Monster Hunter Frontier Water Longsword

I recently found this path to this water longsword, which I've never seen before in Monster Hunter Freedom 2, although maybe it was in Portable 2nd G/Unite :<

http://i423.photobucket.com/albums/p...er/WaterLS.jpg

The problem is that I have no idea what that blue carapace/shell is. The JP wiki says "fin of water dragon?" or something, but as quoted, there's a question mark, and no article for the item in question. Also, I have Plesioth fins, and it's not that. I'm completely clueless at this point, unless the shell is a Plesioth Fin+ or something...

Could anyone help?

It's an upper Plesioth fin