I know of the Venus Basket sponge that two shrimp live inside of. In Japan they give it as a wedding gift to show they hope the marriage last forever.It is very beautiful and very interesting. Also pea crabs live inside of mussel shells and eat what evert the mussel dosen't. Sorry I am on a friend 's computer and I don't have my books and things with me to give you more info write if you want refs. will send them to you when I am back on my own computer.
(:TONI:)