Small Crabs Play a Big Role as Corals’ First Line of Defense from Predators - Oceana USA
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2014-10-07 00:00:00

Small Crabs Play a Big Role as Corals’ First Line of Defense from Predators

A coral crab (Trapezia sp.) actively defends its host, a cauliflower coral (Pocillopora sp.). Yankee Reef, Great Barrier Reef

Just as some people have their own sorts of protections from intruders, some corals have their own creative defenses to guard them from predation. It’s a hard knock life for corals: In addition to ocean acidification, overfishing, and nutrients pollution, corals have to deal with sea stars and snails that can devour them. Fortunately, corals have mechanisms in place to aid them in their defense.

Newly published research out of the Smithsonian Marine Station and Florida Museum of Natural History found that a range of crabs in the genus Trapezia actively defend corals from various forms of predation. The scientists found that “guard-crabs” and stony corals surrounding the island of Moorea in the French Polynesia have a symbiotic relationship: The corals shelter and provide nutrients to their residential crabs in exchange for protection from predators known as corallivores—or in this case, sea stars.

To understand the relationship between crab guards and coral, the researchers selectively removed crabs from their host corals and observed the effects during a two-week period. Predation frequency and tissue loss were greater in corals with crabs removed during this period. Researchers also found that crabs’ defense skills against corallivories differed among sizes. Small crabs, for example, defended host corals from small sea snails but not larger predators, while large crabs defended host corals from large corallivores such as crown-of-thorns sea stars.

“Seemingly small differences among crabs guarding their coral homes can have big effects on coral survival,” explained study author and PhD student Jenna M. Moore in a press release. “Not only does the level of protection provided vary by species, but the smallest crabs were defending the coral from coral-eating snails, a threat that larger crabs ignored.”

This research demonstrates the importance of diversity—specifically functional diversity, or species like Trapezia crabs that have specific traits for influencing ecosystem stability—in ensuring a healthy coral reef. These benefits can have extensive impacts on coral reefs, such as offering protection for the nearby corals when predators bloom.

Over 1.3 billion people—or one-fifth of humanity—live within 62 miles of a tropical coastline. Coral reefs provide nursery grounds and habitat to some 8,000 species of fish and buffer shorelines from storms—an amount that totals $9 billion in coastal protection. The mutual relationship between crab guards and coral is therefore crucial to both ocean health and human livelihood, say the authors.