Trapania aurata and T. vitta

March 13, 2008
From: Bill Rudman

Amongst the species of Trapania posted yesterday [message #21446] the one I have most difficulty with is Trapania vitta. That is not to say it will not turn out to be valid species when we know more about it, but at present I have difficulty distinguishing it from T. aurata.

T. vitta is described as having a white body, including white tentacular anterior foot corners and white dorso-lateral processes. The oral tentacles have a white base but are primarily orange, the gills are white with orange edging on the dorsal side, and the rhinophore clubs are similarly white with orange at the tip and down the dorsal surface. The posterior tip of the foot is described as having "an occasional orange pigment spot".

T. aurata was described on the basis of a specimen from Hong Kong with a similar colour pattern. The main points of difference are that it had a orange-yellow band across the front of the head from the base of each orange-yellow oral tentacle, it had orange-yellow lines along the extra-branchial and extra-rhinophoral processes, and it had a distinct orange-yellow line at the posterior tip of the foot.

Gosliner & Fahey describe the radular morphology of T. vitta as closely resembling that of  T. aurata Rudman, 1987 from Hong Kong "but the outermost denticle of T. aurata is longer with a base that extends the entire width of the tooth ". I don't know what they mean by that, as when I compare SEM photos of T. aurata from both Hong Kong [message #21436] and New Caledonia [message #21437 ] I can see no real difference.

While the colour differences of the type specimens of these two species may seem quite distinct, every specimen found since then seems to be slightly 'intermediate' in colour pattern. For example I found a number of specimens in New Caledonia [see Fact Sheet and message #3774] which seem to match the colour of the Hong Kong specimen except for the absence of the yellow band on the head. Most sepecimens had the orange edging on the gills but at least two did not. As I mention above I could find nothing in the radular morphology to separate Hong Kong and New Caledonian animals.

Of other animals on the Forum, there are a pair from Bali [#313] which match the description of T. vitta except they have distinct orange line on the posterior tip of the foot like T. aurata. Another pair from East Timor [#17725] also look like T. vitta, but they also have the orange mark on the posterior foot, and one has orange on the anterior foot corners as well, which is another feature of T. aurata.

I dont think there is a simple answer to this but I think we should be alert to the possibility that these are all just one variable species. Unfortunately species of Trapania are often found as single animals rather than large populations, so looking for colour variation in a single population is not easy. It would be valuable if any time a group of Trapania are seen together the finder could take a few photos of groups of animals so we can look for the type of subtle variability I have discussed here.

• Gosliner, T.M. & Fahey, S.H. (2008)  Systematics of Trapania (Mollusca: Nudibranchia: Goniodorididae) with descriptions of 16 new species Systematics and Biodiversity, 6 (1): 53-98

Best wishes
Bill Rudman

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