Fiona pinnata, common name Fiona, is a species of small pelagic nudibranch (sea slug), a marine gastropod mollusk in the superfamily Fionoidea. This nudibranch species lives worldwide on floating objects on seas, and feeds mainly on barnacles, specifically goose barnacles in the genus Lepas.

The anatomy of this species is very unusual. It is currently the only named member of the genus Fiona but a 2016 study showed that this species is a species complex. The family Fionidae was expanded in 2016 to include Tergipedidae, Eubranchidae and Calmidae as a result of a molecular phylogenetics study. Features that are characteristic of the genus Fiona include the similar-looking oral tentacles and rhinophores; the cerata with a membrane and lacking a cnidosac; a dorsal anal opening; a reproductive system with two genital openings; two jaws with a cutting-edge, and a radula with only one central denticle in each row of teeth. That one denticle has a central cusp and a few surrounding cusps.

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