Rostanga dentacus ? from Hong Kong

Dear Felix,
It is definitely a species of Rostanga. The 'furry' mantle and the compact, upright gills are good characters of that genus - as is the red colour, in most species. However identifying them to species just by looking at their external shape and colour is usually quite difficult except in certain parts of the world where there is only one species present. Even that needs to be said with caution because when I started looking at the 'single' species known from southeastern Australia, Rostanga arbutus, it turned out to be a complex of species all quite different in radular morphology, shape of egg ribbon, larval development, and choice of food. Since then I am always hesitant to identify a Rostanga just on ther shape and colour of the animal.

So what about your species? Specimens of Rostanga were often identified from many parts of the Indo-West Pacific as R. arbutus, but that species is restricted by its direct developing larvae to southeastern Australia. Of the three species I have reported from Hong Kong, we can ignore R. orientalis because it has rhinophores with vertical lamellae. However both the other two species, R. dentacus and R. bifurcata, have rhinophores similar in shape and colour to your animal. The only difference is that in R. bifurcata the rhinophore lamellae are slightly sloping. So at a guess I would say your animal is R. dentacus, but I emphasise that is a guess. Without a knowledge of their food or egg ribbon, these animals can only be positively identified by radular morphology.

• Rudman, W.B., Avern, G. (1989) The genus Rostanga (Nudibranchia: Dorididae) in the Indo-West Pacific. Zoological Journal of the Linnean Society 96(3): 281-338.

Best wishes,
Bill Rudman