For simultaneous hermaphrodites – equipped with both male and female reproductive organs – reproduction is more complex than the conventional ‘boy meets girl’ story. For a start, each protagonist can fertilize the eggs of the other. Common garden snails, Cornu aspersum, seem to improve their chances of becoming fathers – and this might not sound terribly romantic – by spearing their partner with a so-called ‘love dart’, smeared in mucus. Once in the partner's blood stream, the mucus improves the chances of the biological father's sperm successfully fertilising their partner's eggs by closing off the entrance to a structure that digests unused sperm and causing the sperm to be pumped into a storage organ for later use. But it wasn't clear whether this pragmatic approach to reproduction was peculiar to the common garden snail or whether other species also took advantage of the love dart's benefits. To remedy the situation, Kazuki Kimura and Satoshi Chiba from Tohoku University, Japan, working with Joris Koene from Vrije University, The Netherlands, investigated another species equipped with love darts, the distantly related Japanese Euhadra peliomphala snail, to find out whether the love dart mucus that they inject into their partners also provides a fertility advantage (p. ).
Having collected snails from a nearby local park, the team extracted the glands that produce the love dart mucus from some of the animals and produced a mucus gland extract to test on the remaining snails. They then simulated the love dart stabbing action of an ardent snail with a syringe needle coated in the extract before injecting a minute volume of coloured water that mimicked semen into the snail's genital tract to find out whether the mucus gland extract altered the fluid's fate.
The team tested whether any of the fluid made it into the organ that digests sperm and they found that it was closed in the snails that had been injected with the mucus gland extract. In a real-life situation, the mucus would have saved the semen from destruction, and Kimura and colleagues say, ‘This indicates that in various dart-bearing species the mucus from the dart glands targets the same organ’.
So, having added another species to the list of simultaneous hermaphrodite snails that improve their chances of becoming fathers through the use of love dart mucus, Kimura and colleagues wonder whether these hermaphrodites benefit from sexual selection. ‘Because of the relatively limited literature on sexual selection in hermaphrodites, it was unclear how important sexual selection has been for the evolution of their reproductive behaviours’, they say. However, the fact that love dart mucus targets the same organ in distantly related snails suggests that simultaneous hermaphrodites do benefit from sexual selection. The team adds, ‘This result supports the idea that this conspicuous reproductive trait of land snails had evolved through a conflict over sperm digestion between sperm donors and recipients’.
