Hey all, I have an interest in mutualistic relationships (relationships where both species benefit) and thought I’d share a relationship each month.
Today I’m going to introduce you to the Trapezia-Pocillopora relationship (a crab and coral relationship).
Trapezia is a genus of crabs that live among the branches of corals, particularly of the genus Pocillopora. They form heterosexual pairs and once they set up house, they won't tolerate the presence of more conspecifics (individuals of the same species). Although they don't tolerate conspecifics, they do tolerate Trapezia of other species. It isn't uncommon to observe up to five different pairs of Trapezia species within the same coral head. In the wild, they're only ever found inside their host corals; a crab this small and this front-heavy (they have very large claws) would be quick fish bait.
Originally Trapezia (and its relatives) were considered ectoparasites on coral (Knudsen, 1967). The crabs use a special brush and comb setup on the end of their walking legs to scrape the tissue of their coral host, collecting mucus, bacteria, and coral flesh and then eat that mixture. Sounds like a parasitic relationship right? Turns out it isn't!
Trapezia species protect their coral from predators, quite ferociously. Depending on the size of the crabs, crabs protect against a certain predator of the coral (McKeon et al., 2012). The smallest species protect the corals from a group of snails from the genus Drupella, the medium sized crabs protect corals from the cushion star Culcita novaeguineae, and the biggest and baddest species protect corals from the most-feared Crown-of-Thorns sea star Acanthaster planci (a known killer of whole coral reefs). If you want to see Trapezia in action, check out the YouTube video at the bottom of this post. When the coral head has all sizes of crabs they are completely protected from all these predators. In serious coral predator outbreaks, the only coral heads left are those that are hosting guard crabs.
In return for this great service the corals, as mentioned before, are providing housing. Turns out they also concentrate fats in the tips of their tentacles, but only in the presence of the crabs (Stimson, 1990). The crabs go around clipping off the tips, getting paid for their protective services. Just a real neat relationship.
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Knudsen, J.W. 1967. Trapezia and Tetralia (Decapoda, Brachyura, Xanthidae) as obligate ectoparasites of pocilloporid and acroporid corals. Pacific Science 21: 51-57.
McKeon, C.S., Stier, A.C., McIlroy, S.E., and Bolker, B.M. 2012. Multiple defender effects: synergistic coral defense by mutualist crustaceans. Oecologia 169: 1095-1103.
Stimson, J. 1990. Stimulation of fat-body production in the polyps of the coral Pocillopora damicornis by the presence of mutualistic crabs of the genus Trapezia. Marine Biology 106: 211-218.