The anatomy of this species is very unusual. It is currently the only named member of the genusFiona but a 2016 study showed that this species is a species complex.[12] The familyFionidae was expanded in 2016 to include Tergipedidae, Eubranchidae and Calmidae as a result of a molecular phylogenetics study.[13] Features that are characteristic of the genus Fiona include[14] 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.
^(in German)Eschscholtz, F. (1831). Zoologischer Atlas 4. 19 pp., pls. 16-20. G. Reimer, Berlin. page 14, Plate 19, figure 1.
^Alder J. & Hancock A. (October 1851). "Descriptions of two new species of nudibranchiate Mollusca, one of them forming the type of a new genus". Annals and Magazine of Natural History(series 2) 8 (46): 290-302, pls. 9-10.
^(in German)Bergh, L. S. R. (1894). "Die Opisthobranchien. Reports on the dredging operations off the west coast of Central America to the Galapagos, to the west coast of Mexico, and in the Gulf of California, in charge of Alexander Agassiz, carried on by the U. S. Fish Commission steamer “Albatross”, during 1891, Lieut. Commander Z. L. Tanner, U.S.N., commanding. Bulletin of the Museum of Comparative Zoology, Harvard 25 (10): 125-233, pls. 1-12. Species description is on the page 130-132. Table 1, figures 13-15.