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Researchers find a missing piece in coral reef restoration: crabs

They transplanted native crabs to degraded coral reefs patches to help clear out the seaweed. It worked better than they imagined.
February 17, 2021

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Coral biologists already have their work cut out for them trying to protect reefs from ocean acidification and temperature rise. But unchecked seaweed growth is also harming corals by taking up prized habitat and preventing new coral growth. Now, researchers in the Florida Keys have found a potential solution in the form of a hungry helper: the Caribbean king crab.

As the largest crab in the area, it isn’t surprising that the Caribbean king can eat seaweed at higher rates than other grazers. But low densities of the crustaceans mean they don’t put much of a dent in seaweed overgrowth. The authors of a recent study in Current Biology predicted that populating coral reef habitat with historical numbers of the native crab might help clear up the “seaweed dilemma.” 

To test their hypothesis, the researchers introduced 84 tagged crabs to small, isolated coral patch reefs with about 85 percent seaweed coverage. A year later, they measured how the crabs changed that coverage compared to undisturbed reefs. And to understand how trimming away seaweed affected the ecosystem, they also counted the number of juvenile corals and coral reef fishes two years after crabs were introduced.

The decline in seaweed was so striking that the team repeated the year-long experiment to verify their results. In both cases, seaweed was reduced to less than 30 percent coverage on reefs where researchers introduced crabs. And although human-scrubbed reefs had even less seaweed initially—around 10 percent coverage—it eventually rebounded. A combination of scrubbing and crab grazing reduced seaweed most effectively, which maintained less than 20 percent coverage over the course of the experiment. These decreases in seaweed coverage had a prominent effect on the reef ecosystem. In patch reefs where crabs were released the number of juvenile corals increased by four times and the amount of coral reef fishes increased three to five times compared to control reefs.

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The authors explain that large-scale crab stocking plans may drastically improve coral reef restoration but raising the crustaceans to effective sizes will require time and space. And the problem of coral decline will require much more than increasing seaweed grazers. Still, the study results highlight how the Caribbean king crab could be one piece of the puzzle toward protecting an essential marine species.

Source: Spadaro, A.J. and Butler, M.J. Herbivorous crabs reverse the seaweed dilemma on coral reefs. Current Biology. 2020.

Image: Angelo Spadaro

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