The Marine Mermaid's Light is the 68 episode of the series and the 12 episode of Season Pure.
Coco invites Lucia, Hanon, and Rina to come with her to the Indian Ocean to help search for the Legendary Spiral Shell. It turns out to be Sara's shell that held all her memories, and the four (also Hippo) battle the Black beauty sisters for it.
In the South Pacific Ocean Castle, Coco have a dream that she was in Indian Ocean and Sara's shell was saying gloom, hopeless, it all illusion. Coco wakes up from her dream and went to the Indian but she could'nt find the shell. Coco need help but she could not tell about the shell because she is loyal to a fault. So Coco went to the Pearl Piari and was seen by Lucia, Hanon, and Rina who just came home from school and was happy to see her again. Coco told them if they want to go treasure hunting in the South Pacific Ocean to look for the Legenday Spiral Shell that will bring wonderful happenings to anyone who finds it. The girls and Hippo agree. In the ocean, Coco is swiming faster that regular which made Lucia wondering why. Lucia then heard Seira saying "Someone hurry and find it. Please, someone find it quickly". The mermaid princess then pick up the speed to catch up with Coco but forgot Hippo and left him behind. Hippo that use a minteray to catch up but it was going the wrong way. In the Indian Ocean, the Black beauty sisters found Sara's shell and was delight for it's dark energy. The mermaid princesses realize that Hippo is not with them but they feel that he will come back. The black beauty sisters test the shell on Hippo so he swam fast to get away from the swirling hatred. Coco showed the place where the shell can be hidden and Rina know that where they are is not the South Pacific Ocean but the Indian Ocean. Coco explain that the area there in is also called the "graveyard of the sea". The princesses spilt up to find the shell but have no luck finding it. Lucia notice something wrong with Coco and asked why is she worried. Before Coco can answered Hippo screamed. Hippo told Lucia that a black vortex is following him and it was uttering evil chants. Coco figured it out before all the fish swam away in the same direction. The vortex surrounded Coco, Luchia, Hippo, Hanon, and Rina. The black beauty sisters appeared holding Sara's shell causing the mermaid princesses wonder how they got it. The black beauty sisters used the shell's power to attack the mermaid princesses. Hippo take his form as Hippocampus to try to protect and give some time for the mermaid princesses.
Lucia asked Coco to explain because what she said about the spiral shell was a lie. Coco aplogized for deciving them and told them the truth. Hippo was than overwhemed by the power and was reverted back into his peguin form. The black beauty sisters then sang their song weaken the mermaid princesses. Sara's shell then played Tarou's song letting the mermaid princess have their chance to sing their song. The sisters retreated and the shell slowing flow to Coco who hold it in her hands. The mermaid princess then see Seira in the shell. She thanks the princesses for finding the shell, thus allowing Sara's heart to rest in peace. She also reveal to be the one who gave Coco those dreams and thank the princess once again before disappearing.