“Commensalism” is defined as an interaction between individuals of two species in which one individual gains something while the other neither gains nor loses. A possible example is that of the sea urchin crab, Echinoecus pentagonus, that lives in the anal pore of sea urchins in the Indian and Pacific Oceans. Here are two photos of the tiny beast (ca.1.3 cm, or 0.5 inch, across) in situ. I can imagine that the crab gains protection (and possibly some food) from living in the hole, while the urchin suffers at best mild debilitation. (Photo by Keoki and Yuko Stender from MarinelifePhotography.com)
And here’s the reverse situation, a decorator crab (there are several species that have this common name) off Indonesia carrying a fire urchin (probably Astropyga radiata) on its back. The crab clearly gains protection by looking like an urchin, while it’s not clear what the urchin gets (perhaps a bit of extra food and freedom from competition by getting the ability to move about.
h/t: Matthew Cobb