Male pipefish not only carry fertilized eggs — like their relatives seahorses; they also absorb nutrients from them, essentially cannibalizing their offspring.

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Broad-nosed pipefish swim vertically, like their relatives the seahorses. ((University of Gothenburg))

Gry Sagebakken and her colleagues at the University of Gothenburg found that "pregnant" pipefish, males with fertilized eggs in their brood pouches, sometimes absorb embryos.

After mating, the female broad-nosed pipefish deposits about 100 fertilized eggs in the male's brood pouch, located on its tail.

The males have a network of blood vessels inside the brood pouches that act like a human placenta. Sagebakken, a doctoral student at the university's zoology department, found that while the nutrients usually flow from the father to the young, they can go in the other direction, too.

Sagebakken's colleague and co-author Ingrid Ahnesjo noticed in a previous study that certain pipefish had a lower number of offspring than would have been expected. The scientists thought that either the father or the siblings must be eating the missing eggs.

To trace where those lost offspring were going, the researchers injected female pipefish with proteins that included the radioactive isotope carbon-14.

They found that some of the radioactive isotope ended up in tissues in the father's brood pouch, muscles and liver, and not in the other siblings.

"The male has about 100 embryos in its brood pouch, and anything from zero to all the eggs may be absorbed," said Sagebakken in a statement. "In this way, the fathers are able to use their children to improve their own well-being."

The absorption of nutrients from the young, also called filial cannibalization, benefits the father but also might benefit the remaining offspring by making the father more fit.

"Since caring males may lose weight and fat reserves while caring, brood reduction may actually enhance parental fitness if males can use some of their own offspring as a nutritional resource," the authors wrote.

The research appeared this week in the Proceedings of the Royal Society B.

The researchers say this is the first time that males have been shown to absorb nutrients from embryos through the brood pouch. Cannibalistic infanticide, or savaging, has been observed in other fish and in mammals such as pigs and hamsters.