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Thuridilla gracilis is a species of sea slug, a sacoglossan, a marine gastropod mollusk in the family Plakobranchidae. This species can be up to 25 mm in size. The body is partially covered by the fold of the lateral parapodia. The mantle is black and crossed by multiple fine white longitudinal lines (eight to ten) punctuated sometimes with blue spots. The border of the lapel of the parapodia and rhinophores is often orange. The smooth rhinophores can be whitish without any lines.
It is an Indo-Pacific species that lives in or near coral reefs and eats algae.