Opisthobranch of the Week Data
Siphopteron nigromarginatum is considered to be rare on Okinawa. As of this date I've collected a total of eight specimens; the above uncollected animal is one which was photographed here by Mr. Yokoi Kensuke of the "Blue Try SCUBA & U/W Photo Shop", and his photograph of the animal is used here with his kind permission.
Gosliner (1989) points out that both color and morphology exhibit considerable amounts of variation in Siphopteron nigromarginatum, and in his description of the species he mentions, in part, the following:
The coloration varies particularly in the amount of yellow and orange pigment present. Black pigment is present on the siphon, siphonal crest, and parapodial margins in all material examined, but was absent from the flagellum in one specimen. In its external appearance S. nigromarginatum is most similar to S. citrinum. However, several consistent external and internal differences clearly distinguish the two species. Specimens of S. nigromarginatum always have black pigment on the margins of the parapodia and along the apical margins of the siphon. Siphopteron citrinum lacks black parapodial margins and black pigment is restricted to the apex of the siphonal crest, but is absent from the apical margins of the siphon. In S. brunneomarginatum the parapodia touch or overlap each other whereas in S. citrinum and S. nigromarginatum they are well separated.
It should be noted that the above featured, but uncollected specimen, as well as the eight collected animals, are pigmented with a bright yellow ground color, and appear to be very similar to each other externally. As of this date, I know of no other yellow colored gastropterids found on here on Okinawa (Siphopteron brunneomarginatum, S. citrinum, and S. flavum* are quite similar in appearance, but not yet observed on Okinawa).
* Carlson & Hoff (1974) mention Gastropteron (= Siphopteron) flavum to be one of the most common opisthobranchs found on Guam and more than 600 have been observed; in the case of Gastropteron (= Siphopteron) citrinum, over 400 specimens have been observed.
Siphopteron nigromarginatum is one of nine described species of Siphopteron currently found in Okinawan waters. The nine described species are:
In addition to the above nine described species I've collected several undescribed species, one of which is from 55m.