L’Hippocampe was one of the first films containing footage shot underwater and is also the first about this animal. Painlevé used the first autonomous underwater breathing apparatus invented by Yves Le Prieur, a waterproof case that allowed him to manipulate his 35mm camera and a pair of fins invented by Louis de Corlieu to carry out the underwater shots. This underwater equipment was the most revolutionary of the moment, even though it allowed him to shoot only a few seconds underwater. The film presents the seahorse in its natural and artificial environment, as well as the peculiarities of this animal: its vertical position, unique among the fish, and its reproduction process that defies conventional roles. It is the male who receives the eggs from the female, fertilizes them, carries them in his womb and feeds them. After thirty days, the male seahorse gives birth in a very dramatic way.