One morning I noticed two worms stuck together on the ground. Before I got a good eyeful, my dog rudely interrupted them and they hastily retreated into their burrows. Were they soil mates discussing Nietzsche or niches over a cup of dirt?
It didn’t take much burrowing into the interwebs to realize they had been dirty dancing, earthworm style. I don’t know what species of earthworm they were, but according to my research, most have been around since the last Ice Age and are generally hermaphrodites—employing the services of both male and female reproductive bits.
Being hermaphroditic enhances the chances of finding a suitable mate for creatures like worms, with limited access to partners or the internet. And they may prefer a partner because mixing genes is a good way to hedge genetic bets in ever-changing environments. Even so, self-fertilization (parthenogenesis) may be more common than generally reported.
Mature earthworms prefer to mingle on warm (around 13°C), damp nights in the fall or spring, which in Vancouver could mean February. Worms cannot see, but can sense the good vibrations of a soil mate nearby. They reach out headfirst to touch somebody, either going all out or still keeping their butts covered.
Perhaps they exchange pleasantries, but more importantly, they exchange sperm. They press segments 9, 10 and 11 of their bodies against the clitellum (the pink band) of their partner. Whitish glands in these segments release sticky stuff and special long bristles (copulatory chetae) anchor their bodies together and stimulate their partner. Each worm produces a tube of mucus that keeps the sperm streams separate as they travel from one partner to the two pairs of sperm receptacles of the other. Once each has had its fill, they return to their separate solitudes without so much as a text message.
For months after mating, a worm produces cocoons (egg capsules) in which eggs become fertilized and develop. Like a riddle wrapped in a mystery wrapped in an enigma, the worm secretes another mucous tube that covers the clitellum and the male and female openings. Within this tube, the clitellum secretes the cocoon. About a dozen eggs pass from the female openings into the cocoon. The worm pushes the cocoon forward between the mucous tube and its body. At the sperm receptacles, the cocoon collects sperm to fertilize eggs within the cocoon. The worm pushes the cocoon over its head until the cocoon comes off and the elastic ends close up. It darkens and hardens into tiny lemon-shaped capsule about 5 mm long.
The cocoon contains albumin that nourishes the developing embryos. The cocoons are deposited below the frost line where it will be warmer. After about two weeks, if it is warm enough and if the right kinds of bacteria are present in the soil, one or more mini-worms hatch, depending on the species. If the conditions are not right, it will remain dormant. Once hatched, it’ll take about six weeks before they’ll be ready to get busy on their own.