In Part 1 of this series, I presented some background information on the mating positions of terrestrial pulmonate snails and had a picture of a pair of mating Oxyloma retusa, a land snail in the family Succineidae. To learn more about the behaviors of this species, I've been watching and photographing mating Oxyloma retusa since Friday evening.
Glenn R. Webb published his observations of the mating behavior of Oxyloma retusa in his journal Gastropodia (1:102, 1977). I have so far observed 4 mating pairs and some details in the snails' behavior that Webb missed have already emerged.
The pair in the 1st picture started mating right at the edge of the glass plate they were on, and then the snail on the bottom slid over the edge of the plate and they ended up hanging upside down while continuing to mate.
This unusual position allowed me to get really close with the camera. In the next picture, the shell of the snail on top (which was actually the snail on the bottom before they flipped over) is not visible and the shell visible at the bottom belonged to the snail that was actually on "top". The arrow points at the spot where their genital openings were facing each other.
Olympus E-500 with a 35 mm Zuiko lens + 25-mm extension tube; 1/160 s, f13, ISO 160; light was from an Olympus FL-36 flash; both the camera & flash were hand-held.
Notice how close the snails' bodies were to each other at the genital openings. I can't tell by looking at this picture (the actual high resolution image enlarges to a much larger size) if the mating was unilateral or reciprocal. In unilateral mating, only one snail, usually the one on top, inseminates the other, while in reciprocal mating, both snails inseminate each other (remember they are hermaphrodites). I even examined another pair in a similar position under the stereomicroscope and still couldn't tell how many penises were taking part in their mating.
Webb, on the other hand, claimed that he could tell what I couldn't.
The the animals relaxed and the two penises could be seen to conjoin the pair in reciprocal coitus.There are other details in my observations that differ from those of Webb. I need more data, however, before I can decide that what I am seeing are consistent behavioral traits.