"I think this is what was recently posted on the Sea Slug Forum as Pleurobranchus caledonicus. See http://www.seaslugforum.net/showall.cfm?base=pleucale. The middorsal brown spot seems to be characteristic. It has a wide distribution including the Marshall Islands (see http://www.underwaterkwaj.com/nudi/notaspideans/e151.htm) and Hawaii (http://www.underwaterkwaj.com/nudi/hawaii/hn07.htm).
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Comment from Scott Johnson
I think this is what was recently posted on the Sea Slug Forum as Pleurobranchus caledonicus. See http://www.seaslugforum.net/showall.cfm?base=pleucale. The middorsal brown spot seems to be characteristic. It has a wide distribution including the Marshall Islands (see http://www.underwaterkwaj.com/nudi/notaspideans/e151.htm) and Hawaii (http://www.underwaterkwaj.com/nudi/hawaii/hn07.htm).