak crown-of-thorns starfish densities. And further, because of their flexible diet of various kinds of microalgae, drifting seaweed, and invertebrates , the urchins can survive in a semi-starved state. This results in the barren reefs they create persisting for decades.
Tricky to manage
From a conservation perspective, the way to tackle long-spined sea urchins currently differs from tackling crown-of-thorns starfish for three main reasons:
1. Fluid borders
Crown-of-thorns starfish are monitored and managed by federal institutions on the Great Barrier Reef, including monitoring by the Australian Institute of Marine Science and management by the Great Barrier Reef Marine Park Authority .
In contrast, long-spined urchins are spread across multiple jurisdictions, stretching from northern New South Wales to southern Tasmania. Authorities in each state manage urchins in different ways and to varying degrees, as populations change from native to invasive and overabundant. But these urchins are a national problem, given that urchin larvae travel south for hundreds of kilometres on the prevailing ocean currents, with no regard for state borders.
2. Commercial interests
Crown-of-thorns starfish have no economic value . But urchins can be harvested for their roe, considered a delicacy in countries such as Japan. While demand for this high-value seafood product continues to grow , it is not yet at the scale required to combat the problem. Developing the urchin industry further would see more urchins removed from the water, enhancing reef protection.
3. Inconsistent funding
Sustained, large-scale investment in management is necessary to tackle these prickly problems. The roughly $20 million-per-year federally funded crown-of-thorns starfish control program is locked in years in advance. But funding for urchin control in Tasmania is about 40 times lower .
Researchers, managers and politicians alike have been calling for federal investment into the urchin fishery, but to date no substantial funding has been delivered. Investment into developing a profitable and larger sea urchin industry would deliver a long-term, low-cost solution to the urchin problem.
Natural defences
To protect our precious tropical and temperate reefs, we must take action.
One strategy is to protect their natural predators.
A 2026 study showed the spangled emperor is the key predator of crown-of-thorns starfish on the Great Barrier Reef. This fish species is found throughout the Indo-Pacific region but is sought after by fishers .
On the Great Barrier Reef there are six times more spangled emperor in areas protected from fishing. This suggests protecting spangled emperor populations can help control crown-of-thorns starfish.
Similarly, the highly valuable southern rock lobster is a predator of long-spined sea urchins in Tasmania. Although these lobsters are widespread, they are highly sought after by fishers . When given a break from intensive fishing, lobster populations can build up, helping to limit urchin populations .
Rebuilding predator populations can form part of a whole-of-ecosystem reef management approach, where priority is given to the overall health and balance of ecosystems and communities .
Plotting a positive future
As our climate changes, Australia's reefs face an uncertain future. Outbreaks of coral-eating starfish and kelp-grazing sea urchins can rapidly transform healthy reefs into degraded ecosystems.
Yet this future is not inevitable. By investing in collaborative research we can find ways to restore and protect these iconic environments.
John Keane receives funding from the state government of Tasmania and the Fisheries Research Development Corporation.
Scott Ling receives funding from the Australian Research Council and has received funding from the Australian government's Saving Native Species program.
Sterling B. Tebbett does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
