The identity of Nembrotha gratiosa

July 21, 2005
From: Bill Rudman

Since posting Herb Segar's photo [#14281] from North Carolina I have checked on Bergh's original description of Nembrotha gratiosa which has confirmed my feelings that this is the same species. Fortunately this was one of the few occasions where Bergh specifically noted that the colour information he provided was of the living animal. He notes that the background colour is yellow with numerous round or oval small grey-green spots [0.6 - 0.8 mm in diameter]. The rhinophores are grey-green above and yellow below, and the gills are yellow below with grey-green edges. He also describes the edge of foot with blackish grey-green line, and the sole of the foot as yellow [rough translation of the German].

I discussed a photo on Mike Miller's website, of what is surely this species,  some time ago on the Forum [#11625]. That photo, from Carol and Bob Cox, was of a small specimen, probably about 1/2 inch long. It was found and photographed on a sponge on a natural limestone reef about 4 miles out of the Destin, Pass, Destin, NW Florida, Gulf of Mexico. 

As I mentioned earlier, one problem I have is resolving the generic position of this species. Bergh's anatomical description includes radular drawings showing a cuspless median tooth and a large bicuspid inner lateral tooth which are typical of a species of Tambja. Burn (1967), on the basis of this radular description, placed this species in the genus Tambja, a decision followed by all subsequent workers. However now that we have photos of the live animal it is clear that it has long rolled oral tentacles which are particularly obvious on the Cox photo. The head of species of Tambja is quite different, with a pair of plate-like lappets, rather than elongate tentacles, flanking the mouth. There are good photos of this on the Forum [T. morosa - #10513; T. olivaria - #12539]. The only genus with large enrolled oral tentacles like this is Roboastra [see Roboastra gracilis ].

For awhile I wondered whether the recently described Tambja stegosauriformis was actually Nembrotha gratiosa, as its colour is similar, and its radular morphology is identical, to Bergh's description. However Bergh specifically described many small spots, rather than the few large spots found in T. stegosauriformis, and he mentions yellow on the gills and rhinophores. In  T. stegosauriformis there is no sign of yellow (or orange) on either the gills or rhinophores.

On oral tentacles this is a species of Roboastra, but from Bergh's description, the radular anatomy is of a species Tambja. Clearly this can only be resolved by dissecting a specimen. One real possiblity is that the radula that Bergh described belonged to a species of Tambja which had been eaten by the Roboastra. Stranger mistakes have occurred in the history of nudibranch taxonomy. I guess we should call it Tambja? gratiosa until the morphology of its radula can be re-examined.

• Bergh, L. S. R. (1890): Report on the nudibranchs. Report on the results of dredging, under the supervision of Alexander Agassiz, in the Gulf of Mexico (1877-1878) and in the Caribbean Sea (1879-1880), by the U.S. Coast Survey steamer "Blake", Lleut.-Commander C. D. Sigsbee, U.S.N., and Commander J. R. Bartlett, U.S.N., commanding.  19(3), 155-181 Pls.1-3. [172-5, Pl.2 figs 1-5. Pl.3 figs 1-4]
• Burn, R.F. (1967): Notes on an overlooked nudibranch genus Roboastra Bergh 1877, and two allied genera (Mollusca: Gastropoda). Australian Zoologist, 14: 212-221.
• Pola, M.P., Cervera, J. L. & Gosliner, T.M. (2005). A new species of Tambja (Nudibranchia: Polyceridae: Nembrothinae) from southern Brazil. Journal of the Marine Biological Association, U.K., 85, 979-984.

Bill Rudman

Rudman, W.B., 2005 (Jul 21) The identity of Nembrotha gratiosa. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/14292

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