Thorunna furtiva  Bergh, 1878

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Thorunna furtiva is characterized by an orange/red marginal line around the mantle edge. There is an orange/red line along the outer edge of the gills and an orange/red line along the anterior and posterior midline of the rhinophore clubs. Thorunna furtiva shows a typical rhythmic waving of its gills.
Pretty similar is Thorunna daniellae which has a smudged purple submarginal line around the mantle edge and on the posterior part of the foot.
More information on Thorunna furtiva are on Bill Rudman's Sea-Slug Forum!