Continuing my theme of Odes to Awesome systems, I want to tell you guys about one of my favorite animals: guard crabs. These crabs live on corals and deter starfish predators from attacking the corals by pinching the starfish. The guard crab cute factor is out of this world:
I’ve been fascinated by guard crabs ever since I read the really thorough studies of Peter Glynn from the 1970’s and 1980’s. He did all kinds of work showing that guard crabs protect corals from predation by Ancanthaster starfish. Not all coral species have guard crabs, and Peter Glynn showed that populations of unprotected coral species are decimated during Ancanthaster outbreaks, while protected species suffer few losses. Furthermore, protected corals sometimes indirectly defend unprotected corals, because starfish won’t cross barriers of protected corals.
Previously, it seemed like some guard crab species were a lot better at protecting corals than others. This is true in other defensive symbiont systems, too (e.g., ants that protect Acacia trees). But I just read a really cool study by McKeon and Moore (2014) that shows that the slacker symbionts may not be as lazy as we thought! Specifically, small guard crabs aren’t very good at protecting corals from starfish predation. However, small guard crabs are good at protecting corals from smaller corallivores, like predatory snails. Furthermore, big crabs are no good at protecting against those smaller corallivores. (This is a cool parallel to the paper I talked about last week, where hydroids protect hermit crabs from limpets but not blue crabs.)
So, corals need functionally diverse guard crab communities – communities with large and small guard crabs – to be protected from multiple corallivore species. Awwwwesome!
McKeon CS, Moore JM (2014) Species and size diversity in protective services offered by coral guard-crabs. PeerJ 2:e574.