Crabs guard coral from army of sea stars

Look between the branches of Pocillopora stony coral of the Indo-Pacific and you may find tiny crabs from the genus Trapezia, the largest of which measure only a few centimeters. At night, the crabs feed on fats found in the tips of the tentacles of coral polyps. In return, the crustaceans remove sediment and protect the coral from predators. And, a new study finds, some of the biggest crabs can even defend coral from crown-of-thorn sea stars, large, multi-armed, spiked, venomous starfish that can devour a reef of stony coral.

During a 2008 outbreak of crown-of-thorns sea stars in Mo’orea, French Polynesia, the starfish quickly devastated the reefs, bringing live coral coverage down from 80 percent to a measly 3.4 percent. But Pocillopora eydouxi weren’t as badly affected. To test why, C. Seabird McKeon and Jenna M. Moore of the University of Florida in Gainesville removed one of the largest species of Trapezia crabs, T. flavopunctata, from 45 Pocillopora colonies. Over the next few weeks, they compared what happened to those corals with 45 similar colonies that were still home to big crabs.

Eighteen percent of corals with large crabs were attacked by sea stars, compared with 64 percent of the corals that had their crabs removed. Undefended corals lost an average of 22 percent of their tissues, compared with only 2 percent among the defended ones, the researchers report September 30 in PeerJ. The protective effect benefited other coral species as well — coral living near Pocillopora also tended to go uneaten.

Many of the undefended corals still housed smaller species of crabs, but they were ineffective against the starfish. Those smaller crabs, however, still play a role in coral defense, lab experiments showed. Small, 4- to 6-millimeter-long species were able to fend off coral-eating snails that larger, 9- to 11-millimeter-long species couldn’t. But those larger species could protect the coral from Culcita sea stars that were too big for the tiniest of crabs to hold at bay.

Typically, a single mating pair of Trapezia crabs nestles into the arms of a Pocillopora colony. But each colony can hold multiple species of crabs in addition to young crabs. The larger the coral colony, the more room for crabs; the largest can hold up to five different species, each protecting the coral from different threats. That may help to explain the results of earlier research that found huge differences in the overall survival of large and small Pocillopora colonies over the course of a year.