Picture credit : Simon Dowling
The Coral Crab, also known as the Trapezia Crab, a genus of crabs that live among the branches of corals, particularly of the genus Acropora and Pocillopora. They form heterosexual pairs and once they set up house, they won’t tolerate the presence of more Coral Crabs of the same species. Although they don’t tolerate their own species, they do tolerate Trapezia of other species. It isn’t uncommon to observe up to five different pairs of different Trapezia species within the same coral head.
You will only find them inside their host corals, which protects them from fast becoming food from a passing fish. They are usually quite small (around 1-4cm) with large front claws to defend their home.
Originally the crabs 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. This behaviour represented Parasitic behaviour and hence they were believed to be damaging to corals…. until further discoveries were made.
It turns out that these crabs are excellent guards protecting the coral against certain predators of the coral (McKeon et al., 2012). The smallest species protect the corals from a group of snails from the genus Drupella, and the largest ones protect corals from the most-feared Crown-of-Thorns sea star.
In return for this great service the corals, as mentioned before, are providing a home for the crabs. However the corals 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. Its an amazing example of how 2 completely different organisms can benefit so well from each other.
If you would like to be learn about functions and diverse symbiotic relationships amongst marine life, then the first step is to take the Reef Check Course and gain knowledge of fish, invertebrates and substrates.