This National Geographic video is about the Knysna seahorse.
From Scientific American:
The Mr. Moms of the Fish World
By Adam Marcus
Male pipefish, seahorses and their kin are the stay-at-home dads of the fish world, rearing their young in placentalike pouches from the time they are fertilized eggs until they can swim away.
New research shows that these involved fathers not only shelter their young but transfer key nutrients to their offspring via their own versions of a placenta, helping to supplement what the embryos received from their mother in the egg yolk.
“In this study, we clearly demonstrate embryonic uptake of paternally derived nutrients in two pipefish species,” says researcher Jennifer Ripley, a biologist at West Virginia University in Morgantown (W.V.U.). “This is the first time we actually have evidence for this placental-like role in these fishes. It has been hypothesized for years but not demonstrated until now.”
To test whether males of two pipefish species, Syngnathus fuscus (northern) and S. floridae (dusky), pass nutrients to their young, Ripley and her colleague Christy Foran, a biologist at W.V.U., injected brooding fish—those carrying embryos in various stages of development—with the lipid (fat) palmitic acid and the amino acid lysine, two important embryonic building blocks. The substances—both of which are essential for development and must be supplied by the dad if egg reserves are not sufficient—were tagged with isotopes (a traceable version of an element) that allowed the researchers to track them with spectroscopy from father to embryo.
Seahorse video: here.