Opisthobranch of the Week Data
Siphopteron brunneomarginatum is considered to be common in Okinawan waters; Atsushi Ono reports (pers. comm.) seeing over fifty individuals, however, the species has not as yet been seen in the waters of Okinawa's main island. [Carlson & Hoff (1974) report in their description of the species, collecting five specimens from Guam waters]. The above featured animal was found and photographed on a rock associated with a reef environment by Atsushi in May of 1999 from Gahi Island, Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha. Atsushi has kindly furnished the above image of this 5mm animal as well as two addition images and are used here with his permission. The second of these additional images illustrate the broad dark brown parapodial border perhaps more typical of the species.
This attractive little gastropterid was originally described by Carlson & Hoff (1974), as Gastropteron brunneomarginatum. In Terry Gosliner's revision of the family (1989) the species was placed in the genus Siphopteron.
The following partial description of the species' external morphology is taken from Gosliner (1989):
External Morphology: The living animals are 3-5 mm in length. The body is uniformly light greenish yellow with chocolate brown lines extending along the margins of the parapodia to the foot, as a transverse band on the visceral hump, which extends on to the flagellum, when present, and on the apex of the siphon.
The head shield is short and triangular with an involuted siphon at its posterodorsal end. The siphon contains a medial crest whose apex is slightly elevated above the rest of the siphon. The visceral hump is elongate and posteriorly rounded. An elongate, conical flagellum is present on the right side of the visceral hump . . . . The parapodia are elongate and high. Near the middle of the body the margins of the two parapodia touch, or overlap each other. The gill is small and consists of 3 or 4 simple plicae.
Siphopteron brunneomarginatum similar in external appearance to that of S. nigromarginatum. Rudman (2001) makes the following comments concerning S. brunneomarginatum and S. nigromarginatum in the Sea Slug Forum:
Siphopteron brunneomarginatum 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. The yellow of the body is produced by closely spaced opaque yellow specks on a translucent clear background. It is superficially similar to S. nigromarginatum but that species is more rounded in shape, and has a much thinner marginal line of black rather than brown. S. brunneomarginatum is much more elongate in shape with the parapodia usually touching or overlapping, while in the much more inflated S. nigromarginatum the parapodia are always separated.