Siphopteron cf. nigromarginatum from Japan

Dear Nishina,
These little gastropterids seem quite difficult to identify. I think we still have quite a lot to learn about their colour variation. I don't think this animal is Siphopteron brunneomarginatum. In Carlson & Hoff's description they say it is a uniform yellow with a dark brown parapodial border, dark brown tip to the siphon, dark brown flagellum and a dark brown line across the visceral hump. I think your animal is closer in colour to S. nigromarginatum Gosliner, 1989, which is similarly coloured but differs in having black rather than dark brown markings and having a less uniform background colour to the body. Gosliner describes the background colour as:
"The general body colour is pale yellowish white to lemon yellow. Scattered orange mottlings may be present on the head shield and visceral hump...."
This seems to fit the colour of your animals a lot better.

There are a number of species which are yellow with black markings so it is possible you have been finding more than one species. Hopefully Terry Gosliner and Clay Carlson can give us their opinion on your animal.
Best wishes,
Bill Rudman