@Fire Fist:
Poison is very different from straight acid. There's poisons that can dissolve, yes, because they have an acid base. But pure acid can dissolve just about anything.
Thing is that Magellan can dissolve anything as well. His kinjite, or how I call it "magic paramecia poison", infects and destroys everything it comes in contact with (and it spreads like a disease, even on non-organic things).
His ultimate is able to destroy all of Impeldown if he doesn't watch out, thats why I doubt that there is a higher level of "erosiv power".
I believe a simple acid fruit would be too redundant (if the focus is on eroding stuff with no freaky extra's). After all we are speaking of a Yonko here, her abilities must be absolutely terrifying and on the highest level possible. But who knows what Oda has in store, at least I can't imagine what else an acid fruit could do except what Magellan did a few hundred chapters ago.
@Cooldud_21:
There is a difference between protecting the people you care about and declaring a country to be your turf. Luffy has been doing the former all this while (Alabasta, Skypeia). He could have easily handled the Fishman Island situation in the same way. A rookie pirate declaring an important crossroads at the Grand Line to be his turf looks a little too soon for me. Yonkou aside, I would assume there are at least ten other pirate crews in the New World that could fight on an even keel with the Straw Hats. This is opposed to Newgate who was head and shoulders above any pirate in the world.
Also, this puts Luffy under an obligation. Basically the Straw Hat flag gets a permanent outpost, which is inherently against the idea of freely sailing the sea doing whatever you want that Roger espoused. It can be argued that Luffy is his own man, but this idea of freedom is one of the core values that he shared with Roger, and made people recognize him as Roger's rightful heir.
In my opinion, a declaration like, "If you try to hurt my friends, I'll kick your ass" would have sufficed here.
Hm, I thought the whole territory thing was only to redirect Big Mam's attention and wrath. That's why he desperately screamed that he was the one who ate all the 10 tons of candy.
What wonders me is that Luffy basically lied to Big Mam like a sneaky bastard, I don't remember him lying to anyone before? With him being the pure hearted Son-Goku type of guy.