Euplectella, home of commensal shrimp:
This little shrimp takes up residence as a larva.
In fact, two shrimp take up residence, and they're of opposite sex. Once inside the glassy tower, they develop into adults and are too large to exit the sponge. They spend their lives in wedded, commensal bliss, protected by the sponge and getting a constant current of food-filled water, but doing the sponge no benefit nor harm.
In Japan, a glass sponge is considered a traditional (and symbolic) wedding gift.
