Think about it. The conch could be used as a weapon? So are dozens of items that you are allowed to carry in your carry on.
Many years ago my wife and I were in St. Eustatius scuba diving. The guide book mentioned a place on the island where conch fishermen would punch a small hole in the conch, push out the meat and throw the empty shell onto a pile of other conch shells. We drove to this place which was desolate and sure enough we saw 2 mountains of conch shells just there for the taking. We took maybe 3 or 4 of the best ones and brought them back with no problems. While this was pre 9/11 it was not pre hijacking.