Mermaid's Forest (2003)

According to an ancient Japanese legend, mermaid flesh may grant immortality if eaten. However, there is a much greater chance that consumption will lead to death or transformation into a damned creature known as a Lost Soul. Mermaid Saga tells the tale of Yuta, an immortal who has been alive for five hundred years. Throughout the series, he wanders across Japan searching for a cure and meets others whose lives have also been ruined by mermaid flesh.