Mexichromis pusilla – Verconia varians

In No. 2 of this series Mexichromis pusilla (Bergh, 1874) and Verconia varians (Pease,1871) will be discussed. Both species were described back in the 19th century and have gone through a couple of name changes. The original genus for both was Chromodoris. There had been much confusion in the literature between them up until Rudman, 1986.

Chromodoris pusilla > Durvilledoris pusilla > Mexichromis pusilla

Chromodoris varians > Noumea varians > Verconia varians

Both are small, Verconia varians to 15 mm and Mexichromis pusilla slightly larger to 20 mm. Both have a pink coloured mantle with V. varians sometimes having a peachy tint. The mantle margin in both is white to cream with V. varians sometimes having a pink tint and M. pusilla sometimes a yellow tint. Mid-dorsal white pigment is present in both with V. varians having three large spots/stripes that may be all connected, between rhinophores and gill whilst M. pusilla has two large spots/stripes, one between rhinophores and the other anterior to the gill. Both also have a submarginal purple region anteriorly from rhinophore to rhinophore and another posterior to the gill. Both may exhibit two to three large undulations of the mantle edge laterally. V. varians may or may not have a very thin purple line submarginally joining the larger anterior and posterior purple areas. M. pusilla however has a noticeable narrowing of the mantle margin on both sides just posterior to the rhinophores and again anterior to the gill, opposite the central white patches. This creates what appears as four “tongues” that are infilled with watery purple pigment that becomes more solid against the creamy mantle margin. This feature is the most distinctive and reliable difference between these two species, and other similar species, externally. Gills and rhinophores of V. varians are red/orange but the top half may be purple (as shown here). Gills and rhinophores of M. pusilla are pink much like the mantle colour. Many species of the Verconia genus show intraspecific variation in the colour of the mantle from reddish-pink to violet-purple.

Mexichromis pusilla – A close up of one of the four “tongues” intruding into the mantle margin that have a purple infill. This feature is a defining external characteristic of this species.

With regard to the aforementioned name changes here is some of the nomenclature trail:

Mexichromis pusilla

Bergh is credited with “describing” this species and he used the name Chromodoris ? pusilla. The question mark over the genus was understandable as the description consisted entirely of a small colour illustration he had received and published but with no accompanying text, apart from the name he appended. Bergh never received a specimen to examine, hence no proper description was made. Rudman plucked it from obscurity in 1986 in a paper that “redescribed” it along with many other Chromodorididae species of a similar colouration. In doing so he relocated it into a genus that he had recently raised (1984) called Durvilledoris – becoming Durvilledoris pusilla. Molecular sequencing of the Chromodorididae family in 2012 by Johnson & Gosliner saw Durvilledoris species swallowed up by Mexichromis. Gosliner, 1987 had misidentified Mexichromis pusilla (then Durvilledoris pusilla) as Noumea varians.

The image of Chromodoris pusilla that Bergh had published in 1874, that comprised the whole description of the new species.
Bergh, L. S. R. (1874). Neue Nacktschnecken der Sudsee. 2. Journal de Museum Godeffroy 6. pl. 1, fig. 18. “Image from the Biodiversity Heritage Library. Contributed by Smithsonian Libraries. www.biodiversitylibrary.org”

Verconia varians

Pease described this species as Chromodoris varians in 1871 from a specimen collected in Tahiti. (In 1874 Bergh described Chromodoris gloriosa that Rudman considers is the same species. It was collected at the same time from the same region and by the same collector – Andrew Garrett.) Pruvot-Fol, 1951 synonymized Noumea gloriosa (Bergh, 1874) with Chromodoris varians repeated by Marcus & Marcus, 1970.  Rudman, 1986 also recognized both as the same species and redescribed it as Noumea varians. In 2015 Noumea, Risbec, 1928, a genus of Nudibranchia, Chromodorididae, was discovered to be a junior homonym of Noumea Fauvel, 1874 a genus of beetle. Being thus preoccupied, the genus was changed to Verconia Pruvot-Fol, 1931 having been recognized as a synonym of Noumea Risbec, 1928 by Johnson & Gosliner, 2012. Willan & Coleman, 1984 had “misidentified” what is now Verconia alboannulata as Chromodoris varians. This was prior to Rudman, 1986 where the latter was properly identified and redescribed and the former described for the first time as Noumea alboannulata.

Chromodoris varians – The image accompanying Pease’s 1871 description.
Pease, W. H. (1871). Descriptions of new species of Nudibranchiate mollusca inhabiting Polynesia. Nº2. American Journal of Conchology. 7(1): plate 9, figure 2. “Image from the Biodiversity Heritage Library. Contributed by Smithsonian Libraries. www.biodiversitylibrary.org”

Chromodoris gloriosa – Junior synonym of Verconia varians. This is the image accompanying Bergh’s 1874 description.
Bergh, L. S. R. (1874). Neue Nacktschnecken der Sudsee. 2. Journal de Museum Godeffroy 6. pl. 1, figs 6. “Image from the Biodiversity Heritage Library. Contributed by Smithsonian Libraries. www.biodiversitylibrary.org”

(Note: Sometimes with Bergh’s descriptions the text might be published prior to the associated image/s or vice versa. This was often at the whim of the publisher.)

David A. Mullins – January 2021

References:

– Pease, W. H. (1871). Descriptions of new species of Nudibranchiate mollusca inhabiting Polynesia. Nº2. American Journal of Conchology. 7(1): 11-19. page(s): 19, plate 9, figure 2.

– Bergh, L. S. R. (1874). Neue Nacktschnecken der Sudsee. 2. Journal de Museum Godeffroy 6. pl. 1, figs 6 & 18.

– Bergh, L. S. R. (1875). Neue Nacktschnecken der Sudsee. 3. Journal de Museum Godeffroy 8. pp. 27-29.

– Pruvot-FoL, A.(1951). Revision du genre Glossodoris Ehrenberg. Journal de Conchyliologie, vol. 91, pp. 76- 164

– Marcus, E. & Marcus, E. (1970). Opisthobranch Mollusks from the Southern Tropical Pacific. Pacific Science, 24: 155-179.

– Willan, R. C. & Coleman, N. (1984). Nudibranchs of Australia, Neville Coleman, AMPI: pp 26-27.

– Rudman, W. B. (1984). The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the genera. Zoological Journal of the Linnean Society 81: 115-273.

– Rudman, W. B. (1986). The Chromodorididae (Ophistobranchia: Mollusca) of the Indo-West Pacific: Noumea purpurea and Chromodoris decora colour groups. Zoological Journal of the Linnean Society. 86(4): 309-353.

– Gosliner, T. (1987). Nudibranchs of Southern Africa. Sea Challengers, & Jeff Hamann in association with California Academy of Sciences.

– Johnson, R. F., Gosliner, T. M. (2012). Traditional taxonomic groupings mask evolutionary history: A molecular phylogeny and new classification of the chromodorid nudibranchs. PLoS One 7 (4): e33479.

– MolluscaBase eds. (2020). MolluscaBase. Noumea Risbec, 1928. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=206191