Opisthobranch of the Week Data
Polycera japonica is considered to be common in Okinawan waters, at least in waters of the Keramas. I've not personally seen this attractive little polycerid in waters of Okinawa's main island but Atsushi Ono reports the above featured animal is one of approximately thirty he has seen (per. com.). The animal was photographed on sand and coral rubble by Atsushi in December of 1998 from the Sakubaru Dive Area, Aka Island, Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha. Atsushi's photograph is used here with his kind permission.
The following is from Baba's original description of the species (Baba, 1949):
Small, 6-8mm in length. Velar processes about 10, slender and digitiform. Pallial ridge with a row of (7-8) mammiform papillae on either side. Similar papillae also on back, sides and tail-crest. Branchial plumes 3, non-retractile; oral tentacles small, lobiform; foot corners rounded. General body-colour brownish green, a small purple ring down the tip of the frontal processes. Round [sic] the base of the branchial plumes is a marked band of purple. Also there is an oblique band of the same colour on each side of the tail. Jaw-plates with a characteristic wing-like flange. Radula colourless, the formula 9-10 X 2-188.8.131.52.2-3. First lateral small, roughly hamate, and with a spur about half-way down. Second lateral larger, hamate, bearing a huge spur near the base. The outer laterals simply scale-like.
Baba based his description of the species on four animals (6~8mm) collected from shallow waters of Sagami Bay during August ~ September, of 1940. I've included an illustration taken from Baba's original description of the species.
Marshall & Willan (1999) illustrate Polycera cf. japonica and point out that in Baba's original description of P. japonica, he describes the species as having a purple ring around the frontal pustules as well as around the base of the gills. The authors point out that they have never seen these conditions in the Australian material.
The only other described Polycera species from Okinawan waters of which I'm aware are P. abei, P. fujitai, and P. risbeci, all from the Kerama Islands.