Author: (Bergh, 1874)

Order: Nudibranchia  Family: Chromodorididae

Maximum Size: 20 mm

Sightings: Sunshine Coast

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Mexichromis pusilla (Bergh, 1874)

On first seeing this nudibranch you could be forgiven for thinking it might be a species of Verconia (formerly Noumea). The pink body with its white midline stripes and mantle edge and touches of purple are certainly suggestive of several of that genus. The external clue to delineating it from those similar-looking Verconia is the presence of two pair of deep scallops into the white mantle margin partially filled with purple pigment, one pair situated just behind the rhinophores and the other just in front of the gills. It is none-the-less even more surprising to learn it is actually a species of Mexichromis.

Mexichromis pusilla is a small dorid nudibranch of the Chromodorididae Family attaining a maximum size of 20 mm. Shape overall is elongate-oval with the foot extending posteriorly up to a quarter of the body length past the mantle with the mantle otherwise having a generous overhang laterally. The mantle edge undulates with a noticeable pinching midway between rhinophores and gills. Mantle colour is pink, the intensity of which varies among specimens, and the margin carries a wide white to cream band that significantly narrows at two places on both sides, just behind the rhinophores and just in front of the gills. Rudman in his “redescription” of this species refers to this feature as “a pair of pink ’tongues’ on each side.” These tongues, or scallops, that intrude are partially infilled with purple from a widening of the submarginal thin purple line just inside the broad white margin. There is often also a widening of this purple line along the anterior and posterior curve of the mantle. In some specimens this purple line is continuous right around the mantle but in others is only present in the aforementioned areas. White pigment is also present on the mantle as two broad midline stripes or ovals, one extending posteriorly from between the rhinophores and another anterior to the gills. It is interesting to note that these white patches are positioned in between the opposing pairs of scallops into the mantle margin. The lamellate rhinophores are pink with the stalks being a translucent pink. The gills are translucent white and edged in pink. Specimens I have seen possess from seven to nine gill leaflets arranged in a circle around the anal papilla. Rudman states that the gill circle is open posteriorly but I have not been able to discern that in the living examples I have seen but having said that in most instances the gills have not been fully expanded. The body under the mantle is pale pink to purple often becoming somewhat darker purple towards the white band upon the edge. Defensive glands are situated on the mantle edge around the entire periphery with some of a larger size located posteriorly. Whilst there are no confirmed records of its prey it could be hypothesized Mexichromis pusilla feeds upon sponges of the Dysideidae Family based on the records of other species of Mexichromis.

Mexichromis pusilla is widely distributed in the Indo-Pacific region but not commonly sighted. The history of its identification is most interesting. Originally called Chromodoris pusilla by Bergh in 1874, his description consists only of a published colour illustration with the name appended thereto and even then accompanied by a question mark. Apparently he never even received the preserved specimen sent to him only the coloured illustration. Although, to us now, this might seem entirely inadequate as a species description, compared to many of those early brief descriptions that were unaccompanied by an illustration, the picture certainly serves to “paint a thousand words”. 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 at the time (1984) called Durvilledoris – becoming Durvilledoris pusilla. In raising that genus he did comment, a little presciently perhaps, that: “Of those genera with relatively small radulae, in relation to the size of the body, Mexichromis has similarly shaped teeth but differs in details of the reproductive system and arrangement of the mantle glands,….”.

So then we come to the molecular sequencing of the Chromodorididae family in 2012 by Johnson & Gosliner whereby it was revealed that species of Durvilledoris are most closely related to Mexichromis antonii, the type species of Mexichromis that is, most curiously, an Eastern Pacific species. However there is more to the story. There are two highly supported clades in Mexichromis, what were the Durvilledoris species and Mexichromis antonii in one clade and what were the Pectinodoris and the balance of the Mexichromis in the other. The authors stated: “These clades could be given two names, but it is much less disruptive and confusing to maintain the name Mexichromis for all clade members.”

So both of these genera by Rudman, Durvilledoris and Pectinodoris, are swallowed up in Mexichromis. Oh the vagaries of unfolding taxonomy!

Previously known as Durvilledoris pusilla.
Originally described as Chromodoris pusilla.

David A. Mullins – September 2020

References:
– Bergh, L. S. R. (1874). Neue Nacktschnecken der Sudsee. 2. Journal de Museum Godeffroy, 2: 91-116

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

– Johnson, Scott & Johnson, Jeanette (2005-2016). Mexichromis pusilla page, Kwajalein Underwater website available at: http://www.underwaterkwaj.com/nudi/chromodorids/e134.htm

– Rudman, W. B. & Bergquist, P. R. (2007). A review of feeding specificity in the sponge-feeding Chromodorididae (Nudibranchia: Mollusca). Molluscan Research, 27(2): 60-88.

– Yonow, N. (2008). Sea Slugs of the Red Sea. Pensoft Publishers.

– 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.

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – Critter ID with NudiNotes Column, Issue: #373 (August 2019): 12 by David A. Mullins.