In the 1st part of this series, using some of the main literature references, I gave a brief description of how the slug Limax maximus mates.
One steamy August nite last summer, while checking out the evening creatures in my backyard, I saw 2 L. maximus going single file up a pine tree. I had never seen L. maximus mate before, but because I had read about it, I realized immediately that this was how it was supposed to start. Someone once said that chance favors the prepared mind; that was indeed true my case (having a camera at hand was also quite helpful).
Luckily the slugs didn't climb high, but stayed at a height that was very comfortable for picture taking.
After their climb ended, they started circling around each other.
Such circles always rotate in a clockwise direction, because the genital openings of dextral slugs are on the right sides of their heads. If they were rotating in a counterclockwise direction, their genital openings would not face each other.
This was followed by a sudden embrace and the twisting of their bodies around each other.
While they were rolling down the trunk, one slug had already everted its penis and the tip of the other slug's penis was visible.
Soon, their penises were fully everted and intertwined. Here is a close-up of their tips.
Finally, the penises expanded into a bell-shaped structure with a cool blue color. I suspect this is when they actually exchange spermatophores (cases of sperm).
The bell was present for only about 3 minutes and was followed by the withdrawal of the penises. You can see the tips of the disappearing penises of the still intertwined slugs in the next picture.
It was all over when one of the slugs quickly left the scene of their affair. But the other one remained behind and ate up the copious slime that they had secreted. The entire process, from the moment I took the 1st picture until the picture below, took 43 minutes.
The mating of L. maximus and several other species in the genus Limax, appears to be a very resource intensive way of exchanging sperm by a couple of hermaphrodites. Why not just do it on the ground like most other slugs? In fact, Limax flavus indeed mates on the ground.
Without elaborating, Leonard (2006) suggested that the mating behavior of L. maximus may have evolved under sexual selection. A talk by Gerhard Falkner & Barbara Klee at the World Congress of Malacology in Antwerp last July also discussed the possibility of sexual selection as the driving force behind the evolution of long penises in some members of the genus Limax. There is so much more to learn about these slugs. I can hardly wait until the next mating season!
Here is another set of pictures of mating L. maximus over at A Snail's Eye View.
The next part of this series will take a look at the genitalia of a dissected L. maximus.
Janet L. Leonard. 2006. Sexual selection: lessons from hermaphrodite mating systems.
Integrative and Comparative Biology 46:349-367. pdf