Some say they inspired the myth of Cupid鈥檚 arrow. And for a fellow snail at least, these little white arrows certainly do look like the perfect nuptial gift. They are white, approximately 1 centimetre in length (see above) and made of calcium carbonate, just like snail shells, so would surely be much appreciated: 鈥渓ike a box of chocolates鈥�, as a researcher put it.
Unfortunately, the real motivation behind spearing your partner during sex turns out to be much less romantic. The goal is to cheat your chosen mate and father as many offspring as possible over the ensuing months. And as has just been revealed in Invertebrate Reproduction and Development, Issue 41, page 35, earthworms may go in for body piercing for the same reason.
For the garden snail Helix aspersa, reproduction is an unusually competitive affair. Not only do you have to find and woo a partner, but after sex your sperm have to compete with sperm from other matings and win the race to fertilise the eggs.
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The act of sex itself is arduous enough. After finding a mate, courting may take up to six hours, and involves circling, tentacle touching, and lip and genital biting. Finally they get together and, being hermaphrodites, insert their penises into one another. As they live in shells, their genitals are conveniently located on their heads.
But ensuring fatherhood does not end here. This sex extravaganza may be repeated with around six partners in one mating season. Sperm from each partner have to fight their way up the reproductive system past a cavity called the bursa copulatrix. Only the fittest will arrive at their destination, a special vessel where they may be stored for as long as four years while they are gradually used to fertilise the snail鈥檚 eggs. As many as 99.98 per cent of them will die and be digested before they reach their goal, the storage organ.
This is where the love darts come into play. Their job is to fix the contest in favour of the dart-thrower鈥檚 sperm by ensuring that more of them get to the storage vessel. The darts are fired as sex begins. As they plunge into the partner鈥檚 body like hypodermic needles, they transfer a chemical called an allohormone.
This makes the female part of the reproductive system contract, sealing off the entrance to the bursa copulatrix, and diverting the sperm straight to the storage organ. A snail that has been speared will store twice as much of its partner鈥檚 sperm as a snail that has not.
Allohormones can clearly be effective. So it wouldn鈥檛 be surprising if snails were not the only creatures to use them. Earthworms 鈥� also hermaphrodites 鈥� have now been shown to pierce their partner with a row of 40 needle-like setae during mating. And they, too, inject a substance into them, though its effect is not yet clear.
One thing is certain, though. For snails and worms, mating is a tough and manipulative business.