Marsupial frogs

Female Brazilian tree frog (Flectonotus pygmaeus) with
brood pouch enclosing the developing embryos
(Mauricio Rivera Correa ShareAlike 2.5)

What happens when frogs abandon their amphibious lifestyle for a more terrestial one? Frogs in the Family Hemiphractidae from South and Central America have evolved some ingenious solutions.

In hemiphractids, the embryo develops on the back of the mother either in a mucous-filled depression or in a closed pouch - as shown above for a Brazilian tree frog (the embryos are under the bumps).

Froglet of a marsupial frog (Gastrotheca ovipera) showing
the external gills. From Nathan 1932 (here)

The embryos may develop into tadpoles and be released to water-filled cavities in plants or skip the tadpole stage and develop directly into froglets. In species of the genus Gastrotheca, embryos have 1-2 pairs of external gills that serve for respiratory gas exchange with maternal tissues in the brood pouch. This would satisfy most people's definition of a placenta. The gills are shed around the time of birth.

Marsupial Frogs by William E. Duellman 2014
Johns Hopkins University Press ISBN 978-1-4214-1676-5

The biology of marsupial frogs is described in painstaking detail in this new book - the destillation of a lifetime's work by William E. Duellman (details here). It is superbly illustrated but at USD 120 a bit pricey. More than half the content comprises species accounts and no doubt it will find a place on the bookshelf of specialists. But it is well worth checking out for its insights into the reproductive biology of frogs. Who knew, for example, that the oocytes of Flectonotus pygmaeus have up to 2000 nuclei reduced during oogenesis to a single one?

For other fun facts on frog reproduction see this video.