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Caribbean coral reefs that are being taken over by seaweed could be saved by giant herbivorous crabs.

While climate change threatens many coral reefs around the world, algal overgrowth, including seaweed, is also an issue. “In the Caribbean it’s a well-known problem,” says Mark J. Butler IV at Florida International University.

Butler says the seaweed covers up corals, shading them from light and preventing young corals from growing. The seaweed also creates harmful chemicals that repel larval reef fish and may cause disease and shut down reproduction in corals.

“The main way that people are trying to restore coral reefs is to transplant corals onto reefs but that doesn’t get rid of the seaweed problem,” says Butler.

Historically, herbivores such as sea urchins would keep the seaweed in check, but in the early 1980s almost the entire Caribbean population died suddenly – exactly why is unknown – and the seaweed took over the reefs. As an alternative, Caribbean king crabs (Maguimithrax spinosissimus) aren’t picky eaters and will finish off seaweed that other animals avoid, which could help restore the reefs.

Butler and co-author Angelo Jason Spadaro, also at Florida International University, collected hundreds of the crabs from nearby habitats. They added the crabs to 24 reefs in the Florida Keys, then monitored their effects on the corals and fish after two years.

“We were really astounded by the first year’s results where we saw on average about an 80 per cent reduction in the seaweeds on those reefs where we put the crabs,” says Butler. The pair also saw more new corals and a greater number of fish on the reefs.

While the crabs are native to the region, they are not naturally abundant on reefs as few survive to adulthood. “Coral reefs are tough neighbourhoods,” says Butler. “There are a lot of hungry mouths to feed on a coral reef and the tiny crabs are eaten by quite a few different species.” To get around this, the pair suggest the crabs could be reared in captivity until they are large enough to escape predators.

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