Unlikely romances between humans and mermaids have been the basis for several anime films, like Hayao Miyazaki’s “Ponyo” and Masaaki Yuasa’s “Lu Over the Wall.” Now, Hans Christian Andersen’s “Little Mermaid” tale gets its latest animated spin in “ChaO,” a new film from Studio 4°C that opens in Japan Aug. 15 after premiering in June at the Annecy International Film Festival in France, where it won the Jury Award.The romantic comedy takes place in a near-future Shanghai where humans live in relative peace alongside the merpeople who populate the city’s canals and surrounding seas. Its protagonist is Stephan (voiced by Ouji Suzuka), a mild-mannered engineer who works at the city’s top manufacturer of maritime vessels. One day, Stephan meets a mermaid princess named Chao (Anna Yamada), who is instantly smitten with Stephan, claiming they’ve met before, though he has no idea what she’s talking about. Sensing a public relations coup for his company (a shipbuilder married to the daughter of a mer-king!), Stephan’s boss (Ryota Yamasato) pushes his young employee to marry Chao. Even though he grumbles about the need to wed “a talking fish,” the meek Stephan nonetheless goes through with the nuptials. The unlikely pair then begin a crosscultural coupling that brings challenges as Chao navigates life above the sea and Stephan slowly unlocks the repressed memories of when he first met his bride.“ChaO” was directed by Yasuhiro Aoki, 56, a veteran animator and director of episodes of properties like “Batman: Gotham Knight” and “Tweeny Witches” in his feature film debut. For “ChaO,” which took nine years from conception to release, Aoki was tasked by Studio 4°C producer Eiko Tanaka (“Tekkon Kinkreet,” “Children of the Sea”) to create a “totally new” original anime film.