The Great Barrier Reef is home to a fascinating predator-prey relationship that may hold the key to controlling destructive outbreaks of crown-of-thorns starfish (CoTS). A recent study published in the Proceedings of the National Academy of Sciences reveals that small, hidden decapod crabs, often overlooked, are critical in regulating CoTS populations. This discovery challenges conventional beliefs that only larger reef predators like fish and triton shells suppress CoTS outbreaks.
The Crown-of-Thorns Starfish Problem
The crown-of-thorns starfish (Acanthaster spp.) is one of the most damaging coral predators in the Indo-Pacific. These corallivores experience boom-and-bust population cycles, leading to massive coral loss during outbreaks. While previous research focused on higher-level predators, new findings suggest that cryptic predators—small crabs hiding in coral rubble—may be playing a vital role in preventing these outbreaks before they start.
eDNA Sheds Light on a Hidden Battle
Scientists employed environmental DNA (eDNA) analysis to uncover this overlooked predator-prey interaction. By analyzing the gut contents of decapod crabs across a 1,000 km stretch of the Great Barrier Reef, researchers found CoTS DNA in seven different decapod species. The presence of CoTS DNA indicates that these crabs are preying on juvenile CoTS before they mature into coral-consuming adults.
Further analysis showed that reefs prone to CoTS outbreaks had significantly lower densities of these crabs, suggesting a correlation between crab abundance and starfish population control. This new understanding of lower-trophic interactions could be pivotal in managing CoTS outbreaks and preserving reef health.
Why Juvenile Predation Matters
Crown-of-thorns starfish are most vulnerable during their juvenile stages when they reside in coral rubble nurseries. The study found that decapods could consume 1.6 to 3 times more juvenile CoTS in areas where outbreaks are less frequent. Some of these crabs are capable of consuming up to 20 juvenile CoTS per day, compared to reef fish and triton shells, which typically consume one adult CoTS every few days or weeks.
This suggests that focusing conservation efforts on maintaining or enhancing cryptic predator populations could be an effective strategy for natural CoTS population control.
The Role of Habitat Complexity
The study also highlighted the importance of habitat complexity in determining the density of predatory decapods. Coral rubble, which serves as both shelter and a hunting ground for these crabs, varies significantly between reefs. In areas where rubble is more abundant and structurally complex, crab populations thrive, leading to higher predation rates on juvenile CoTS.
Conversely, reefs with a history of severe CoTS outbreaks tend to have reduced rubble habitats, potentially leading to lower predator densities and a lack of natural population control for the starfish.
Implications for Reef Conservation and CoTS Management
The discovery of decapods as key regulators of CoTS populations has significant implications for reef conservation. Current CoTS management strategies focus primarily on manual removal of adult starfish and the protection of their known predators like triton shells. However, this study suggests that enhancing the populations of cryptic decapods could provide a more sustainable, natural solution.
Potential management strategies include:
- Protecting and restoring rubble habitats to support decapod populations.
- Investigating the feasibility of reintroducing or increasing decapod populations in outbreak-prone areas.
- Expanding eDNA monitoring to track cryptic predator populations and assess their impact on CoTS control.
A Shift in Coral Reef Management Approaches
For decades, the “predator removal hypothesis” has dominated discussions about CoTS outbreaks, suggesting that overfishing of large predators has led to uncontrolled starfish populations. However, this new study introduces the “variable predator hypothesis,” which emphasizes the role of both high- and lower-trophic predators in regulating CoTS populations.
This paradigm shift underscores the need for a holistic approach to reef conservation that accounts for all levels of the food web. By protecting the entire ecosystem—including the smallest, most hidden predators—scientists and conservationists can improve the resilience of coral reefs against future CoTS outbreaks.
Conclusion: Small Crabs, Big Impact
The discovery that small, cryptic decapods play a crucial role in controlling crown-of-thorns starfish populations is a game-changer for coral reef conservation. As scientists continue to refine eDNA techniques and study predator-prey dynamics at different trophic levels, this new understanding could revolutionize how we manage CoTS outbreaks and protect fragile marine ecosystems.
By focusing not only on large, charismatic predators but also on the hidden warriors of the reef, conservationists can develop more effective, sustainable strategies to maintain the delicate balance of marine life. Protecting these tiny yet powerful predators might just be the key to safeguarding the future of the world’s coral reefs.
More information: Kennedy Wolfe et al, eDNA confirms lower trophic interactions help to modulate population outbreaks of the notorious crown-of-thorns sea star, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2424560122