Tiny crabs protect reefs by eating baby crown-of-thorns starfish

Coral reefs face threats from many sources, but the crown-of-thorns starfish is often singled out for its ability to chomp through living coral at staggering rates.

Scientists have puzzled over this starfish’s boom-and-bust population cycles, which can leave once-colorful coral communities in serious trouble.

Crown-of-thorns starfish numbers can spike quickly, damaging reefs from Australia to other parts of the Indo-Pacific.

Previous explanations have focused on shifting ocean conditions or lack of predators, yet a recent study highlights a surprising group that appears to gobble up young starfish before they mature.

Small crabs help control starfish numbers

“Using our newly developed eDNA techniques to detect COTS, we found COTS DNA in the guts of seven species of decapod caught across more than 1,000 km of the Great Barrier Reef. Plus we found fewer crabs in areas prone to COTS outbreaks,” said study senior author Dr. Sven Uthicke from the Australian Institute of Marine Science. 

Some reefs seem to carry pockets of these small decapod creatures, which are ten-legged crustaceans that often lurk in cracks and under rubble.

The researchers found that their absence in certain spots matched places where starfish outbreaks flared up.

Crabs eat young starfish

“Consuming juvenile COTS is a highly effective way to reduce COTS populations because, not only are they removed before they can start feeding on coral, but they can be removed in large numbers because they are so small,” said Dr. Uthicke.

Small starfish tend to spend time hiding in rubble and among broken coral fragments. The researchers discovered that crusty guardians live in those same hiding spots, and their diets sometimes include juvenile starfish.

“It’s a numbers game. Some of the crabs can eat 20 juveniles a day, whereas fish or triton may eat one adult every few days or per week,” said Dr. Uthicke.

Differences in how often these crabs show up across the reef may explain why some sections avoid major starfish invasions.

By using eDNA (environmental DNA) methods, scientists found proof of starfish consumption without needing direct observation. Targeting scraps of genetic material helps capture hidden feeding patterns that traditional surveys might overlook.

Wider reef implications

Scientists suspect that fewer small cryptic predators could give young starfish a better chance of surviving to adulthood. Removing juveniles early on may hold the key to keeping starfish populations in check.

A single decapod crab might appear harmless, yet large groups can collectively remove countless baby starfish. This kind of steady predation could halt population spikes before they cause coral damage.

Hidden crabs bring renewed attention to natural pest control on reefs. Managers usually track adult starfish as they munch their way across corals, but juveniles slip below the radar and can later emerge en masse.

Marine experts believe many reef inhabitants remain poorly understood. The study suggests these overlooked animals could influence overall coral health by feeding on starfish in ways that bigger predators might not.

Protecting crab populations

Decapod crabs rely on healthy, structurally complex habitats. When reefs degrade, key crab hideouts disappear, leading to fewer opportunities for them to find and eat juvenile starfish.

Scientists note that preserving coral rubble habitat might help maintain crab populations. Extra protective measures could make a difference, especially in places that have experienced intense fishing or widespread coral decline.

Future work may look at what drives crab populations up or down and whether certain types of rubble or coral attract more crabs. Researchers might also see if artificial reefs provide enough shelter for these decapods to flourish.

Insights like these can improve management. Rather than focusing solely on adult starfish, officials could encourage conditions where small crustaceans thrive and reduce problems before they start.

A sign of hidden connections

Crustaceans are not the only puzzling factor in starfish control, yet their impact sparks curiosity about other nooks and crannies of reef life. Everything from tiny worms to reef bacteria may hold clues.

Each new finding reminds us that reefs are webs of interactions with many unsung heroes. Understanding these links may be essential for long-term reef survival.

Researchers suggest that bolstering small predator populations could be a smart tactic in starfish management. Keeping these animal communities strong may help reefs bounce back faster when facing constant stress.

Local conservation groups already invest in large-scale starfish culls, but it may be easier to back up that work by maintaining conditions for crabs and other protective critters. The goal is to strike a balance that curbs harmful sea star numbers without disrupting other species.

The study is published in Proceedings of the National Academy of Sciences.

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