Opisthobranch of the Week Data
Polybranchia orientalis is considered to be uncommon in Okinawan waters; I've not personally seen this attractive sacoglossan in the waters of Okinawa's main island and Atsushi Ono reports the above featured animal is one of over ten which he has seen in the Keramas* (per. com.). The animal was collected by Atsushi in January of 1999 from a rope-supported buoy within the confines of a Zamami Island bay.
The following description of the species is taken from Marshall & Willan (1999):
This large sacoglossan bears numerous very tightly packed cerata. The bases of the cerata are cylindrical and each unfurls to a flattened disc. Both faces of the cerata as well as their margins are covered with papillae. The rhinophores are long and forked (bifurcate).
The cerata are transparent and patterned with dark green that merges into tan or red at their bases and they can also have some superficial white pigmentation. Some of the papillae bear pale brown spots at their base. The rhinophores are transparent with a faint green pattern and small white pustules. The sole of the foot is speckled with green and it has white patches made up of numerous dots.
Polybranchia orientalis casts off (autotomises) its cerata very readily when it is touched, even lightly. Polybranchia orientalis has a resemblance to the species of Cyerce and this is reinforced by its behaviour of autotomising its cerata at the slightest touch. However, unlike Cyerce, the ceratal sculpture of Polybranchia is distinctive and the foot sole has no transverse groove.
Atsushi's above featured animal is quite lightly pigmented with a series of obvious green spots. Rudman (2001) comments in the Sea Slug Forum that a featured P. orientalis is comparably pigmented due to not recently feeding and that the green pigment spots are from the last traces of a previous meal.
* Keramas = the Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha.