Small, hungry crabs may be unsung heroes in the fight to save the Great Barrier Reef, playing a critical role in controlling populations of the destructive crown-of-thorns starfish.
New research published in Proceedings of the National Academy of Sciences of the United States of America reveals compelling evidence of crab predation on juvenile crown-of-thorns, offering fresh insights into natural solutions for reef conservation.
The research suggests that small crabs could play a big role in reef conservation — a natural defence against one of the reef’s most formidable threats.
The crown-of-thorns starfish is a well-known threat to coral reefs. With 21 toxin-tipped arms, a single adult can devour a dinner plate-sized patch of coral each day — up to 10 square meters a year. This relentless coral loss weakens the Great Barrier Reef (GBR) biodiversity and structural integrity.
The marine scientists say visual detection of newly settled juvenile crown-of-thorns starfish (CoTS) is “extremely challenging,” yet they detected CoTS DNA in 12% of cryptic predators collected on reefs more than 1,000km apart during the juvenile recruitment period on the GBR.
Field surveys were conducted at reefs in the southern (Heron Island, March 2023), central (Moore Reef, June 2023), and northern areas of the Great Barrier Reef. (Lizard Island, March 2024).
Differences in the density, distribution, and habitat associations of key rubble-dwelling predators were explored to address the hypothesis that fewer decapod predators are present in regions where primary CoTS outbreaks are thought to originate.
Predators were surveyed in transects (120 m2; 4 x 30 m) placed over the reef and rubble, with 52 in the south at 6 sites; 6 in the central GBR at 2 sites and 57 in the north at 8 sites.
To quantify predator density, all large rubble pieces (≥ 10 cm diameter) within each transect were lifted or overturned for close inspection.
“Using our newly developed eDNA techniques to detect CoTS, we found CoTS DNA in the guts of seven species of decapod (crabs) caught across more than 1000km of the Great Barrier Reef,” says Dr Sven Uthicke, senior author and Australian Institute of Marine Science researcher. “We found fewer crabs in areas prone to CoTS outbreaks,” he told Cosmos.
By combining DNA evidence from this study with predation data from previous aquarium experiments, the researchers determined that rates of juvenile starfish consumption by decapods were around 1.6 to three times lower in reefs prone to COTS outbreaks.
“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,” says Uthicke. “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 a week.”
Tackling surges in CoTS populations is more urgent than ever. Rising ocean temperatures and more frequent coral bleaching events reduce the time reefs have to recover, making them even more vulnerable to the destructive effects of CoTS outbreaks.
“Both the crabs and the small crown-of-thorns are extremely cryptic,” explains Uthicke. “We need to find out more about the crabs, where those crabs are, and what drives the changes in crabs.
“With regard to conservation, the reef is zoned into fished and non-fished areas. In non-fished areas, there may be secondary effects in that crabs are also protected on reefs.
“So, if we know that we could use it for the protection of more reef areas. We could consider trialling in small areas to breed crabs or enhance the crab populations”.
Crown-of-thorns research
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