For certain vulnerable corals, help is in the claws of a crab. Reporting in Proceedings Biological Sciences, researchers found that a species of branching coral benefitted from a reef-dwelling crab, especially when the coral was heat-stressed and wounded. The findings support the idea that positive species interactions can buffer coral reefs from multiple environmental threats.

Corals are what ecologists call “foundation species,” meaning that they’re integral to habitats and food webs. Understanding ecological factors that shield coral reefs from harm could inform conservation and restoration efforts.

A hoof-clawed reef crab appears as a dark splotch hidden among the coral branches. Photo by Devin Rowell

“Foundation species like corals create the base of an ecosystem: They form structures that other species use for shelter, they modify the local environment, and they provide food for other organisms. Learning how these species respond to stress can help us design better strategies to conserve them — and, in turn, other species that rely on them — in an era of global change,” said first author Julianna Renzi, who conducted the research as a graduate student at the Duke University Marine Laboratory, part of the Nicholas School of the Environment.

Renzi is particularly interested in mutualism, a relationship between species that helps both. Many studies have investigated how positive species interactions reduce the effects of a single environmental stressor, like rising ocean temperatures. However, less work has explored how these beneficial relationships can counteract multiple, coinciding stressors, according to the authors.

For their study, Renzi and colleagues collected samples of Acropora aspera, a type of branching coral, from a designated research area in Australia’s Great Barrier Reef during a heat wave. They placed the coral samples in tanks fed with seawater and exposed them to one or a combination of treatments, including physical wounds; a harmful algae; and the Australian crustacean Cyclodius ungulatus, or hoof-clawed reef crab. For each treatment, they measured coral tissue loss over the course of a month as a proxy for coral health.

The team found that high water temperatures appeared to trigger tissue loss in the coral samples. However, the amount of tissue loss varied depending on the experimental treatment. For example, the presence of algae increased the probability of significant tissue loss by six-fold compared to treatments without algae. By contrast, crab tankmates decreased the risk of major tissue loss by more than 60% compared to treatments without crabs.

Surprisingly, crabs seemed particularly beneficial to the wounded corals, which experienced less tissue loss than either uninjured corals exposed to crabs or injured corals without a crab. What’s more, coral wounds rarely grew when crabs were present but, in some cases, appeared to expand in the absence of crabs.

The team documented the extent of tissue loss in coral samples exposed to different treatments. Photo by Julianna Renzi

To better understand the relationship between crabs and coral, the researchers conducted additional experiments on crab behavior. Lab studies suggested that crabs avoided living coral tissue, instead feeding in areas of recent tissue loss. Out on the reef, the team observed that wounded corals lost patches of algae faster than uninjured corals. The finding suggests that crabs and other organisms might remove the algae —  in effect, pruning their habitat.

But why would crabs be attracted to coral wounds in the first place? Perhaps they’re lured by nutritious mucus that corals release when they’re injured, the authors suggested.

“Corals probably experience small wounds, like the ones we used in this study, relatively frequently from fish bites and physical abrasion. These wounds are probably not super detrimental, but they may be enough to release this mucus and attract coral-associated organisms, like C. ungulatus,” said Renzi, who is now pursuing a Ph.D. at the University of California Santa Barbara.

The team’s findings support an ecological concept that positives species interactions like mutualism may be particularly important under environmental stress and could potentially contribute to some species’ resilience against stressors.

“This work challenges a paradigm about corals,” said Brian Silliman, Rachel Carson Distinguished Professor of Marine Conservation Biology at the Nicholas School, who advised Renzi during her master’s work. “The temperature at which corals succumb to heat stress is generally thought to be innate and inflexible. But this work shows that an intricate biological partnership greatly increases the ability of corals to resist heat stress. The crabs don’t affect heat tolerance directly — rather, they appear to remove the stress of injury by cleaning coral wounds.”

The results also have implications for coral reef restoration strategies. For example, adding mutualistic crabs to new coral colonies could potentially minimize tissue loss from diseases associated with transplanting.