* in earthworms
Earthworms are hermaphroditic, yet they still have to find mates. Although each worm produces both sperm and ova, a worm cannot self-fertilize. Two worms must find each other (not an easy task), exchange sperm, then later use the partner’s sperm to fertilize its own eggs.
Today my daughters noticed two earthworms in a crack of our driveway, in flagrante delicto. I grabbed my cameras, and caught a few good images. My daughters were fascinated by what they saw, and a bit troubled that their father was taking pictures of animals engaged in sex.
Here’s what was going on. The worm secretes sperm from pores on Segment 15, and can store sperm from another worm in receptacles on Segments 9 & 10. Its own ova are stored in oviducts that open on Segment 13.
Once they meet, the two worms line up in opposite directions with their ventral surfaces touching; Segments 9 and 10 of each worm are aligned with the clitellum of its partner. Setae (little hair-like structures that serve as anchors during locomotion) are extended to penetrate the body of the mate, holding the worms together. Mucus is also secreted in abundance, helping to secure the worms, and providing a moist environment for the transfer of sperm. Sperm released from each worm is carried via an external seminal groove to the seminal receptacles of the partner; after an hour or so the two worms part ways.
Later the clitellum secretes a thick mucous “cocoon,” from which the worm wriggles out backwards. As the cocoon passes over the oviduct an egg or two are released into it; then as it passes the seminal receptacles stored sperm is released into it.
Once the worm has crawled fully out of the cocoon the ends of the cocoon seal, the outside hardens, and the egg, now fertilized, develops into a young earthworm that will emerge from the cocoon in anywhere from 3 weeks to several months (perhaps longer), depending on the moisture and temperature of the soil.
I relied on these sources for information used here – I do not know this material myself:
Worm sex in cross-view 3D:
This train of thought has been percolating in my mind all day – I’ll try to make it coherent. Sexual reproduction, that is, the need to exchange genetic material with a partner, must be very advantageous to the earthworm. The sexual anatomy is such that self-fertilization would not be difficult. When the worms copulate and exchange sperm, the partner’s sperm is stored a few segments in front of the opening from which the worm will deposit its egg into the mucous cocoon. The worm releases its own sperm from openings a few segments behind this female opening from which the egg is released; its sperm then travel externally to the storage receptacles on its partner. How hard would it be (have been) for the worm either to release its sperm to its own storage receptacles, or instead simply to release its own sperm directly into the mucous cocoon? This would avoid the risk associated with coupling with another worm in a relatively open setting (certainly more open that the worm’s burrow) for a period that typically lasts more than an hour. Clearly the benefit gained from the exchange of genes must outweigh the risk inherent in copulation.
And one more thought: one reason given for the inability to self-fertilize is that sperm and egg mature at different times, with sperm maturing first. I don’t understand this argument, as during sexual reproduction both worms allegedly use the partner’s sperm to fertilize an egg shortly after copulation; if the sperm that one worm released is not ready for prime time, surely the sperm that it received is equally immature. Something about this doesn’t ring true. Feel free to email any comments to me at firstname.lastname@example.org.