Researchers find a missing piece in coral reef restoration: crabs

Coral biologists already have their work cut out for them trying to protect reefs from ocean acidification and rising temperatures. But uncontrolled algae growth also harms corals by taking up valuable habitat and preventing the growth of new corals. 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 region, it’s no surprise that the Caribbean king can eat algae at higher rates than other grazers. But the low densities of crustaceans mean that they don’t interfere much with algal blooms. The authors of a recent study in Current biology predicted that populating coral reef habitat with historic numbers of native crabs could help solve the “algae dilemma”.

To test their hypothesis, the researchers introduced 84 tagged crabs to small, isolated coral reefs with around 85% algae cover. A year later, they measured how the crabs changed this cover compared to undisturbed reefs. And to understand how the size of the algae affected the ecosystem, they also counted the number of juvenile corals and coral reef fish two years after the crabs were introduced.

The algae’s decline was so striking that the team repeated the year-long experiment to verify their results. In both cases, algae was reduced to less than 30% cover on the reefs where the researchers introduced crabs. And although the human-scrubbed reefs had even less algae to begin with – around 10% cover – they eventually rebounded. A combination of brushing and crab grazing most effectively reduced algae, which maintained less than 20% coverage during the experiment. These decreases in algal cover had a predominant effect on the reef ecosystem. In the patch reefs where the crabs were released, the number of juvenile corals increased fourfold and the amount of coral reef fish increased three to five times compared to the control reefs.

The authors explain that large-scale crab stocking schemes can dramatically improve coral reef restoration, but raising the crustaceans to effective sizes will take time and space. And the problem of coral decline will require much more than an increase in algal grazers. Still, the study results underscore how the Caribbean king crab could be one piece of the puzzle toward protecting a critical marine species.

Source: Spadaro, AJ and Butler, MJ Grass Crabs Overturn Algae Dilemma on Coral Reefs. Current biology. 2020.

Image: Angelo Spadaro

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