Moorea: The Coral Reef Community 

From the researcher's perspective...

China white with bright red spots and green eyes, there is little mistaking Trapezia rufopunctata. This is the best known of the Guard Crabs associated with Pocillopora, with a range from French Polynesia in the Pacific Ocean all the way to the East Coast of Africa in the Indian Ocean. Like other Trapezia, T. rufopunctata is an aggressive guard of its host coral, and will do its best to chase away coralivores that might eat the coral.

What does the crab get from this arrangement? The first benefit is shelter; Trapezia are found nowhere other than in live Pocilloporid corals. The second is food; the coral produces fat that is pushed up into the tentacles of the individual polyps. These tentacles are grazed on by the crab, reducing its need to ever leave the coral for food. This type of relationship, where both partners benefit, is a mutualism. You may know about "cleaner fish" and "cleaner shrimp", or clown fish and anemones, all of which are involved in mutualisms. As reef scientists explore and study the biodiversity of tropical oceans, we find more and more of these positive relationships.

Chris Meyer, Seabird McKeon, Hannah Stewart, and Matthew Johnson