January 18, 2022

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Caribbean crabs love seaweed. That could help Florida’s corals.

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Caribbean king crabs are very efficient at removing seaweed from coral reefs because they eat all kinds of algae. Too much algae can kill corals because they block out the light.

Among the many problems facing Florida coral reefs, from pollution to climate change, is the explosion of algae that smothers once healthy tracts, blocking baby corals from growing.

One possible solution: More ‘reef goats’ — native crabs that graze on algae and seaweeds.

A study by Florida International University showed that increasing the abundance of the Caribbean king crab, found in Florida waters and throughout the Caribbean, can reduce algae overgrowth and help restore the natural balance, improve coral health overall.

“We’ve had a lot of experience raising these crabs for human consumption, but I started looking at what their ecological role was on coral reefs. And they are like reef cows, or better yet, reef goats, because they will eat almost any type of algae, and they eat a lot of it,” said Mark Butler from Florida International University, a co-author on the study.

He said that when seaweed covers up corals, it blocks the light and prevents baby corals from settling and growing on reefs. Seaweed also produces chemicals that can make corals more susceptible to disease and affect their reproductive cycle. Seaweed also releases chemicals that larval reef fish avoid, which hurts fish communities in the important nursery habitat that corals provide.

“We hypothesized that the removal of seaweeds by grazing crabs would also have a positive, cascading effect on reef fish community composition,” said the study titled, “Herbivorous Crabs Reverse the Seaweed Dilemma on Coral Reefs” and published in the journal Current Biology today.

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Pollution and a warming ocean can cause algae overgrowth on reefs, which can smother the corals living on them. This Florida Keys patch reef is covered with the green algae Halimeda. Courtesy of A. Spadaro.

Butler, who worked on the research with Angelo Jason Spadaro, a professor at the College of the Florida Keys, said the voracious Caribbean king crabs (Maguimithrax spinosissimus) are powerful coral reef grazers.

They eat all kinds of algae, even certain types of seaweed that most species avoid. The parrotfish, for example, spends nearly its entire life eating algae off coral reefs, but won’t touch certain algae species like the Halimeda, a green macroalgae that has calcium carbonate inside its tissue, making it inedible to most herbivores.

The crab’s coral-cleaning services were “particularly noteworthy given that our experiments were conducted on reefs in shallow, nearshore waters where seaweed growth is high and dominated by calcareous green algae (Halimeda),” Butler said.

Caribbean king crabs already inhabit coral reefs in Florida, but not in numbers that are abundant enough to make a difference. Butler wanted to know what coral reefs would look like if more crabs were introduced.

In two experiments conducted at separate locations about nine miles apart in the Florida Keys, Butler and Spadaro transplanted crabs onto several reefs, leaving others without added crabs, as control in the study. They monitored how the corals and the fish community on those corals did for a year. The scientists also compared how effective crabs were at eating algae and cleaning up corals versus scrubbing reefs by hand to remove algae.

Crabs reduced seaweed cover by 50% to 80%, resulting in a three to five-fold increase in the number of little baby corals that settled on those reefs, and boosting fish community abundance and diversity, Butler said.

He said more research may be necessary to check what would happen if a reef was overloaded with crabs. “We’re going to have to figure out what is the exact density we should keep,” Butler said, adding these crabs tend to stay in the reefs, so the potential for them to spread is low.

Most coral restoration efforts focus on cultivating corals in labs or nurseries in the ocean, and then transplanting them to ravaged reefs, which act as crucial natural protection infrastructure for coastlines against erosion and storms.

But these strategies don’t address the seaweed issue, Butler said. He is hoping that the National Oceanographic and Atmospheric Administration and partners will consider his research as part of a multimillion-dollar effort to restore seven iconic coral reef sites in the Florida Keys National Marine Sanctuary, a project called Mission: Iconic Reefs.

“So we are hoping to get funding so that we can set up a big aquaculture facility to scale this up,” he said.

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Adriana Brasileiro covers environmental news at the Miami Herald. Previously she covered climate change, business, political and general news as a correspondent for the world’s top news organizations: Thomson Reuters, Dow Jones – The Wall Street Journal and Bloomberg, based in São Paulo, Rio de Janeiro, Paris and Santiago.

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