Rostanga calumus from S.E. Australia

Dear John,
This was one of the messages that forced me to prepare some pages on this genus. The short answer is I am pretty sure that this is R. calumus. Although there are many records of R. arbutus from southern Australia, as far as I can determine it does not occur further south than southern New South Wales. One fairly good external difference is that R. arbutus has fairly 'typical dorid' shaped rhinophore clubs which taper gradually to the tip and have nearly horizontal lamellae. In R. calumus the rhinophore clubs, as can be seen in your photo, have a flat-topped club in which the lamellae are almost vertical. Both your photos and mine show dull brownish patches scattered over the orange-red mantle. I am not sure if that will prove to be a good character for the species, as we have not seen enough material to be sure.

We have only one report of the egg ribbon of this species but it is enough to show it is quite different from that of R. arbutus. So any photos of species ofRostanga on sponges and associated with egg masses would be very welcome.

When I first started looking at Rostanga in the Pacific it was thought that there was only one species, Rostanga arbutus, from Australia to Japan and across to the Red Sea. When Geoff Avern and I published our review, we showed there were at least 9 species, 7 of them new. They can be distinguished on radular morphology, sponge diet, shape of egg ribbon and development type, and to some extent on rhinophore shape. Unfortunately what can't be used is their external colour, which is why it is very difficult to identify species of Rostanga from photographs.

They are a perfect group to illustrate how important it is to try and get information on aspects of these animals' natural history such as their food and their egg ribbons. The only other way to confidently identify them is to kill them, cut them open and look at their teeth - which is not a very useful technique if you want to study them alive.
Best wishes,
Bill Rudman