Heat stressed corals do better with crabs around

Nature is ripe with interactions, everywhere you look. Think of the fungal-algal partnership, we call lichen, or clownfish and sea anemones (where’s Nemo?) swapping shelter and nutrients, or coral polyps and algae forming the Great Barrier Reef. Interactions contribute to healthy ecosystems in so many ways.

Corals are highly sensitive to rising temperatures, but the fundamental relationships with their symbiotic algae break down during marine heatwaves, leaving bleached and dying reefs. But nothing exists in isolation. Researchers have found that such corals are more likely to survive heatwaves if they have crabs — 60% more likely.

Just 450 or so species of hard corals support one of the most biodiverse ecosystems on earth — the World Heritage listed Great Barrier Reef, comprising nine thousand or so species of animals and plants, in dense webs of interactions, challenges and stressors.

Corals are the primary habitat-forming species and are critical to that ecosystem. Understanding how they deal with multiple stressors and what that could mean for coral restoration attracted the attention of PhD student Julianna Renzi, of the University of California at Santa Barbara, and colleagues at the Universities of New South Wales and Queensland.

Is mutualism the answer?

To test whether observed mutualisms — relationships that benefit both species — could help corals deal with multiple stressors, researchers chose the shallow water, green staghorn coral Acropora aspera, and the Hoof-Clawed Reef Crab, Cyclodius ungulates, with which it’s thought to have a mutualistic relationship.

Green staghorn corals are found on reef flats and in lagoons, growing down to 5 metres depth. Listed as vulnerable by the IUCN, the species is widespread but uncommon and prone to bleaching, inshore reef destruction, coral diseases and predation by Crown-of -Thorns Starfish.   

Pieces of live Green Staghorn Coral were placed in multiple flow-through tanks set up at the University of Queensland’s Heron Island Research Station. Drawing water directly from the reef flat, the tanks captured the temperatures and water conditions of the local reef ecosystem. Corals in the separate tanks were exposed to combinations of physical wounding, contact from a selection of potentially-harmful seaweed (macroalgae) and presence of Hoof-Clawed Crabs.

Physical wounding, which mimicked fish predation and damage from tourism, caused the corals to exude a thick mucus which attracted consumers, including crabs, writes Renzi. And seaweeds naturally carry microbes which can cause corals’ wounds to become infected.

Researchers also did field surveys and experiments looking at relative abundance of the crabs and the weeds on the reef flat, and the relationship between wounding, weed removal and crab abundance.

Coincidentally, writes Renzi, a marine heatwave occurred at the start of the experiment, allowing the researchers to look at how the species’ interactions affected coral tissue loss during the elevated temperatures.

Mutualism and the Hoof Clawed Crab

The researchers found that corals were more than 60% less likely to suffer significant tissue loss if Hoof-Clawed Crabs were present.

Attracted to the wounded coral, crabs fed on extruded mucus, on dead, infected tissue at wound margins, and grazed on algae. Live tissue was untouched. More coral tissue was lost if algae were present, but even this was reduced if crabs were around.

Mutualism reduced the risk of infection for wounded coral and competition from algae, despite the marine heatwave bleaching or killing 80% of corals in the Heron Island lagoon, and in field and tank experiments, writes Renzi.

The authors admit that larger field studies are needed to test these results but contend that understanding such positive species interactions could be applied to improving coral reef restoration.

The paper appears in Proceedings of the Royal Society B: Biological Sciences

More from the coral at Heron Island