If snails used Facebook, all their relationship statuses would say "It's complicated." It's also slimy, violent, and life-shortening. Most species of snail that live on land are hermaphroditic—that is, they have a complete set of female and male sex organs. When they mate, both partners inseminate each other. The act may come after a courtship period. And in certain land snails, this courtship includes the launching of "love darts," which are much less cute than they sound. A love dart is a sharp dagger that a snail builds in its body out of calcium carbonate. Before mating, the snail thrusts this dart out of itself and straight into its partner's flesh. A love dart is not related to a penis (each snail has one of those, too) and doesn't carry sperm. It's pure weaponry. You might see a fired dart still jutting from a snail's neck while it copulates, as in the right-hand snail below:
Love darts are coated in mucus, like poison-tipped arrows. In some species, scientists have discovered that this mucus acts on the female organs inside the snail receiving the dart. It seems to encourage the female parts to store the sperm they're about to receive. This means love darts may help potential snail fathers edge out the sperm from all the other snails their partners have mated with. So stabbing one's mate with a love dart is in the best interest of the sperm donor, but not necessarily the sperm recipient. It give the sperm donor a paternity boost instead of letting the recipient pick and choose which sperm it stores. It's also quite violent. "In species bearing a relatively large dart, snails retract [their bodies] rapidly when they are stabbed, suggesting strong pain," says Kazuki Kimura. A researcher at Japan's Tohoku University, Kimura wanted to find out whether love darts truly hurt their recipients—not just in the moment, but over the rest of their lives. Kimura gathered young Bradybaena pellucida snails from the wild in Japan. Once the snails were sexually mature, he began to play matchmaker. He let 50 snails mate with 4 partners each, a fairly low number for the active B. pellucida. In one more quirk to this snail's mating system, it only makes love darts after the first time it mates. So virgin snails don't shoot darts. That meant Kimura could arrange the hookups such that half his snails never got shot with a love dart, because they only mated with virgins. The other, less lucky group of snails mated with experienced partners and got shot every time. After the mating sprees, Kimura monitored the snails for the rest of their lives (only a few months). He counted the number of their eggs that hatched. And he found significant differences between the two groups: snails shot with love darts had fewer offspring and shorter lives. These slimy sex weapons do serious harm to snails. If any snail could avoid getting darted, it might live a longer and more productive life. But using love darts seems to be non-optional in this species, Kimura says. The snails' male halves are trapped in a competition with their partners' female halves so extreme that it shortens all of their lives. Kimura says he's now studying how such a hard weapon evolved in such a soft and squishy animal. He means the calcium carbonate dart itself—though the snail's attitude toward its lovers is pretty tough too.
Images: Top, "Weinbergschnecke Paarung" by Jangle1969 at the German language Wikipedia. Licensed under CC BY-SA 3.0 via Wikimedia Commons. Bottom, "Garden snails and love dart" by Eynar - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons. NOTE: Neither picture shows the study species.
Kimura, K., & Chiba, S. (2015). The direct cost of traumatic secretion transfer in hermaphroditic land snails: individuals stabbed with a love dart decrease lifetime fecundity Proceedings of the Royal Society B: Biological Sciences, 282 (1804), 20143063-20143063 DOI: 10.1098/rspb.2014.3063