Introduction: Fishes of the Syngnathidae family are rare in having male pregnancy: males receive eggs from females and egg development occurs in the male brood pouch that diverged during evolution. The family is divided into two subfamilies: Nerophinae and Syngnathinae.
Methods: We compared histologically five types of the brood pouch in Syngnathinae: an open pouch without skinfolds (alligator pipefish); an open pouch with skinfolds (messmate pipefish); a closed pouch with skinfolds (seaweed pipefish); and closed pouches with a sac-like pouch on the tail (pot-bellied seahorse) or within a body cavity (Japanese pygmy seahorse).