Dear Ayumi,
Thanks for these photos. I am afraid I can't tell whether the pale whitish lumps are small sponge colonies or not. They could be, or they could be the tubes of small polychaete worms encrusted with sand grains, or even small bryozoan colonies. All these animals can form lumps like this on branching algae.

We don't know what any gastropterid [Gastropteron, Siphopteron and Sagaminopteron ] feed on, but I think they are closely related to the carnivorous aglajids, philinids and scaphandrids, which all feed on particular prey, including polychaete worms, flatworms, sea slugs, and small bivalves. The shape of the teeth in gastropterids is very similar to that of philinids and scaphandrids so I am pretty sure they feed on something similar and are not sponge feeders.

Perhaps they are feeding on polychaete worms living in burrows or tubes? There certainly seem to be a couple of holes visible in your close-up photo. These Siphopteron are quite small so it is difficult to see precisely what they are doing in the field. Unfortunately though, I think that is the only way we are going to discover what they eat. All I can say is good luck and please keep looking.

Best wishes,
Bill Rudman

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

Thanks Jun,
The animals in your mating photos seem to show a nice variation betwen the yellow and orange forms. One has a quite yellow head shield but orange parapodia.
Best wishes,
Bill Rudman

Dear Haruo,
This is indeed a wonderful photo. These gastropterids swim in such an irregular, jerky fashion, that photographing them is very difficult.
Best wishes,
Bill Rudman

Dear Nishina & Jun,
Thanks very much. Yes this is indeed S. flavum.
Best wishes,
Bill Rudman.

Dear Yasuhiro,
Firstly, the animal I identified as S. fuscum is almost identical to the original description, so I am pretty sure that it is correctly identified. I have seen some published photos [See Atsushi Ono's Opisthobranchs of Kerama Islands in which the dark brown background colour can be a very pale yellowish brown.

In your photos, the short, blunt, posterior projection is very similar to that described for Siphopteron flavum (Tokioka & Baba, 1964). That animal was described as "bright chrome yellow, and minutely dotted with white. The posterior edge of cephalic shield is ivory black... as is posterior protuberance"

In a later publication (Baba & Tokioka, 1965) another specimen is described as lemon yellow and with reddish-purple rather than the black of first specimen. There is also a reddish purple spot on the posterior shield.

Carlson & Hoff (1974) report on 600 specimens they have observed in Guam and they describe the colour as yellow with bright white spots scattered over the dorsum and parapodia. In their specimens the parapodia are lined with dark brown as is the back edge of the funnel. the dorso-median crest and posterior protuberance are tipped in black.

If the Guam animals are the same as the Japanese specimens then the species is quite variable in colour.

Yasuhiro's animals have elements of both descriptions and they look a bit different from Atsushi Ono's photos at the top of this page. Any opinions or information on the colour variation, and the length of the posterior flagellum within S. flavum would be very welcome.

Best wishes,
Bill Rudman.

Dear Atsushi,
Thank you for the photos and the information.

I asked Atsushi if he would mind sending photos of Siphopteron flavum and Siphopteron tigrinum as his photos show the typical siphonal crest of species of Siphopteron so well. It certainly suggests that Siphopteron sp.1. and Siphopteron sp.2., which seem to lack this crest, are most possibly not Siphopteron.

I also asked Atsushi whether he has ever seen any sign of a gastropterid feeding. To me this is another great unsolved mystery. Perhaps like Chelidonura they are flatworm feeders. If anyone sees a feeding incident, please let us know!

Has anyone got any views on Atsushi's colour variant? Apart from the obvious colour differences, I wonder if the apparently right lateral position of the posterior flagellum is significant? Terry Gosliner states that S. flavum is peculiar in having a centrally placed flagellum, as can be seen in the upper photo of a typically coloured animal.

Best wishes,
Bill Rudman.