Nudi Notes

VEXATIOUS VARIABILITY – No. 4 Mexichromis mariei

May 17, 2024

VEXATIOUS VARIABILITY – No. 4 Mexichromis mariei

Mexichromis mariei, (Crosse, 1872) originally described as Goniodoris mariei back in 1872 from a single specimen collected in New Caledonia, can be quite variable in presentation. “This is one of the most variably colored species of dorid nudibranchs in the Indo Pacific. Originally described long ago it has been called many other species due to its wide range of color, ….” (Behrens, 2023). In some cases these variations would appear to be associated with different regions. Eight specimens with differing presentations are offered here in the following two composites (not all of the variations recorded have been included here). 

 

Above: Four variations of Mexichromis mariei.
Clockwise from top left: Tulamben, Bali, Indonesia; Tulamben, Bali, Indonesia; Lembeh, Sulawesi, Indonesia; Sunshine Coast, Queensland, Australia.

 

Above: Four more variations of Mexichromis mariei.
Clockwise from top left:
Sunshine Coast, Queensland, Australia; Tawali, Milne Bay, PNG; Sunshine Coast, Queensland, Australia; Sunshine Coast, Queensland, Australia.
(Images by kind courtesy of Terry Farr.)

 

Looking at the above examples the features that show variability could be listed as:
– Main colour of the notum
– Colour of the foot margin
– Colour of the rhinophores
– Colour of the gill
– Colour of the tubercles
– Size, concentration and distribution of the tubercles
– Width of the band on mantle margin and also whether it is continuous or discontinuous 

So this list would seem to indicate a great deal of variation or differences between members of this one species. How does one make sense of this? Are these differences significant.? Could all be accounted for under intraspecific variation? Several years ago I received some valuable advice from Dave Behrens. Dave told me it is more helpful to look for what is the same in comparing specimens rather than to create a construct of differences.

In assessing the above list of variations, colour is the most noticeable. Colour variations however are a well-known polymorphic feature of many species even though, conversely, it can at times be an aid to differentiation of known species. So too are characteristics such as the size and distribution of tubercles and widths of colour bands. 

Many early sea slug descriptions were short, only concerned with the external appearance of the animal and made from a single specimen. It is only to be expected then, that the degree of intraspecific variation was unknown, unappreciated and unrecorded. It has taken time and many surveys to reveal that diversity of appearance and it may well eventually transpire that taxonomic cryptics exist in those populations.

Further confusion is added to the mix due to the existence of additional, somewhat similar-looking species within the Mexichromis genus that can also present some difficulty in separating their identity. This relates mainly to the highly variable appearance of each of these as well, where certain characteristics may overlap. There are four other species involved here, shown in the following composite:
Mexichromis festiva (Angas, 1864)
Mexichromis macropus Rudman, 1983
Mexichromis multituberculata (Baba,1953)
Mexichromis katalexis Yonow, 2001

 

Above: Other species of Mexichromis with a similar appearance.
Clockwise from top left: Mexichromis festiva, Sunshine Coast, Queensland, Australia; Mexichromis macropus, Sunshine Coast, Queensland, Australia; Mexichromis multituberculata, Tulamben, Bali, Indonesia; Mexichromis katalexis, (or a close resemblance, given the external features to work with.) Tulamben, Bali, Indonesia.

 

A comprehensive molecular sequencing of the Chromodorididae family was undertaken in 2012 (Johnson & Gosliner, 2012) wherein Mexichromis mariei and Mexichromis multituberculata were shown to be good and separate species. Mexichromis festiva was included in the results as a hypothesized member. Mexichromis katalexis was not mentioned in the study. There is some disagreement amongst taxonomists with some believing M. katalexis is just a further degree of polymorphism within Mexichromis mulittuberculata, but this has not as yet been proven to be so. Mexichromis macropus has an “interrupted” yellow/orange mantle margin, but these “interruptions” present as distinct transverse bars rather than a broken band. Additionally this species is usually recorded in Australia south of Port Headland in Western Australia and south of Bundaberg, Queensland. Specimens of M. macropus reported in the tropics are of questionable identification. Mexichromis festiva of course has spots rather than raised tubercles and its distribution occurs between Southern Queensland and Southern NSW.

To round off the images, here is the original illustration that accompanied the description of Goniodoris mariei, that is now known as Mexichromis mariei.

 

Above: The illustration accompanying the original description of Mexichromis mariei (as Goniodoris mariei). “Image from: Crosse, J. C., (1872). Description d’une genre nouveau et d’espèces inédites, provenant de la Nouvelle-Caledonie. J. Conch., Paris. Series 31, (20) Pl. vii Fig. 5. Courtesy of the Biodiversity Heritage Library. Contributed by Smithsonian Libraries. www.biodiversitylibrary.org”

 

Recording all the many variations of a species can be just as rewarding as finding a new species even though it may be accompanied by a degree of frustration. Both circumstances deliver a: “That’s different!” moment for the nudibranch aficionado.

David A. Mullins – May 2024

References:
– Angas, G. F. (1864). Description d´éspèces nouvelles appartenant à plusieurs genres de mollusques nudibranches des environs de Port-Jackson (Nouvelle-Galles du Sud), accompagnée de dessins faits d’après nature. Journal de Conchyliologie. Series 3 (12): 43-70.

– Crosse, J. C., (1872). Description d’une genre nouveau et d’espèces inédites, provenant de la Nouvelle-Caledonie. J. Conch., Paris. Series 31, (20): 148-154.

– Allan, J. K., (1947). Nudibranchia from the Clarence River Heads, north coast, New South Wales. Records of the Australian Museum, 21(8):433463, PIS 41-43.

– Baba, K. (1953). Three new species and two new records of the genus Glossodoris from Japan . Publications of the Seto Marine Biological Laboratory, 3 : 205-211.

– Rudman, W. B. (1973) Chromodorid opisthobranch Mollusca from the Indo-West Pacific. Zoological Journal of the Linnean Society, 52(3): 175-199, 7 figs, 2 col. pl.

– Rudman, W. B. (1983) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris splendida, C. aspersa and Hypselodoris placida colour groups. Zoological Journal of the Linnean Society, 78: 105-173.

– 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. (1999, January 16) Mexichromis festiva (Angas, 1864). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/mexifest and associated messages.

– Rudman, W. B. (1999, January 16) Mexichromis macropus Rudman, 1983. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/meximacr and associated messages.

– Rudman, W. B. (1999, January 16) Mexichromis mariei (Crosse, 1872). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/meximari and associated messages

– Rudman, W. B. (1999, January 16) Mexichromis multituberculata (Baba, 1953). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/meximult

– Yonow, N. (2001) Results of the Rumphius Biohistorical Expedition to Ambon (1990). Part 11. Doridacea of the families Chromodorididae and Hexabranchidae (Mollusca, Gastropoda, Opisthobranchia, Nudibranchia), including additional Moluccan material. Zoologische Mededelingen uitgegeven Rijks Museum van Natuurlijke Historie te Leiden 75: 1-50.

– Coleman, N. (2008). Nudibranchs Encyclopedia. Neville Coleman’s Underwater Geographic Pty Ltd, Springwood, Qld. Pp 186-187.

– 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.

– Gosliner, T. M., Valde ́s, A ́. & Behrens, D. W. (2018). Nudibranch & Sea Slug Identification – Indo-Pacific, 2nd Edition. New World Publications, Jacksonville, Florida, USA.

– Behrens, D. (Images Michael Miller) (Feb., 2023) Mexichromis mariei. Available from: http://slugsite.us/bow2007/nudwk1280.htm