The Shell Thrones are where the mermaid princesses are born as a child. Later on, after the princess reaches an appropiate age, they will sit on their shell throne once again to take on their duties as the Mermaid Princess of their sea. This happens at the 'coming of age' ceremony in order to get in touch with Aqua Regina.
In each kingdom, there is one shell throne, each in a different colour fitting for their princesses and their own mermaids:
- North Pacific Ocean Castle - Lucia's kingdom has a pink shell throne
- South Pacific Ocean Castle - Coco's kingdom has a yellow shell throne
- North Atlantic Ocean Castle - Rina's kingdom has a green shell throne
- South Atlantic Ocean Castle - Hanon's kingdom has an aqua (blue) shell throne
- Arctic Ocean Castle - Noel's kingdom has an indigo (deep blue) shell throne
- Antarctic Ocean Castle - Karen's kingdom has a purple shell throne
- Indian Ocean Castle - Sara's and Seira's kingdom has an orange shell throne
When a Mermaid Princess is about to be born, the essences of the ocean will gather around her shell throne. They appear as spheres containing all emotions that a young princess needs, like love, friendship, and the ability to believe. Those spheres, together with the spirit and the pearl of the new princess, will become the Mermaid Princess' heart. In order to complete the birth not only the spirit of the new princess, but also the fragments of her heart contained in the ocean's essences are needed. When those come together the new princess will be born. After the essences of the ocean and the spirit of the princess have come together, the princess' pearl has to be given to the princess to complete her birth in the manga. In the anime, it's the exact reverse: First the pearl is needed, then the essences of the ocean and the spirit of the princess have to gather. This process is shown with Seira's birth, just before she was aborsbed into Mikeru.
After a Mermaid Princess reaches an appropiate age they will have their coming of age ceremony. In order to contact with Aqua Regina they have to sit in their shell throne once again, and open their heart up to her.