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Baby fish grow up differently depending on the liquid their father鈥檚 sperm swam in. The finding shows that fathers can influence their offspring through chemicals in the semen, as well as through the sperm themselves.

鈥淭here is something in the seminal plasma, maybe some non-genetic factor, that is modifying the offspring,鈥� says Jukka Kek盲l盲inen at the University of Eastern Finland in Joensuu.

Among sexually reproducing species, males produce sex cells called sperm that are carried in a liquid called semen. The sperm merge with eggs produced by females, which then develop into embryos.

Kek盲l盲inen and his colleagues studied a freshwater species called European whitefish (Coregonus lavaretus). Females lay eggs, onto which the males release sperm. Because fertilisation takes place in open water, the team was able to separate the effects of sperm and semen.

They collected semen from 10 males and modified it: some samples had the liquid removed, while others were mixed with liquid from other males, and some were unchanged. The team then used the samples to fertilise the eggs of five females, in all possible combinations, and tracked the offspring鈥檚 progress.

Semen secrets

They found that the embryos hatched significantly earlier if a male鈥檚 sperm was mixed with seminal liquid from another male聽rather than if the sperm and seminal fluid both came from the same male.聽What鈥檚 more, those offspring were able to swim against a current for longer. In contrast, removing the seminal liquid entirely made the embryos less likely to survive.

The results show that seminal fluid plays a crucial role. 鈥淚t鈥檚 not just a medium for the sperm,鈥� says Kek盲l盲inen. 鈥淚t can have important effects on the next generation.鈥�

The finding fits an emerging body of evidence for the , says Kristin Hook of the University of Maryland in College Park. 鈥淲e know in other animal systems, there are components of the ejaculate that are under immense selective pressures to be different and varied,鈥� she says. 鈥淚n flies, males have components of their ejaculate that allow them to control the females鈥� interest in re-mating.鈥�

If some males have better than others, that could encourage females to mate with multiple males, says Hook. 鈥淭here鈥檚 an enhancement effect of the seminal fluid from certain males,鈥� she says, and females may want to ensure .

However, it isn鈥檛 clear what components in the seminal fluid are affecting the offspring.

There are many possibilities, says Leah Houri-Ze鈥檈vi of Rockefeller University in New York. She points to showing that .