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According to the description are the ridges and tubercles quite reduced and differ from most species of Halgerda in being the same translucent white colour as the background colour of the mantle. Another unusual feature is the yellow pigment on the notum is not associated with the ridges but is scattered randomly along the flat midregion of the mantle. Halgerda onna has black lines on both the anterior and posterior sides of the gill branches. The rhinophore clubs appear to be black and the stalks white. Irregular spots of dark brown pigment are scattered on the dorsum, especially near the mantle border.
This species is a good example why a species should NOT be described from a single specimen, there are 3 pictures of Halgerda onna from Japan
on umiushi-zukan.com, all of them have a yellowish/orange background colour (not white) and not "quite reduced ridges and tubercles".
More informations and a picture of the holotype of Halgerda onna are at Robert F. Bollands OKINAWA SLUG SITE!