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This specimen differs by the brownish colour and by numerous white spots, it was identified by Dr. Richard C. Willan.
There are only a few pictures of this species online,
6 pictures on http://www.umiushi-zukan.com,
5 pictures on https://seaslug.world/
2 pictures on http://nissiesphotoalbums.web.fc2.com/
5 pictures on http://www.masea.info/,
1 picture from New Zealand on http://www.fotosearch.com/,
1 picture from Poor Knights Islands, New Zealand on http://www.oceanwildlife.com/,
and 2 pictures from Port Stephens, New South Wales, Australia on http://www.seaslugforum.net/.
The pictures from New Zealand show that this species grows up to 35 cm, though Baba described it as a 'small Aplysia' growing to about 70 mm. Let me quote Bill Rudman:
"At this stage A. sagamiana is the only species with these distinctive white spots surrounded by dark brown rings, so I will tentatively identify your animals as that species, until we know more about it from Japan."
In most specimens from Japan (except the type specimen, Baba (1949, pl, 4, figs 15, 16) figured a rosy pinkish orange animal with just a single white spot quite low down on the side of the right parapodium. That spot did have a brown ring around it) are the white spots not brown encircled, I guess this is no distinguishing feature as Bill suggested.
More information on Aplysia sagamiana are on Bill Rudman's Sea-Slug Forum!
How to cite:
Köhler, E. (2017), published 20 January 2017, Aplysia sagamiana Baba, 1949
available from https://www.Philippine-Sea-Slugs.com/Anaspidea/Aplysia_sagamiana.htm