Re: Rostanga calumus? from S.E. Australia

Dear Audrey,
Thanks for these photos. Firstly about your disagreement with the egg colour. At this stage in out knowledge of these animals, it is much too early to talk of who's right or wrong. My mention of colour was a report of an observation by a field naturalist whose information I respect. It is quite usual in this genus for there to be some variation in the colour of the eggs and the body. In fact ypur two photos show quite a difference in egg ribbon colour. Rostanga usually matches the colour of the sponge colony it is feeding on, and since sponges show quite a colour variation from colony to colony, this can lead to considerable variability in colour within a species.

One problem I have is with your identification. The egg ribbons and the slug look very like Rostanga bifurcata to me. If you have a look at the messages and photos of R. bifurcata you will see very similarly coloured animals and egg masses.

Unfortunately we don't have a photo of an egg mass which can be reliably identified as R. calumus so it is not possible to compare the egg ribbons of the two species. As I have said elsewhere, in parts of the world where more than one species of Rostanga occur, it is extremely difficult, if not impossible to identify species without looking at the radula. Only in species with very distinctive rhinophores, and/or distinctive egg ribbons can we positively identify them. It is possible that Rostanga calumus has an egg ribbon very similar to that of R. bifurcata but until we have been able to positively identify an animal which has laid eggs, from its radula, we will not be able to say with certainty just what its egg ribbon looks like.

At present I think your animal is most probably R. bifurcata. If you find another specimen laying eggs and are willing to preserve it, I would be very happy to examine its radula, as it would be very useful to know what the egg ribbon of R. calumus looks like.

Best wishes,
Bill Rudman