*Warning: may be outrageous overstatement for sake of alliterative headline
Garden snails (Cantareus aspersus) are sado-masochists. During mating, they whip out darts made of calcium carbonate and stab each other through the nearest bit of flesh. Although it sounds unlikely, this behaviour helps in fertilisation.
Garden snails (and most other land snails) are hermaphrodites—Arthur and Martha—and indulge in simultaneous reciprocal mating. Both individuals in a pair swap sperm during copulation and use it to fertilise their eggs. Sperm is transferred in packages (spermatophores), which are placed in the partner's bursa copulatrix.
You might think it was safe, all neatly bundled in a spermatophore and tucked away in the right place. But it isn't. The bursa copulatrix is a dead end off the reproductive tract. It releases enzymes that digest the sperm. If the little fellas don't wriggle fast enough, they are recycled into nutrient.
But the dart helps to slow down the enzymes. Before it is stabbed into the partner, it is coated in mucus that retards the digestive process. This means that more sperm escape and make their way along the reproductive tract to fertilise the ova.
Although the garden snail is the most-studied species, it isn't the only one that uses a dart in mating. Among the others are the Bradybaenidae, including the Japanese Euhadra subnimbosa. Copulation in this species has left those studying it almost speechless. Forget the sluggard garden snail, which discards the dart after a single use. Euhadra subnimbosa stabs its partner 'a staggering 3311 times' on average. That's just over 2.5 times per second. Even Bradybaena similaris, the increasingly accurately named tramp snail, stabs its partner 900 times during copulation. Sometimes the action is so violent, the dart passes right through the body and emerges from the foot.
A snail's pace, indeed.
Authors Joris Koene and Satoshi Chiba suggest that the repeated stabbing and associated transfer of mucus improves the chances of the sperm getting to their intended destination. If the strategy works well in the garden snail, it should be even more effective in Euhadra and relatives.
They have even captured it on video. (Not surprisingly, you need a subscription to the journal to view the flick.)
Read more (if you dare)
Koene, J. & Chiba, S. (2006). The way of the Samurai snail. American Naturalist 168 (4): 553 – 555. (PDF about 850kB)
