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Pretty similar are:
Platydoris formosa which differs by its bright red rhinophores and by distinct black lines outlining the gills and
Platydoris cruenta which differs by its hair-like brown lines on the mantle.
It was posted on 02 June 2004 at these websites as Platydoris cf. formosa (Alder & Hancock, 1866) [Doris]
More information on Platydoris cinerobranchiata are on Bill Rudman's Sea-Slug Forum!