Siphopteron flavum mating from Japan.

Dear Jun,
Thanks for these interesting photos. I don't know of any published information on mating in gastropterids. I think what is happening is the penis is coming out of the side of the head [see your lower right photo] and inserting in the female opening, which is on the right side of the body.

In most cephalaspideans the penis is physically separated from the rest of the reproductive system. The 'female' part of the reproductive system opens in the mantle cavity, or in cases like Siphopteron, where the mantle cavity is almost completely gone, it opens on the right side, where the last remains of the mantle cavity are found. In cephalaspids, and the anaspids [Sea Hares] there is an external groove from the female genital opening, which runs forward along the right side of the 'neck' to the opening to the penial sac on the right side of the head. The penis, prostate, and associated glands lie in this sac, inside the body except during mating, when the necessary parts are everted. The groove from the female opening carries the animal's sperm forward to the penis. During mating the penis is everted and inserted in the partner's female genital opening in the mantle cavity or on the right side of the body.

I would suspect what is happening in your photos is the penis everted from the right side of the head and is inserted in the female opening on the right side of the partners body below the visceral hump. It seems that part of the female system everts partially as well. In your upper photo the penial apparatus is narrow near the head and very wide near the other animal's visceral hump. You would expect it perhaps to taper the other way around, with the thin bit near the tip. However as Terry Gosliner's (1989) review shows, the penial complex in Siphopteron is quite complicated. As well as a tapering penial papilla there is an accsessory organ armed with spines. In fact he named one species S. quadrispinosum after the impressive set of 4 large spines it possesses. If I'm right I think the bits you can see in your photo are the female opening [fo],
and through the transparent wall of the penial sac, the muscular accessory organ with penial spines [ps]; the prostate gland [pr]; and either part of the penial papilla, or the sperm groove [pp].

Best wishes,
Bill Rudman