Toothy Goby in Coral
The toothy goby or common ghost goby (Pleurosicya mossambica) lives among soft corals and sponges in the Indo-Pacific ocean. The relationship it has with its host is commensal, which means the goby benefits from the protection and habitat in the corals, but the coral doesn't get hurt or benefit from the relationship.
Many of the other 2000 or so species of gobies form such symbiotic relationships, both commensalisms and mutualisms. Gobies and pistol shrimps will live together, with the near-blind shrimp tending their sandy burrow while the goby watches for predators. And cleaner gobies (Elacatinus) clean the mouths of bigger fish that would normally treat them as prey.