Sluggin' Along: Some Enchanted Evening: Mating Three-toothed Snails

Though I've frequently witnessed my Three-band Slugs (Lehmannia valentiana) mating and have learned quite a lot about their reproductive behavior, I've yet to observe a mating pair of Three-toothed Snails (Triodopsis sp.). A few nights ago I got my wish: a couple came together on the cross-tie right under the lantern in the backyard. Now, I'll need to do more research and observation before I understand all the facets of this silent, fascinating act, but here are my initial notes.

Like slugs, snails are hermaphroditic, meaning they possess both male and female sex organs and thus can produce both eggs and sperm. In a similar manner to Three-band Slugs, Three-toothed Snails circle one another during the mating process. However, the snails do so continuously during actual copulation, while the slugs usually coil around each other in place, reserving the circling to the courtship phase. Just before copulation, the Three-band Slugs extrude their penial appendix. Three-toothed snails, on the other hand, appear to go ahead and evert the penis, inserting it into the partner's gonopore and vagina to transfer sperm.

Here is a helpful diagram showing the general morphology of a snail's reproductive system:

D - love-dart

S - stylophore or dart sac

MG - mucus glands

P - penis

EP - epiphallus

FL - flagellum

BTD - bursa tract diverticulum

BT - bursa tract

BC - bursa copulatrix

SRO - spermatophore-receiving organ

SP - spermathecae, sperm storage organ

FP - fertilization pouch (FP)

AG - albumen gland

G - genital pore

HD - hermaphroditic duct

OT - ovotestis

PRM - penis retractor muscle

SO - spermoviduct

V - vaginal duct

VD - vas deferens

The snails mated for a good half hour or so. I missed the courtship phase, as well as the full withdrawal, but I got some good photos nonetheless. Next time I'll know what to look for. Here you can see the penes entwined and inserted into the respective gonopores, as the snails slowly turn round and round.

These next photos are interesting, as you can see the entire penis of the snail on the right. There is an unknown bulge on the side of each penis. At this point both penes appear to be removed from the gonopores, and it looks as if they're retracting...

But no, not quite. The penes are still everted somewhat, and the bulges are touching or perhaps even stroking each other, all while the snails turn in a circle. I'm assuming the sperm has already been exchanged and this behavior is secondary. Yet its purpose is unclear to me.

Throughout this behavior, I noticed a thin, pointed protuberance (like a dagger) that kept appearing on the side of each snail's penis. Resembling a love dart, it pulsated in and out. What in the world is it? What purpose does it serve? I thought at first it might be the flagellum, but I don't know why that particular part would be protruding and retracting like that. This "dagger" isn't coming from the bulge but from the penis , and its movement is quite rhythmic. Fully extended, it's somewhat curved.

The "dagger" is bendable.

Eventually the snails retracted their genitalia completely and separated. As with the slugs, one of the snails remained behind to eat the leftover mucus.