Crustaceans Work Together to Defend Their Coral Homes From Cushion Seastar Attacks

Posted on March 30, 2012

Researchers have found that crustaceans often work together to defend their coral homes from hungry seastars, like the cushion seastar pictured above. Seabird McKeon, a marine biologist at the National Museum of Natural History's Smithsonian Marine Station in Fort Pierce, Fla, says that in these frequent conflicts, the crustaceans are much more effective when they fight together than when they fight alone. McKeon says the strength that comes in numbers is greater than the sum of its parts.

The cushion seastar consumes a coral by pushing its stomach outside its body and laying it over the corall like a cushion. It then hugs the coral close and "eats," letting stomach acids and digestive juices do their work. The coral is defenseless against the coral, but the tiny crustaceans that live among its branches come to its aid, like tiny sea heroes, snipping and prodding an intruding seastar with their claws.

McKeon says, "The coral itself is like a cauliflower head, a main central stem and lots of little branches. Crabs gain protection from fish by living inside the coral structure."

In repeated experiments McKeon and colleagues measured the effectiveness of a single crab pair in preventing a seastar from eating their home coral. He found that one pair of crabs reduced the volume of coral eaten by about 19%, compared to a coral with no defenders. Two pairs of crustaceans working together, however, were able to reduce the volume of coral eaten by as much as 65%. McKeon calls this the Multiple Defender Effect.

Trapezia serenei, a tiny coral-dwelling crab, is pictured above. Once a mating pair of crabs takes up residence on a coral head they do not tolerate the presence of other crabs of their same species. However, crabs of other species are allowed to reside in the coral, as are snapping shrimp. The researchers found that some coral heads may have as many as five different species of defensive crustaceans living on them, all pairs of different species.

McKeon says, "These crabs don't allow others of their same species on their coral, yet the synergy of different pairs fighting together is critical to the defense of the coral. The multiple defender effect is an important new angle on why we must conserve biodiversity in the ocean."

The researchers paper, "Multiple defender effects: synergistic coral defense by mutualist crustacean," by C. Seabird McKeon; Adrian C. Stier of the University of Florida; Shelby McIlroy of the University at Buffalo and Banjamin Bolker of McMaster University, can be found here in the scientific journal, Oecologia.

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