Opisthobranch of the Week Data
Siphopteron citrinum is considered to be very rare in Okinawan waters; Atsushi Ono reports (pers. comm.) seeing only the above single animal and the species has not as yet been seen in the waters of Okinawa's main island.
The above featured animal was found and photographed on a patch of dead coral by Atsushi in April of this year from the SE area of, Gahi Island, Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha. Atsushi has kindly furnished the above image as well as an addition image of the same 3mm animal. Both of these images are furnished by Atsushi and used here with his permission.
This attractive little gastropterid was originally described by Carlson & Hoff (1974), as Gastropteron citrinum. In Terry Gosliner's revision of the family (1989) the species was placed in the genus Siphopteron. Carlson & Hoff (1974) report in excess of 400 specimens being observed from Guam waters between 1970 and 1974. The following partial description of the species is taken from the Carlson & Hoff (1974) publication:
The general body shape is the same as G. flavum. The body is elongate with a high, smooth, rounded dorsal hump, a cephalic shield with a high funnel, and a dorso-median crest which extends well beyond the funnel. G. citrinum has a lo [sic] long pointed posterior projection rather than a mammary shaped projection as on G. flavum. It also has longer parapodia and a slightly shorter tail.
On the living animal the free mantle margin appears to be very narrow, basically a thickened area of the epidermis, extending from the anterior of the dorsal hump back to the dorsal projection, becoming very thin posteriorly.
To the naked eye specimens of G. citrinum appear either yellow or pink with a dark brown protuberance on the dorsal hump and a dark brown tip on the dorso-median crest. Through the microscope animals are basically transparent. Some have a yellow-orange or pink internal coloration, whereas others are entirely yellow. Those with pink are spotted with minute pink spots on the parapodia but have a yellow head, tail and foot. When observed in situ G. citrinum can easily confused with G. flavum.
There are to date the following nine described species of Siphopteron known from Okinawan waters, as well as several undescribed species.