If you’re a hermaphroditic land snail looking for love, it’s better to be on the giving end of a stabbing.
When any of several species of land snails get together to reproduce, a snail drives a hard, sharp object inaptly named a “love dart” through the body wall of its partner during the mating process. The dart doesn’t deliver sperm; that transfer happens in a less violent way. Instead, the love dart transfers mucus from a dart sac into the other snail’s blood.
Scientists who have studied love darts have found that for the stabber, there are definite advantages — the practice can help a snail win out in the sperm race, increasing the likelihood that it, and not another snail, fathers the stabbed snail’s offspring. But the effects of this practice on the snail that gets stabbed hadn’t been studied. So Kazuki Kimura and Satoshi Chiba of Tohoku University in Sendai, Japan, set out to document those effects in Bradybaena pellucida land snails, which are native to Japan.
The key to the study was virgin snails. That’s because when two of these snails mate, they act as male and female simultaneously. Both transfer sperm. Both receive sperm. And both produce love darts that they ram into their significant other. The one exception is during a snail’s first tryst. Those virgin snails are love-dart free, except as a receiver. Only after that first mating will they produce a love dart.
The virgin snails let Kimura and Chiba look at what happened to the darters and darted in the weeks after a mating until the snails died. In the days after the big event, the stabbed snails laid fewer eggs than snails that hadn’t been darted. The difference wasn’t big, but it was enough that it lowered the snails’ overall reproductive rate. The stabbed snails also didn’t survive quite as long as the stabbers.
Harming your partner would seem like a bad idea, especially if that harm has long-term effects. But for the snails, the stabber gets a better chance at fathering offspring. That drives an arms race of sorts, propelling the evolution of darts of different shape and shooting behavior, the researchers note March 11 in the Proceedings of the Royal Society B.