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Outbreaks of a predatory starfish can cause widespread coral mortality. Jane Ballard, one of the authors of a Research Note published in F1000Research, discusses the community dynamics of these habitats and the ecological shifts that can seal the fate of corals.

A battle is waging under the ocean’s surface between corals and predatory seastars. However, our observations highlight other participants that might be just as important in this critical fight for coral survival.

The crown of thorns seastar (COTS) is a widespread predator of corals. COTS are usually found in low densities across coral reefs, but outbreaks of high population densities can occur. During these outbreaks, thousands of seastars swarm and feed on the corals, leaving behind large areas of dead reefs.

Game of thorns

To counter these attacks, some coral species possess mutualistic allies that can deter COTS predation. Pocilloporid corals (e.g. Pocillopora eydouxi) are one such species, with their host trapeziid crabs (a.k.a coral crabs), known for their ability to effectively defend their host coral colonies from COTS assaults.

Trapeziid crabs protect the host coral from diverse predators and also provide additional services such as cleaning algae and sediment from the coral colony. In return, the crabs benefit from habitat and refuge from predators, as well continuous food supply from the coral.

In the area surrounding the island of Moorea, French Polynesia, these “guarded corals” have shown the ability to resist devastating COTS predation for several years while other coral species were killed. However, guarded corals don’t always survive COTS outbreaks.

After monitoring COTS outbreaks for several years on coral reefs in Moorea, we had a hunch that  additional processes might explain the fall of these guarded corals. Widespread decline in living corals had concentrated diverse coral predators around the remaining guarded corals. These resident predators are usually dispersed over large reef areas, but collapse of their prey pushed them to aggregate in remarkably high densities around the only survivors, the guarded corals.

Battle of Moorea

During the COTS outbreak, we observed a shift in the distribution of resident coral feeding butterfly fish (e.g. Chaetodon ornatissimus, C. pelewensis, C. quadrimaculatus, C. reticulatus), for example from a density of approximately four fish dispersed over 200 square meters of reef area to increased concentrations of up to nine fish per square meter around the guarded corals. In one observation, a guarded coral that was already under attack by a COTS  had butterfly fishes aggregated around it in such high numbers, it represented more than a 400 times increase in predation pressure.

These simultaneous attacks are likely to overwhelm the defensive capacity provided by the trapeziid crabs. Unable to fend off such a high number of predatory attacks, could explain the fall of the guarded corals.

Healthy coral reef ecosystems often support greater numbers of predators. However, as food resources diminish during COTS outbreaks, the larger number of predators could possibly accelerate the decline of coral reefs as they overwhelm the defenses of the so called guarded corals.

This suggests that healthier reefs might be more susceptible to collapse under COTS attacks. Our observations show that COTS outbreaks need to be controlled early on in order to conserve our coral reefs and to prevent these cascading effects accelerating coral decline.