The egg ribbon of Rostanga calumus

April 23, 2002
From: Bill Rudman

On my recent field trip to southern New South Wales and nthn Victoria I was lucky to turn over a large flat rock and find a Rostanga with two egg ribbons which looked like one of those in Audrey Falconer's recent message.

Data: S. of Secret Bch, Mallacoota, nthn Victoria, 26 February, 2002. Under rock at low tide. 24mm long alive, AM C408090.

Even though I named these species, I must admit to having been uncertain of its identity until I had a chance to look at its radula, which I have just been able to do. In a separate message I have posted a couple of SEM photos showing that it is Rostanga calumus. I kept the Rostanga alive for 3 days in the hope it would lay another egg ribbon and so prove its 'ownership' but unfortunately it wouldn't oblige. The difference in colour of the egg ribbons reflects the stage of larval development, the ones in the lower right ribbon being just about to hatch as veliger larvae. Although we can't be 100% certain that this is the egg ribbon of R. muscula, until we find one in the act of laying, I am pretty confident that it is. This is a useful piece of information as it is one of the few species whose egg ribbon morphology was unknown.

Looking again at Audrey Falconer's recent message I wouldn't be surprised if her 2 egg masses were from different species. From my Mallacoota photos it looks like the egg ribbon on R. calumus is about one and one half whorls and the free edge has relatively large folds. The photo in Audrey's message from Marengo, Victoria [lower right] seems identical. However the other photos, which show an egg ribbon being layed in an aquarium, shows an egg ribbon which is more than 3 whorls and has the free edge bearing many small tight folds. This is more like that found in Rostanga bifurcata. The animal also looks quite like R. bifurcata as well. The egg ribbon of R. bifurcata usually has many more whorls than this, but strange things happen when egg ribbons are layed in strange places.
Best wishes,
Bill Rudman

Rudman, W.B., 2002 (Apr 23) The egg ribbon of Rostanga calumus. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/6795

Factsheet

Rostanga calumus

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