Opisthobranch of the Week Data
The above featured little gastropterid, Siphopteron pohnpei, was photographed by Atsushi Ono from the waters of Aka Island in the Kerama Islands, one of a series of islands in the Kerama Islands Group, which are located 30 ~ 40 kilometers west of the Okinawa Capitol, Naha. This featured animal was photographed by Atsushi during August of 2003 in 6m of water from a sandy bottom. Gosliner (1989) also reports its being found on sand in Hawaii. Clay Carlson, commenting in the Sea Slug Forum, mentions that it is found in Guam on the sea grass, Enhalus acoroides. I've not personally seen S. pohnpei here in the waters of Okinawa's main island and Atsushi reports (pers. comm.) seeing only four individuals (two of the four were collected) and of course considers the animal to be rare in the Kerama Islands.
The following description of external morphology and discussion of Siphopteron pohnpei is taken from Gosliner (1989):
External morphology: The living animals are 3-5 mm in length. The general body color is variable and ranges from brown to red. Brownish animals were observed to copulate with red ones. Animals with brown color may also have opaque white and yellow spots. Frequently, brown pigment is absent from the posterior end of the foot, leaving a translucent area on the posteriodorsal portion of the animal. The head shield is short and triangular with an involuted siphon at its posteriodorsal limit. The siphon contains a medial crest that terminates in an elongate, cylindrical extension. The visceral hump is elongate, ovoid with no flagellum or other elaboration in any of the observed specimens. The parapodia are well developed. The gill is situated on the right side of the body, ventral to a prominent crescentic ridge. It is minutely bipinnate, consisting of six primary leaflets.
Discussion: Siphopteron pohnpei is known only from its original description (Hoff & Carlson, 1983). The brown specimens . . . closely resemble the original description. Initially, the brown and red specimens appeared to represent two distinct species. However, all aspects of the external and internal morphology are identical in both color phases. This, combined with the fact that both color forms copulate readily with each other, suggests that they are all members of a single, variable species.
The radular morphology is identical in the specimens from Ponape and Hawaii.
The morphology of the penis, . . . , is similar to that of Siphopteron brunneomarginatum and S. citrinum.
The eggs, unlike those of other known members of the genus, are white rather than yellow.
I've added a second page showing a different individual, also photographed by Atsushi from the same site and date, but from 7m.
Siphopteron pohnpei is one of nine described species of Siphopteron currently found in Okinawan waters. The nine described species are: