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It might be a strange yellowish-orange variant of Phyllidia willani or of Phyllidia sp. 3 in IPN at page 292, which differs by the opaque white granular appearance and by transverse black lines or patches.
Pretty similar are:
Phyllidia ocellata which is very variable in colour, but has usually a yellow-orange ground colour of the mantle, and has large black rings,
Phyllidia babai which has a white granular appearance and black rings.
Phyllidia exquisita which has a pale bluish granular background colour, black swirls, and yellowish-orange pigment at the mantle margin
Phyllidia willani which has a white granular appearance, and lacks orange pigment at the mantle margin
I can't tell for sure how to distiguish the phyllidiids with the whitish granular appearance:
Phyllidia babai, Phyllidia exquisita, Phyllidia willani, Phyllidia sp. 01 and Phyllidia sp. 3 in IPN at page 292. I guess they are variants of one species.
Bill Rudman thinks Phyllidia babai, Phyllidia exquisita, and Phyllidia willani to be variants of the very variable Phyllidia ocellata.
More than 20 years after Brunckhorst (1993) needs the family Phyllidiidae Rafinesque, 1814 a worldwide revision based on anatomical and molecular studies, not like the recent publication Stoffels, Bart E.M.W. et al. (2016) which focusses on the "coral triangle" and excludes most oceans.
Brunckhorst, D.J. (1993) The systematics and phylogeny of Phyllidiid Nudibranchs (Doridoidea).
Records of the Australian Museum, Supplement 16: 1-107.
Gosliner, T.M, D.W. Behrens & Á.Valdés. 2015. Nudibranch &Sea Slug Identification Indo-Pacific.
New World Publications Inc. Jacksonville, FL. 408 pp.
Stoffels, B. E. M. W., van der Meij, S. E. T., Hoeksema, B. W., van Alphen, J., van Alen, T., Meyers-Muñoz, M. A., and van der Velde, G. (2016). Phylogenetic relationships within the Phyllidiidae (Opisthobranchia, Nudibranchia).
ZooKeys, (605), 1-35. Advance online publication. http://doi.org/10.3897/zookeys.605.7136
How to cite:
Köhler, E. (2020), published 18 October 2020, Phyllidia sp. 01 Genus: Cuvier, 1797
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Doridina/Phyllidia_sp_01.htm