STRIKING that happy balance between giving and receiving in a relationship can be fraught with difficulty. But not, it seems, for hermaphrodite sea slugs. These gentle soft-bodied animals, blessed with both male and female genitalia, solve the battle of the sexes by engaging in 鈥渟perm trading鈥�.
They donate sperm only on the condition that they receive it, so thwarting the male desire to fertilise and run. During sex, each slug inserts its penis into the other and one transfers a small package of sperm. The transfer of further sperm will only proceed if the other partner reciprocates by transferring a package of its sperm.
That hermaphrodite sex worked this way was suggested 20 years ago but this is the first time it has been demonstrated. Nico Michiels and colleagues at the University of T眉bingen, Germany, sealed off the sperm ducts of Chelidonura hirundinina sea slugs so that they could insert the penis but not transfer sperm.
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In 57 staged sexual encounters, sea slugs paired with a 鈥渃heating鈥� partner, unable to transfer sperm, were more likely to abandon sex than animals paired with a 鈥渇air trader鈥� (Current Biology, vol 15, p 792).
鈥淚 expect that sperm trading is widespread in hermaphrodites,鈥� says Michiels. 鈥淭hese sea slugs have found a way to optimise sperm transfer so that both partners benefit.鈥�
