Reef crabs help heat-stressed corals survive

A branching coral under pressure from heat and physical damage appears to gain crucial support from a particular reef-dwelling crab, offering fresh insight into how positive species relationships can shield corals from multiple hazards. 

These findings reveal a protective role for the crustaceans when corals face intense stress, highlighting the possibility that cooperative interactions may bolster the resilience of vulnerable reef habitats.

Reef-dwelling crabs and foundation corals

Published in the journal Proceedings of the Royal B Biological Sciences, the study emphasizes that corals are what ecologists call “foundation species,” forming the bedrock of entire ecosystems. 

Study first author Julianna Renzi conducted the research as a graduate student at the Duke University Marine Laboratory, part of the Nicholas School of the Environment.

“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,” said Renzi.

“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.”

Mutualism and overlapping stressors

Renzi concentrates on the concept of mutualism, in which two species each benefit from interacting with the other. Much existing research focuses on how one beneficial partnership mitigates a single threat, such as higher ocean temperatures.

However, few studies have explored whether such alliances can ease multiple overlapping stressors. 

In this case, the team selected a branching coral known as Acropora aspera from a research zone in Australia’s Great Barrier Reef during a period of elevated water temperatures. 

Within controlled seawater tanks, the scientists subjected coral samples to different stress combinations, including physical wounding, harmful algae, and the presence of Cyclodius ungulatus (the hoof-clawed reef crab). 

Coral tissue loss over a month served as the main measure of coral health, allowing the team to see precisely which factors worsened damage and which helped the coral endure.

Unexpected benefits for wounded corals

The data indicated that high temperatures alone instigated tissue loss, but the extent depended heavily on other conditions.

Corals placed alongside harmful algae were six times more likely to suffer serious tissue loss, while those housed with reef crabs had more than a 60% lower chance of severe damage. 

Even more unexpected was how strongly the crabs’ presence helped corals that had sustained wounds. Those corals displayed significantly less tissue loss than uninjured corals sharing tanks with crabs, and also less tissue loss than wounded corals without crab companions. 

While corals lacking crabs sometimes saw injuries expand, corals with crab neighbors rarely experienced further deterioration around the wounds.

Reef crabs removing algae

To unravel what motivated the crabs to linger near wounded coral, the researchers examined their behavior in more detail. They noticed that, in laboratory tests, the crustaceans avoided live coral tissue, instead feeding in regions where tissue had recently been lost. 

Observations on the reef revealed that areas of algae disappeared faster around corals with wounds, leading the investigators to suggest that reef crabs and other organisms might be methodically removing algae in these damaged spots, effectively tidying up the coral surface.

Another key question arose: what attracts crabs to fresh wounds in the first place? Renzi and colleagues propose that the injured coral releases a nutritious mucus that proves enticing. 

“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.

Mutualism and heat stress resistance

The conclusions back an ecological view that beneficial interspecies interactions, or mutualisms, can become particularly vital when the environment imposes significant hardships. 

“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.”

Reef crabs working under pressure

The researchers note that these discoveries hold promise for coral reef restoration, since introducing mutualistic crabs to newly planted colonies could reduce tissue loss from infection and disease.

One broader lesson is that nature sometimes operates more cooperatively than commonly assumed. 

“We often think of the world as ‘dog-eat-dog’ and assume organisms are constantly competing against each other in effort to survive,” Renzi said. “But in cases like these, positive species interactions may be really important for survival. Evolutionarily, looking out for number 1 may also mean looking out for number 2.”

Altogether, this study suggests that reef-dwelling crabs could prove key to helping certain branching corals remain healthy, even under the combined pressures of heat and injury.

By highlighting such mutually beneficial partnerships, the findings pave the way for innovative approaches to reef conservation and underline the value of collaborative ties in the marine realm.

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