Monday, 11 May 2009

Darts for love

I must apologize as so far my posts have been solely about insects. I hope this one will change the trend. A couple of days ago I saw the first mating Garden Snails (Helix aspersa or Cornu aspersum) of the year. It's been quite a long, cold winter and a dry spring, so, garden plants have been enjoying quite a long snail-free season. Snails have now come out of their winter dens under stones, behind tiles and in general dry places around the garden and are meeting each other ready for reproduction. Garden snails are hermaphrodite, that is, they produce sperm and eggs, but they only rarely self-fertilize, instead they mate several times over spring and summer (in my garden I've seen copulating snails from early May until the end of August. Mating is a reciprocal long-winded affair, the pair of snails exchanging sperm simultaneously. An remarkable aspect of courtship in snails is that it involves the shooting of a relatively large, calcareous, thin and sharp 'love dart' from one snail to its partner before actual copulation. The purpose of this violent affair was poorly understood and debated for a long time. The dart is not a requisite for successful mating (apparently, snails often 'miss') and not all the individuals shoot or a physical stimulant for copulation, as previously assumed. It became apparent that snails shooting their darts effectively had higher success fathering the offspring of the other snail. Recently, research has shown that the dart should be instead called 'the needle': the dart allows mucus to be injected into the other snails head: this mucus contains a chemical that inhibits sperm digestion (apparently, without the dart most sperm is digested), and therefore allows the shooting snail to father more offspring. Therefore the dart is an evolutionary result of sperm competition.
Mating garden snails, the snails on the right has been darted.
Snail laying eggs
Snail eggs

3 comments:

Les said...

Thanks for the interesting and informative post. Good pics too.

Rambling Rob said...

That's amazing - from which part of the snail does the dart shoot from, I wonder. Seems like it must have a rocket launcher concealed somewhere...

Blackbird said...

Thanks guys! The first time I noticed this was when I accidentally dislodged a couple of mating snails when moving them to a location more suitable for photos. A dart came out and that was a shock! They actually only shoot on contact and it comes from the head of the snail (and they try and shoot the head of the other too.