Thorunna daniellae (Kay & Young, 1969) [Hypselodoris]

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Thorunna daniellae is characterized by the smudged purple submarginal line around the mantle edge and on the posterior part of the foot.
This picture shows a 5-6 mm juvenile specimen.
Pretty similar is Thorunna furtiva which has an orange/red marginal line around the mantle edge, 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.
More information on Thorunna daniellae are on Bill Rudman's Sea-Slug Forum!