One type of mutualistic relationship is one in which a species is called a "keystone mutualist." Mangrove trees qualify as keystone mutualists.
A keystone mutualist is a species whose existence is pretty much necessary for a whole ecosystem to exist. If the keystone mutualist is removed, the whole ecosystem collapses. Not too surprisingly, mangroves are a keystone mutualist in the mangrove swamp ecosystem.
Mangrove swamps are large areas where pretty much the only vegetation is mangroves. The mangroves' roots sink down into the mud and stabilize it. When the mangroves do this, the muddy swamp and the mangroves' roots become a habitat for many other creatures. All of these depend on the mangroves.
To read more, search for "red mangrove" in the page that I have linked to.