The paper was written during lockdown 2019–2020, during which I could not access the type of Phyllidia monacha Yonow, 1986 at the Natural History Museum in London (NHMUK) to confirm its generic status. In the first modern revision of phyllidiid genera, Brunckhorst (1993: 64) stated that he examined the holotypes of both P. monacha Yonow, 1986 and Phyllidia dautzenbergi Vayssière, 1912 and that they were the same species; he therefore assigned both of them to the genus Phyllidiopsis Bergh, 1876, which has fused oral tentacles. No new specimens of P. monacha have ever been recovered for reinvestigation and in that paper it was listed as a separate species with new photographs, but retained in Phyllidiopsis.

In fact, the original description (Yonow 1986: 1408) clearly states that “The tentacles are simple conical structures” and the drawing also shows two separated triangular oral tentacles and a divided anterior foot margin (Yonow 1986: fig. 3B). These oral tentacles are not diagnostic of Phyllidiopsis, which has fused oral tentacles into a single unit. Images of the holotype of P. monacha (Figures 1, 3, 5) and the specimens of P. dautzenbergi (Figures 2, 4, 6) have just been obtained from the NHMUK, and confirm that the original designation of Phyllidia monacha was correct. These images of both species (photographs of the three specimens of P. dautzenbergi were provided) confirm that they are not the same species and that they do not belong in the same genus. Therefore, all references in the paper to Phyllidiopsis monacha should be corrected to Phyllidia monacha: page 2, Introduction; page 23, Check-list and Discussion; page 24, Discussion and Plate 19; and page 32, Appendix 1.

Figures 1–6. 

1, 3, 5 holotype of Phyllidia monacha, NHMUK 1985205 2, 4, 6 specimen of Phyllidiopsis dautzenbergi, NHMUK 20210058, illustrated here as it was not figured by Yonow (1986). This is also the specimen that was dissected by Brunckhorst (1993) but clearly he did not dissect Phyllidia monacha. 1, 2 dorsal views 3, 4 ventral views 5, 6 close-up views of the anterior foot margins. Scale bar: 5 mm (1–4); 10 mm (5, 6).

For completeness, the four relevant specimens from Yonow (1986), with their current museum catalogue numbers, are listed below:

• Phyllidia dautzenbergi Vayssière, 1912, 1 specimen 19 × 5 mm, NHMUK 20210058, Jezirat Seba, Djibouti, Red Sea, 10–15 m depth, 23 June 1983.
• Phyllidia dautzenbergi Vayssière, 1912, 1 specimen 6 × 3 mm (preserved), NHMUK 20210059, South Tower reef, Saudi Arabia, Red Sea, 15 m depth, 16 March 1984.
• Phyllidia dautzenbergi Vayssière, 1912, 1 specimen 6 × 4 mm, NHMUK 20210060, Sha’ab Rumi, Sudan, Red Sea, 10–15 m depth, 6 July 1983.
• Phyllidia monacha Yonow, 1986, holotype 14 × 7.5 mm, NHMUK 1985205, Creek, Jeddah, Saudi Arabia, Red Sea, 8 m depth, 15 December 1983.

I am very grateful to Andreia Salvador, senior curator of Mollusca, NHMUK, for her perseverance in obtaining these images during difficult times. All images are copyright of the Natural History Museum of London, taken by Kevin Webb. Bert Hoeksema provided helpful comments and Plamen Pankov (Pensoft Layout) kindly made up the plate.