Two to Tango

Coral polyps and the algae living inside them, Zooxanthellae, work together to form the Great Barrier Reef. Algae provide energy to the corals through photosynthesis, spurring them to grow and build the reef.

However, corals are highly sensitive to rising ocean temperatures. During heatwaves, the stress caused by the heat disrupts the symbiotic relationship between coral polyps and algae, leading to coral bleaching, a phenomenon that involves corals expelling the algae. This causes the reefs to be more vulnerable because corals don’t exist in isolation.

In some crabs to the rescue, according to a new study.

Researchers found that crabs protect the coral by cleaning it of macroalgae (seaweed) or parasites that might add additional stress besides the heat. They may also provide nutrients to support the corals when going through difficult conditions.

To test whether observed mutualism – a relationship that benefits both species involved – between corals and crabs could help the corals survive, the researchers used flow-through tanks supplied with seawater drawn from a nearby reef flat to see how common coral-dwelling crabs Cyclodius ungulatus, affect corals, Acropora aspera, wounded by heat.

A marine heatwave coincidentally occurred at the start of the experiment, raising the temperature in the reef flat and, consequently, in the researchers’ tanks, contributing to tissue loss in the coral sample.

The extent of tissue loss depended on the experimental conditions tested, including exposure to potentially harmful macroalgae, physical wounding, and crabs.

Seaweeds carry microbes that can infect coral wounds, while physical wounding – simulating fish predation and tourism damage – triggered corals to release a thick mucus that attracted crabs.

Surprisingly, with crabs around, corals were more than 60 percent less likely to suffer significant tissue loss as the crabs fed on the mucus at the wound margins without harming healthy tissue.

While macroalgal contact increased tissue loss, crabs helped reduce it.

The researchers proposed that wounding might help increase coral resistance to heat stress by attracting mutualistic crabs, which could protect corals from different stressors.