Heat-stressed and wounded coral may just find help in the claws of the trusty reef-dwelling crab, which – it would appear – has a particular penchant for nursing a certain species of coral back to good health, all through a bit of light pruning.

This is a breakthrough finding among scientists exploring the concept of mutualism between species, investigating the depth at which positive species interactions can actually buffer coral reefs from multiple environmental threats.

Corals are what ecologists call ‘foundational species’, meaning they are integral to habitats and food webs. Understanding ecological factors that shield coral reefs from harm could go on to inform new and more effective conservation and restoration efforts.

While much work has already been done to investigate how positive species interactions can reduce the effects of a single environmental stressor, like rising ocean temperatures, fewer efforts have been invested in exploring how these beneficial relationships can counteract multiple, coinciding stressors at the same time. 

But it’s this area of mutualism that Julianna Renzi, a graduate student at the Duke University Marine Laboratory, a part of the Nicholas School of the Environment, wanted to explore more.

“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,” she said. “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.”

For this study, Renzi of her colleagues decided to focus their attention on Acopora aspera, a type of branching coral found among Australia’s Great Barrier Reef. Samples were collected from the reef during a heat wave, placed in tanks fed with seawater, and then exposed to one or a combination of treatments, including physical wounds; a harmful algae; and the Australian crustacean, Cyclodius ungulatus, otherwise known as the hoof-clawed reef crab.

For each of these treatments, researchers measured the levels of coral tissue loss over the course of a month as a proxy for coral health.

The team found, initially, that high water temperatures triggered tissue loss in the coral samples. However, the level of this 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, coral samples paired with a friendly crab tankmate experienced tissue loss by as much as 60% less than samples without crabs.

What surprised scientists the most, is that 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.

So, what was it the crabs were doing to nurse these wounded coral samples to better health?

To better understand this, the researchers conducted additional studies on crab behaviour through which, findings have suggested that crabs actively 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, suggesting that crabs – and other organisms – might be doing their bit around the home, removing the algae, and in effect pruning their habitat.

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

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

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

Brian Silliman, a Rachel Carson Distinguished Professor of Marine Conservation Biology at the Nicholas School, who advised Renzi during the study, said: “This work challenges a paradigm about corals. 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.”

However, these results do have implications for coral reef restoration strategies. For example, adding mutualistic crabs to new coral colonies could potentially minimise tissue loss from diseases associated with transplanting.

“We often think of the world as ‘dog-eat-dog’ and assume organisms are constantly competing against each other in effort to survive,” said Renzi. “But in cases like these, positive species interactions may be really important for survival. Evolutionarily, looking out for number one may also mean looking out for number two.”

Renzi’s study An abundant mutualist can protect corals from multiple stressors, was published this month in the scientific journal, Proceedings of the Royal Society B.

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