Board Thread:General Discussion/@comment-25850402-20150723062747/@comment-24674051-20161017143714

Which we did see a form of the seashell communication magic in season 3. In "Nasty Habits", Neal uses a shell to summon a giant squid. and again, in Ariel, Regina uses a shell to summon Ariel (specifically Ariel, since Regina was expecting her to show up). So I'm not 100% certain that the shell communication is limited to Mermaids, but is actually more a form of general magic that is accessible to those that know about it (generally sea people, or sea-faring people, like pirates).

The variation of this that we saw in "Strange Case" though, does tend towards specialized magic, since it not only involved a sending shell but also a receiving shell (which Killian carried).