[Whatigot]

My moneys on it's either a mis id'd whelk or the shrimp was already in trouble.
It does look a little smooth to me as well for a nassarius.


Diana could also be right, and they could just be a lot more voracious then the smaller variety and not getting enough food in a smaller tank but the other two possibilities seem more likely to me.

Definitely do some research on the larger Nassarius' to see if your is the exact same.
If it is, you might want to feed your tank more or buy faster shrimp.