Nature created the fae to protect the natural world. And Fate created mermaids to create and protect the seas. Sereia was the first of her kind, the first mermaid that fate created. She was born to create the oceans and she did just that. She cried tears and those tears formed a pool. This pool was where all oceans sprang from. The pool overfilled and the first ocean came into being and around it, she used her powers to create the underwater kingdom of Astoria.
From there she took vials of the tears and placed them in the places that her father told her too. Taking the tears and dropping them onto the ground new bodies of water springing up. And these became the oceans, lakes, rivers, streams and other such places on the world known as Asmiara. Her father was a controlling man or creator as some might say. And as such when she decided to create more of her kind to help her guard the waters of the world, she created only females.
Using seeds placed in crystals she would drop one tear from the pool, the pool that never runs out and is where all mermaids return to ensure their health and longevity once a year. Mermaids are also said to carry a vial of tears with them on land for its healing properties. The pool is the strongest healing place, however, tears can heal small things on land and can help a mermaid if need be. Time, however, will lessen the effect the more time the tears spend away from the pool the less effective they will be.