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.

Although the pipefish embryos did not take up much of the lipid, for reasons that are not clear, Ripley says, the amino acid readily moved from father to offspring. Moreover, the researchers say, the amount of amino acid that was transferred rose as the embryos matured—suggesting that as the nutritional needs of the babies grew, their fathers provided more of the substance.

Marine biologist Amanda Vincent, director of Project Seahorse at the University of British Columbia in Vancouver, says she's "very excited" by the latest findings. "There really does appear to be some sort of negotiated trade-off between the male and female contributions" to the development of the embryos, she says. She notes that it would be interesting to determine whether the position of eggs deposited into the pouches makes a difference in uptake of nutrients and embryo development.

Ripley says she hopes to explore how changes in the environment, such as the rise and fall of toxins in seawater, affect the paternal transfer of nutrients in pipefish and their kin. "We are trying to draw parallels between this pouch and the mammalian placenta so that in the future we can test emerging contaminants and chemicals," she says. "In this way, we may be able to predict the potential harmful effects [of these substances] on human development."