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I asked David W. Behrens: "Dave, any ideas on this one?" His reply:"Sorry - no idea at all. Dave"
I keep it as Doridoidea sp. 02 Superfamily: Rafinesque, 1815, until I know more...
It is characterized by the greyish-brown background colour with some reddish spots, and by pustules or tubercles near the longitudinal midline.