Why Is Shirahoshi Important?

It appears that Shirahoshi can summon them through her will and feelings even though she wasn't directly calling for them. This ability is what classifies her as one of the Ancient Weapons that the World Government fears so much; she is the current incarnation of the weapon Poseidon.

What kind of mermaid is Shirahoshi?

Princess Shirahoshi, also known as the Mermaid Princess, is a giant smelt-whiting mermaid and the youngest of King Neptune's children.

Will Shirahoshi destroy the world?

Shirahoshi is the reincarnation of the Ancient Weapon Poseidon and with it, she can control all the Sea Kings of the world. She is known to have enough power to destroy the entire world if she wishes to.