It’s hard out there for corals. On top of ocean acidification, overfishing, and pollution, they have to deal with corallivores—predators that include:
- the GIANT Crown-of-Thorns sea star Acanthaster planci
- the medium-sized sea star Culcita novaeguineae
- and the tiny snail Drupella cornus
Fortunately, corals have defenders that come in all different sizes too. That’s one of the takeaways of a new study published in PeerJ and co-authored by C. Seabird McKeon and Jenna Moore, of the Smithsonian Marine Station, in Fort Pierce, Florida and the Florida Museum of Natural History.
McKeon and Moore conducted a series of experiments that effectively pitted small, medium, and large species of crabs—all from the coral-dwelling genus Trapezia—against the three previously-mentioned corallivores. (As always, size is relative and the crabs in this investigation were on the littler side; the carapace width of the largest ones was just over one centimeter.)
These studies, which included one field and two lab experiments, were conducted on the Island of Moorea, in French Polynesia. McKeon and Moore used the coral Pocillopora, which is found in the nearby reefs and throughout the Indian and Pacific Oceans. Not only do these coral provide Trapezia with shelter, they also feed the crabs lipid droplets. In return, the crabs protect them. Try to touch the coral, and McKeon says you’ll get pinched.
But just how well do the crabs defend their coral hosts? After each corallivore-crab face off, the scientists measured how much coral tissue the predators devoured. For comparison, the researchers conducted all the corallivore experiments both with and without the guard crabs. Here are a couple of their key findings:
- The smallest Trapezia crabs provided a strong defense against the tiny Drupella snails. Surprisingly, the medium-sized crabs did not.
- Only coral colonies with the largest Trapezia guard crabs, T. flavopunctata and T. rufopunctata, stood a chance against the mighty crown-of-thorns sea star. On average, the crab-protected coral lost only 2% of its tissue, compared to 22% of the non-protected coral.
What this tells McKeon and Moore is that diversity matters—as similar as they may be, not all Trapezia crabs are doing the same thing in their coral habitats. “The different crabs are providing multiple lines of defense and each one is necessary to keep the corals safe,” says McKeon.
Editor’s Note: Head to the Ocean Portal to read more about other interesting research on the Island of Moorea.
By Tina Tennessen, Smithsonian's National Museum of Natural History