Dictionary of Ichthyology  

marsupium = the name applied to the brood-pouch in Syngnathidae and Solenostomidae. In Syngnathidae it consists of a vascularized groove formed by flaps of skin along the underside of the tail of males (subcaudal marsupium); in Solenostomidae it is a pouch formed by the pelvic fins, provided with many long filaments, and found only in the female (ventral fin marsupium.
Hippocampus abdominalis, New Zealand, Porto Bello, Otago Peninsula, South Island, CMNFI 2000-0047, 87 mm from top of head to tail curl. Photograph by Brian W. Coad.

           Canadian Museum of Nature