‘Reef goats’: Seaweed-loving giant crabs could help save corals in the Florida Keys
Adding more Caribbean king crabs to coral reefs overgrown by seaweed can help coral communities recover and improve the abundance of species in reefs, an FIU study shows.
Among the many problems, including pollution and climate change, facing Florida coral reefs is the explosion of algae, which smother once-healthy tracts, blocking baby corals from growing.
One possible solution: More “reef goats” — native crabs that graze on algae and seaweed.
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, improving 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,
who works at Florida International University and is a coauthor of 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.
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 a 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 abundance and diversity, Butler said.
He said more research might be necessary to check what would happen if a reef were flooded 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 those 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.