This tale follows on from that piece on leopard slug courtship from a few days ago. I commented then that copulation in garden snails is generally preceded by (among other things) pushing 'darts' into each other's bodies. There've been various explanations for this odd behaviour (I mean, it sounds painful!), including the suggestion that the dart acts as some sort of 'wedding present' (nuptial gift), which might make the pierced partner more inclined to mating. Or that it indicates how ready the dart-shooting snail is to mate. But data from a 2001 study (Pomiankowski& Reguera) suggests another reason for this behaviour.
Snails have quite intimate, elaborate courtship rituals that involve a lot of close physical contact before actually mating. After about 30 minutes of mutual stimulation, one snail pushes a sharp pointed dart into the other. (This is often described as 'shooting', but it isn't - it's more of a hard push.) The darts aren't essential for copulation - virgin snails don't have darts, but still mate successfully. (As do snails that miss the mark - apparently around 33% of darts either don't hit the partner at all, or fail to enter their body.) So why go to the trouble of making darts (which aren't re-used, so an amorous snail must be constantly making new ones)?
It seems that the dart carries mucus along with it, & this mucus seems to paralyse the partner's female reproductive tubing. This lets more sperm make it to the sperm storage organs, where they're stored until needed to fertilise the eggs. This is important - when garden snails (Helix aspersa) mate they produce & pass to their partner a spermatophore containing 1-10 million sperm, but only about 0.025% survive in the partner's female reproductive tract (Pomiankowski & Reguera, 2001). Most of them end up in the no-return area of the bursa copulatrix, where they're digested & absorbed. But in a study of mating pairs, virgin snails that were firmly pierced by their partner's dart contained twice the stored sperm of non-stabbed virgins. And yet successful shooters appeard to transfer fewer sperm than did unsuccessful shooters. This suggests that successful shooters can afford to reduce the amount of sperm transferred because the penetration of dart mucus ensures a higher rate of sperm storage. Koene & Schulenburg (2005) suggested that this may well lead to something of an arms race between the manufacture of a 'love dart' that maximises the shooter's success, and the female spermatophore-receiving organs (because the 'female' partner's reproductive success may benefit by using sperm from as wide a range of partners as possible).
But there's a lot we don't know about the finer details of snail reproduction. For example, snails may vary in how their female tracts respond to the paralysing mucus. And what's the story in those snail species that don't shoot their partners during foreplay? Hard questions to answer...
A. Pomiankowski & P. Reguera (2001) The point of love. Trends in Ecology & Evolution 16(10) 533-34
J.M.Koene & H.Schulenburg (2005) Shooting darts: co-evolution and counter-adaptation in hermaphroditic snails. BMC Evolutionary Biology 5:25 doi:10.1186/1471-2148-5-25