The extraordinary genus Myja is not a tergipedid, but related to the Facelinidae s. str. with the addition of two new species from Japan (Mollusca, Nudibranchia)

Morphological and molecular data are presented for the first time in an integrative way for the genus Myja Bergh, 1896. In accordance with the new molecular phylogenies, the traditional Facelinidae is paraphyletic. Herein is presented the phylogenetic placement of true Facelinidae s. str., including the first molecular data for F. auriculata (Müller, 1776), type species of the genus Facelina Alder & Hancock, 1855. The taxonomic history of F. auriculata is reviewed. The genus Myja is related to the clade Facelinidae s. str., but shows disparate morphological traits. Two new species of the genus Myja , M. karin sp. n. , and M. hyotan sp. n. , are described from the Pacific waters of Japan (middle Honshu), and M. cf. longicornis Bergh, 1896 is investigated from Thailand. According to molecular analysis and review of available morphological information, the genus Myja contains more hidden diversity. The family-level relationship within aeolidacean nudibranchs with emphasis on the family Facelinidae is outlined. The problem of the relationship between Facelinidae Bergh, 1889 and Glaucidae Gray, 1827 is discussed. The family Glaucidae has precedence over Facelinidae and is phylogenetically related to the core group of Facelinidae s. str., but has a profoundly modified aberrant external morphology, thus making a purely molecular-based approach to the taxonomy an unsatisfactory solution. To accommodate recently discovered hidden diversity within glaucids, the genus Glaucilla Bergh, 1861 is restored. The family Facelinidae s. str. is separate from, and not closely related to, a clade containing the genera Dondice Marcus, 1958, Godiva MacNae 1954, Hermissenda Bergh, 1879, and Phyllodesmium Ehrenberg, 1831 (= Myrrhine Bergh, 1905). The oldest valid available name for the separate ex-facelinid paraphyletic clade that contains several facelinid genera is Myrrhinidae Bergh, 1905, and resurrection of this family name under provision of the ICZN article 40.1 can preliminarily solve the problem of paraphyly of the traditional Facelinidae . “ Facelinidae ” s. l. needs to be further divided into several separate families, pending further study.