Caribbean king crabs can help restore coral reefs

Rising temperatures, pollution, overfishing, and disease are all major threats to the survival of the world’s coral reefs, which have been declining at an unprecedented rate over the last few decades. In a new study published by Cell Press, researchers describe how Caribbean king crabs can provide the conditions needed for coral reefs to recover.

When the health of a coral reef is compromised, seaweed starts to move in and take over. Without an abundance of herbivorous fish available to limit the growth of the seaweed, it smothers the remaining corals and the reef collapses.

Mark Butler of Florida International University has been studying coral reefs and other habitats in the Florida Keys for more than 30 years. In collaboration with study first author Angelo “Jason” Spadaro and other colleagues, Butler recognized the potential for the Caribbean king crab to aid in the restoration of coral reefs. 

The Caribbean king crab not only eats seaweed at an extraordinarily rapid pace, but also eats types of seaweed that other species avoid. However, these crabs are not naturally present in large enough numbers to keep the seaweed under control. The researchers set out to investigate whether the presence of more of these crabs could restore the ecological balance of coral reefs.

“Experimentally increasing the abundance of large native, herbivorous crabs on coral reefs in the Florida Keys led to rapid declines in seaweed cover and, over the course of a year or so, resulted in the return of small corals and fishes to those reefs,” said Butler. “This opens up a whole new avenue for coral reef restoration.”

The research was focused on 12 isolated patches of coral reef, which were split into three groups: unmanipulated control reef, reefs stocked with crabs, and reefs where seaweed and algae were removed before crabs were introduced.

Initially, 85 percent of the reef was covered with seaweed. The addition of crabs helped to dramatically reduce the seaweed, dropping it to less than 50 percent coverage in the first year.

“When Jason showed me the results he had compiled from our first year of experiments, I couldn’t believe it – they looked too good,” said Butler. 

“You should have seen the look on Jason’s face when I said, ‘Nobody will believe these results. We have to repeat the experiment for another year at another location.’ Not exactly what a grad student wants to hear when trying to finish up! But we did it and the results were the same.”

The second experiment produced similar results – the crabs alone reduced seaweed cover by about 50 percent. By scrubbing the reef first, seaweed declined by about 70 percent.

The findings show that herbivorous crabs can be used as an effective tool for coral reef restoration. According to the researchers, the crabs essentially improve the habitat conditions for corals and fishes.

“One wonders what the tropics would be like without the mind-boggling complexity and beauty of shallow coral reefs,” Butler says. “For example, the third longest barrier coral reef in the world that fringes the Florida Keys now has less than two percent coral cover.”

“Conquering the challenge of climate change coupled with local reef restoration, like development of stocking programs for herbivorous crabs, are immediately necessary to reverse this decline. Our findings mean little if they don’t result in tangible new restoration efforts.”

Butler said that coral nurseries have already been established to help restore the reef. To put their research into action, the team is trying to find the resources needed to set up nurseries to raise large numbers of crabs. 

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer