Known from tropical western Pacific (Papua New Guinea, Okinawa, Japan).
Hachijo Island, Japan, 7m, 8-10mm long. 1 July, 2001. Photos: Nishina Masayoshi.
Gosliner describes the 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.... An area of opaque white is generally present at the base of the head shield just anterior to the beginning of the visceral hump. Black pigment adorns the apex of the siphon, the tip of the siphonal crest, the parapodial margin, and the flagellum and its base."
Similar in colour to Siphopteron brunneomarginatum (Carlson & Hoff) but that species 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. In his message below Terry Gosliner also notes that S. nigromarginatum is a far more delicate species than S. brunneomarginatum and has a thinner marginal line. Also the parapodia usually overlap in S. brunneomarginatum, but do not in S. nigromarginatum.
• Gosliner,T.M. (1989): Revision of the Gastropteridae (Opisthobranchia: Cephalaspidea) with descriptions of a new genus and six new species. The Veliger, 32(4): 333-381.
Rudman, W.B., 2001 (July 30) Siphopteron nigromarginatum Gosliner, 1989. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/siphnigr
February 11, 2006
From: Takao Urasawa
Dear Dr Rudman
Thank you for you welcome advice every time.
This Siphopteron is very small. It's body is lemon yellow and red. -A beautiful colour pattern.
Locality: Laulau Beach, 6m, Saipan Northarn Mariana Islands, Pacific Ocean, 05 February 2006. Length: 1.5 mm. Photographer: Takao Urasawa
firstname.lastname@example.orgTakao Urasawa, 2006 (Feb 11) A juvenile Siphopteron nigromarginatum. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/15773
I thought at first this was Siphopteron flavum but I am pretty sure it is a juvenile Siphopteron nigromarginatum which has not yet developed all its colours. Have a look at Yasuhiro Shirai's photos  which show a similar colour pattern - where your animal is translucent clear, his animal has a bluish pigment. The two red-brown spots on the edge of the parapodia are I guess the beginnings of the black line along the edge of the parapodia which give this species its name.
In many opisthobranchs black pigments start as a reddish brown colour in the juveniles. Even in adults the normal black color can be replaced by orange as occurs in Chromdoris hamiltoni and other chromodorids with black lines. I am sure a chemist could explain what is happening here, but it seems that when te black pigment is not present in sufficient quantity we get these red-browns or oranges.
August 12, 2002
From: Nishina Masayoshi
Here are some photos are Siphopteron nigromarginatum from Hachijo Island Japan.
In upper photo S. nigromarginatum is at rest, while in the lower photo I moved it to another place for photographing.
Date: 21 April 2002
Location: Hachijo Island, Tokyo Japan
Water temp: 21C degree
Photo: M. Nishina
email@example.comMasayoshi, N., 2002 (Aug 12) Siphopteron nigromarginatum from Hachijo. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/7717
We are starting to get a good idea about the colour variation in this species now
August 23, 2001
From: Yasuhiro Shirai
Thank you for your message. I saw Mr. Nishina's photos of Siphopteron cf. nigromarginatum. I took some photos similar to Mr. Nishina's individuals in spring of this year and last year at Tanabe Bay, Kii Peninsula. But my individuals of photos have bold marginal line. These photos are of different individuals. Are these Siphopteron nigromarginatum? If not, could you please identify them for me?
Date: May, 2000 and 2001
Place Tanabe Bay, Kii peninsula, Japan
Depth: 7 to 15m
Size: 5 to 10mm
Shirai, Y., 2001 (Aug 23) Siphopteron nigromarginatum? from Kii Peninsula. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5011
Thanks for these photos which are indeed Siphopteron nigromarginatum. It is apparently quite variable in colour with the black parapodial border being very heavily pigmented in some animals and almost absent in others.
I don't know if you have seen the very large new book by Dr Okutani (2000) - Marine Mollusks in Japan. In it, this species is identified as Gastropteron bicornutum Baba & Tokioka, 1965, but I think this is a misidentification. In that animal the posterior visceral hump has a secondary papilla (hence the name bicornutum), and the brief colour notes in the original description mention orange spots rather than an orange background colour. There is also no mention any black pigmentation.
August 4, 2001
From: Nishina M. & J. Imamoto
I and Jun Imamoto are always surprised by your information. This kind of Siphopteron is not so rare at Hachijo Island. I said that this animal is S. brunneomarginatum with confidence, because I did not know that the kind called S. nigromarginatum existed. So I completely mixed up those species. Black and a dark brown distinction are difficult for me. I think that the color is influenced by the light.I will be very happy if you have a good photos of S. brunneomarginatum and S. nigromarginatum.
I cannot discriminate whether my animal is a S. brunneomarginatum and whether it is a S. nigromarginatum. The description about the color of the hump is helpful. Scattered orange was present on the head shield and visceral hump. Also with the naked eye, it has been checked clearly. The purple pattern(line?) of a clear metallic tone was also conspicuous. But I thought that it was a kind of color variation.
By the way, I guessed the meaning of the Latin name "nigromarginatum" as possibly meaning "black margin"? I have sent some images by Jun Imamoto of S. flavum (I guess) in a separate message.
Nishina Masayoshi, Jun Imamoto
firstname.lastname@example.orgNishina, M. & Imamoto, J., 2001 (Aug 4) Re: Siphopteron cf. brunneomarginatum. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5006
Dear Nishina & Jun,
These little gastropterids are difficult to see and there are quite a few similarly coloured species. Black and brown are often difficult to determine. Fortunately Clay Carlson and Patty Jo Hoff have just sent a photo of their Siphopteron brunneomarginatum so it now easier for you to see the differences.
Concerning the meaning of the Latin name "nigromarginatum" - yes 'black margin' or 'black edge' will do, Just as "brunneomarginatum" means 'brown-edged'.
August 2, 2001
From: Terry Gosliner
I was pleased to see Nishina Masayoshi's photos of the Siphopteron from Japan. It certainly looks like the specimens of S. nigromarginatum that I originally collected from Papua New Guinea. Bob Bolland has also found specimens from Okinawa. It is a far more delicate species than S. brunneomarginatum and has a thinner marginal line. Also the parapodia usually overlap in S. brunneomarginatum, but do not in S. nigromarginatum. The color can vary from yellow to yellow with orange pigment as you noted.
All the best,
email@example.comGosliner, T., 2001 (Aug 2) Re: Siphopteron from Japan. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4986
August 1, 2001
From: Nishina Masayoshi
I think this animal is Siphopteron brunneomarginatum. It is rare in Sagami Bay but rather common at Hachijo although it is very seasonal. We usually are able to see it between June to Sep. I found two together on some green algae. This is one of them which I moved temporarily to photograph.
Date: 1 July, 2001
Place Hachijo Island, Japan
Nishina, M., 2001 (Aug 1) Siphopteron cf. nigromarginatum from Japan. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4943
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.