Marionia levis and Marioniopsis fulvicola

May 20, 2005
From: Bill Rudman

I have been surprised by the almost immediate deluge of message concerning the tritoniid which I have identified as Marionia levis[#13785, #13832, #13830, #13809]. By coincidence, I have a manuscript in preparation discussing the identity of this species and was planning to leave the discussion of a nomenclatural problem until that was published, but now that the species has attracted such interest it seems silly not to discuss it here.

A new species of tritoniid, Marioniopsis fulvicola was recently described from the Red Sea (Avila, Kelman, Kashman & Benayahu, 1999). It was associated exclusively with the soft coral Parerythropodium fulvum fulvum on which it fed. Two colour morphs, yellow-brown and grey, were found, matching the colour forms of the soft coral on which they are found. This colour match, and their gills which look remarkably like the extended polyps of the soft coral, make them extremely well camouflaged. The external shape and colour of this species matches the photos on the Forum which I have identified as M. levis. One colour feature not mentioned by Avila et al, are dark brown spots found on the gills stalks and rhinophores of most specimens. However, Dr Benayahu, who found this species while studying the soft coral, had earlier sent me some preserved specimens, and in them the dark spots, deep red in alcohol, are clearly visible.

Eliot's description of M. levis, based on 6 specimens from Zanzibar and Wasin Ids, East Africa, also fits this species well:
'The living animals were high and narrow in shape, with a flat back. The sides were described as white, mottled with translucent patches. The ground colour of the back was a light purplish brown, with stripes of the same colour but darker and others of white. The branchiae and rhinophores were pink with dark red blotches.'

However there are some a major differences in the internal anatomy. For example the radular formula of  M. levis is approx 47 x 80 + 1.1.1.+ 80 and M. fulvicola is 40 x 42 + 1.1.1 + 42. Another difference are the number of gizzard/stomach plates, approximately 30 in M. fulvicola and approx 150 in M. levis. The Benayahu specimens I have looked at match the anatomical description of Avila et. al. It would seem there are either two species, very similar externally, but quite different internally, or that Eliot's description of the internal anatomy is wrong.

My belief is that Eliot's description is incorrect.  In his description he writes: 'An uninjured alcoholic specimen 26 mm long, 10 high, and 7 broad in the widest part, but one which was dissected was about twice as large.' As the largest live specimen recorded by Avila was 40 mm (26 mm preserved), I suspect the large specimen Eliot dissected (approx 52 mm long preserved) and from which I assume he obtained the radula and gizzard plates, was a different species. Although Eliot for a time lived in Zanazibar, his material came from a variety of collectors, and his knowledge of the living animals was often from notes made by the collectors. Eliot was describing seven similar species in this paper, and it is quite possible that either he or one of the collectors mixed two species together.

If this is so, which part of Eliot's description should we follow - the external description or the anatomical description? In practical terms, the anatomical description is not sufficient to identify the species - for that matter very few species can be identified solely from a description of their internal anatomy. That is why we have so much difficulty in identifying many species described by Rudolph Bergh, and more recently, Eveline & Ernst Marcus, which were based solely on preserved decolourised animals. We could look for type material and choose one specimen to designate as a lectotype, but since the specimens, if they exist, will be preserved and decolourised, we still won't know which represent the colour pattern in his illustration. The only sensible solution is to accept the colour illustration as representing the species and to treat the anatomical information as problematic. 

For those reasons I consider Marioniopsis fulvicola to be a synonym of Marionia levis. Avila et al. provide us with valuable information on the natural history of this species, with about 25% of the colonies of the soft coral food, Parerythropodium fulvum fulvum, at the study site being occupied by at least one animal. Becaue they are so cryptic, specimens were often only discovered after the soft coral colony was touched, which caused the polyps to retract, and so the slug to be exposed. This animal also appears to sequester secondary metabolites from the soft coral in their tissues, presumably for defence. 

• Avila, C., Kelman, D., Kashman, Y., and Benayahu, Y. (1999) An association between a dendronotid nudibranch (Mollusca, Opisthobranchia) and a soft coral (Octocorallia, Alcyonaria) from the Red Sea. Journal of Natural History 33: 1433-1449.
• Eliot, C.N.E. (1904) On some nudibranchs from East Africa and Zanzibar. Part V. Proceedings of the Zoological Society of London, 1904(2): 83-105, Pls.3-4.
• Yonow, N. (2000)  Red Sea Opisthobranchia 4: The orders Cephalaspidea, Anaspidea, Notaspidea and Nudibranchia: Dendronotacea and Aeolidacea. Fauna of Arabia, 18: 87-131.

Copyright © Australian Museum, 1998-2010