Thuridilla gracilis from S. E. Queensland

May 14, 2010
From: David Mullins

Concerning message #22294:

Dear Bill,

On a dive trip to Flinders Reef off Moreton Island last year we found this specimen of Thuridilla gracilis in a life and death struggle with a specimen of Gymnodoris okinawae.

The G. okinawae had a mouthful of parapodium which can be seen through it's transparent buccal wall. The Thuridilla dragged the Gymnodorid around and around at top speed for approximately half an hour before finally managing to shake it off losing a piece of the parapodium in the process.

The longitudinal white lines on the parapodia tend to have a greenish tinge above the blue ovals which changes to an orange margin. I can see where confusion can easily arise when trying to classify these creatures according to the coloured markings on their body. This is especially so when you are presented with a series of specimens that apparently show a graduation of pattern and colour that seemingly leads from one species into another.

Off Mooloolaba on the Gneering Shoals and at Mudjimba Island the most abundant species is (what we call) T. splendens seen in large numbers on every dive (see message #20630) but not T. neona at all. At Flinders Reef, a mere 50 kilometres to the south, we have not sighted T. splendens at all however T. neona is found on every dive and now this T. gracilis.

I don't know if any conclusions can be drawn from these observations?

Locality: Flinders Reef, off Moreton Island,, 10 metres, S. E. Queensland, Australia., Pacific Ocean, 06 March 2009, Coral encrusted Reef. Length: 12 mm. Photographer: David Mullins.

Kind regards,

David Mullins

Mullins, D.A., 2010 (May 14) Thuridilla gracilis from S. E. Queensland. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear David,

Thanks for the gymnodorid observation - they definitely seem to have no way of judging a 'suitable' prey size.

Concerning different 'species' at different localities. As you are no doubt aware I don't think we have got all the species of Thuridilla sorted out at present so it's a bit hard to be sure what we are discussing. Perhaps blue spots and blue patches are extremes of one species? If at least some species of Thuridilla have an abbreviated larval development then perhaps aggregations of 'look alikes', as you describe here, represent clusters of siblings which have not spread far from where they hatched?  Just an idea ........

Best wishes,
Bill Rudman

Rudman, W.B., 2010 (May 14). Comment on Thuridilla gracilis from S. E. Queensland by David Mullins. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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