Vicious

She could have just found an empty clam shell though

"This is how we do it"

Look at this stuff, isn't it neat?
Wouldn't you think my collection's complete?
Wouldn't you think I'm the girl
The girl who everything.
Look at this clam isn't it sweet?
I think I'll make myself a treat.
It's life is important, but it would look amazing on me.
A sacrifice would make my outfit complete.
I've got gadgets and gizmos aplenty
I've got whosits and whatsits galore
You want thingamabobs?
I got twenty
But who cares?
No big deal
I want more....

You think mermaids don't have stores? I don't know, they might not. I don't know much about their culture.

well what do you think she does for food? probably why shes friends with flounder. juuuust waiting...

Ahem, friendly nerd here, I mean no harm.
Judging from the texture and size of Ariel's shell bikini, the type of mollusk she viciously ripped open was most likely a scallop. Scallops have a flat "hinge" of sort where the two shells come together, but the rest of the shell has ridges much like those seen in Ariel's bikini. Additionally, scallops can grow up to 9 inches but can be smaller than that - more than enough room to cover a petite mermaid's boobs. Finally, clams of this size and not the enormous tropical ones are most often white or a pale yellow or blue - not purple. However, scallops are known for their pretty markings and color, so why wouldn't she choose one?
Scallop: https://cdn2.bigcommerce.com/server3900/36f60/products/1437/images/565/ps1_purpscallop__66335.1320992834.1280.1280.gif?c=2
Clam: http://www.ocalacameraclub.org/JAlbumGallery/NCFPC%20Gallery%202011/Jan/Color%20B/slides/KF-Color%20B-Clams%20and%20an%20Oyster.JPG

Lucky clam

Look what I found, a wonderful clam!
Watch me rip out his life, to make a sacrafise.
For my beauty, watch how he diessssss
I wonder how humans use their clams?
*song continues as areole rips open the clam slowly and eats the insides*