In graduate school I studied marine animals that live together, each presumably benefiting from such a relationship.
The animal that benefits at the expense of the other, as in slowly consuming its live host, is a parasite. If one outright eats the other, we would call this predation.
What I studied was called commensalism. I sought a host that provided residence for these animals, such as a coral head or anemone and then identified the players. To do so, I laid out a 10 meter square underwater grids and laid them over a reef.