How Sharks Help Protect Corals Without Ever Touching Them
One of the most fascinating discoveries in modern marine ecology is that sharks don’t have to eat something to control it.
A 2025 study published in Communications Biology by researchers from the Australian Institute of Marine Science and the University of Western Australia revealed a surprising connection between shark fishing and outbreaks of coral-eating Crown-of-Thorns Starfish, which are one of the most destructive threats to Indo-Pacific reefs.
The link isn’t direct. It’s behavioral.
When sharks are present, smaller predatory fish, such as snappers and emperors, adjust their behavior to avoid becoming prey themselves. They stay close to the reef floor, where sharks are less likely to attack. Scientists describe this as a “landscape of fear.” In this safer zone, smaller predators feed on seabed-dwelling invertebrates, including juvenile Crown-of-Thorns Starfish.
When sharks are removed through over-fishing, that fear disappears. Smaller predators venture higher into the water column, shifting their diet to fast-moving prey like small fish and squid. While this allows them to grow quickly, it leaves the reef floor unchecked, and starfish populations begin to surge.
Using environmental DNA analysis, researchers confirmed that far more reef fish species consume Crown-of-Thorn Starfish than previously believed, but only when their behavior keeps them near the seabed. The study also found significantly fewer starfish outbreaks in marine reserves where shark fishing is prohibited.
Sharks don’t need to hunt starfish. Their presence alone reshapes behavior across the reef, creating conditions where coral has a fighting chance. My shark gallery celebrates the these architects of fear.