By Gail Gallessich

Tiny crabs that live on South Pacific coral reefs help prevent coral death by providing regular cleaning “services” that may be critical to coral reefs around the world, according to scientists from UCSB.

The relationship between the crabs and the soft animals that build the hard corals is described in the November issue of the journal Coral Reefs. The coral provides protection for the crabs. The crabs provide “housekeeping” for the coral, routinely “sweeping” out sediment that falls onto the coral, according to the UCSB study, which is based on research done near the French Polynesian island of Moorea.

Thus the relationship between the corals and the trapeziid crabs is mutually bene­ficial, or symbiotic. The little crabs, measuring only a centimeter wide, make their home in branching corals, like Acropora.

The accumulation of sediment on coral tissue reduces metabolic and tissue growth rates, increasing the probability of bleaching and coral death. Many corals can remove some sediment but high loads can be deadly.

“Although we don’t know much about these crabs, we do know that they are ‘picky,’ and are always tasting and exploring,” said Hannah L. Stewart, first author of the paper and a postdoctoral researcher at UCSB’s Marine Science Institute (MSI). “They use their front appendages to manipulate and shovel out the sediment.”

Stewart said that this family of crabs is common around the world. “This relationship probably occurs all over the Pacific, and is likely more ubiquitous than we know,” she said. “Species of crabs that associate with corals may be more important than we realized.”

Coral reefs are one of the most productive and diverse ocean ecosystems. They support more than 9 million species and provide a livelihood for millions of people around the globe. And they are threatened by a variety of environmental changes.

For example, higher water temperatures and increased ultraviolet radiation, which are associated with climate change, are sources of widespread coral bleaching. Population increases along coastlines increases the sediment load on coral due to the higher amount of water run-off from development, deforestation with erosion, and expansion of agriculture.

The studies were conducted as part of The Moorea Coral Reef Long Term Ecological Research Site (MCR LTER). Stewart performed the research with Sally Holbrook, professor and vice chair of ecology, evolution, and marine biology; Russell Schmitt, a professor in the same department and the director of the MSI’s Coastal Research Center; and Andrew Brooks, assistant research biologist at the MSI and deputy director of the MCR LTER.

The scientists showed the importance of trapeziid crabs by gently removing crabs from sections of the corals on a coastal reef. This resulted in 50 to 80 percent of those corals dying in less than a month. By contrast, all corals with crabs survived. This symbiotic relationship had not been recognized until this study. For surviving corals that lacked crabs, growth was slower, tissue bleaching was greater, and sediment load was higher.

Laboratory experiments revealed that the corals with crabs not only shed substantially more of the deposited sediments, but also that crabs were most effective at removing grain sizes that were most damaging to coral tissues.