Merillia is the life force of the ocean in the Barbie in A Mermaid Tale movies. The ability to create it is granted at exactly midday during the Changing of the Tides Ceremony, which takes place in the underwater city of Aquellia.

If the person who spins Merillia is upset, then their Merillia becomes weaker and weaker. Without a change, the Merillia slowly poisons Oceana. When Queen Calissa was imprisoned by Eris, her Merillia was not strong enough for the sea to remain healthy. It only improved when Merliah released her.

Merliah gets distracted when at the end of the movie, she has gotten the power to make Merillia