September 1, 2001
From: Nishina Masayoshi
Hypselodoris placida is one of the common species here like Chromodoris orientalis. I think the attached images animal are Hypselodoris placida. If so it has orange rhinophores and gills in Echizen. Therefore I sometimes misidentify it as Hypselodoris sagamiensis. At the same time, we also find specimens of H. placida with yellow rhinophores and gills like Brian Darvell's specimens. Baba described it in his book Opisthobranchs of Sagami Bay as having "rhinophores are yellow, tip of branchial plumes tinged with orange"
These photos were taken at Echizen Coast, Japan on 12 Aug, 2001. Length was between 20mm to 30mm
Nishina, M., 2001 (Sep 1) Hypselodoris placida from Japan. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5173
Thanks for these photos of H. placida. Concerning the importance of colour differences. This is difficult. In some species slight differences in colour can indicate different species. However in most cases it is the total colour pattern of an animal which is important. In H. placida the colour pattern can be summarised as "translucent white mantle with black spots, a submarginal row of blue patches, orange or yellow broken line at mantle edge, orange or yellow edge to gills and upper half of rhinophore clubs". Although the yellow may sometimes be orange, and sometimes the spots around the mantle edge may be absent the rest of the pattern remains, and is different from other black-spotted species such as C. orientalis and H. sagamiensis.
There is no standard rule to say which differences are important in differentiating species - it differs with every species. This is why the range of photos you are sending is so valuable in furthering our understanding of these animals. It is also why I sometimes have to say "I am not sure" to some of the messages that are sent to the Forum.