Thuridilla neona and colour patterns

October 31, 2005
From: Skip Pierce

Hi Bill,
My usual disclaimers up front here and I have never seen a Thuridilla on the hoof, but I think this recent discussion [#15121] illustrates some of my points, again AMONGST THE ELYSIIDS ONLY [message #15109]. I continue to have no expertise about the other taxa. I can't tell from your blow up of the recent T. neona photos whether we are looking at pixels or we can actually see the pigment containing cells I referred to in my previous message (although we have all been seduced by digital photography - and the Forum would be impossible without it - digital resolution compared to film is awful and will be so until the size of a pixel approaches the size of a silver atom -but that's another story), but if the orange and blue pigments in Thuridilla are in round, unattached hemocytes as they are in Elysia (and as it looks like they might be in the blow up), it is an example of my thought that taxonomic discussions about positions, widths, shapes, etc of color bands (By the way I don't know about the black lines, the species we work on do not have such) made up of these kind of cells can be misleading. Those cells can and do move around in the hemocoel within the same specimen, so the banding pattern changes. If, on the other hand, the pigments are located in an epithelial (or at least in a non-moveable) cell type, then genetics is in play and the patterns of bands and spots are of taxonomic merit.

Skip Pierce

Pierce, S.K., 2005 (Oct 31) Thuridilla neona and colour patterns. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Skip,
Since most of these species have been described from few specimens, it certainly illustrates the need to know more about them before either naming them or trying to identify them. I may be speaking too soon, but some species, such as T. moebii and T. decorata, seem quite constant in colour pattern, but many of the others, with blues and yellow-orange-red colours seem to have problems. When you talk about observing colour 'granules' changing position does this also include a non-food related colour disappearing? I am thinking there is a big difference in surface area covered between a thin blue line and a series of large blue patches, or between a large undulating orange band of variable thickness and a  thin orange line. Where would the extra pigment granules go?

Looking through earlier comments I have made when identifying photos of Thuridilla on the Forum, my recurring theme has been 'I am not sure'. We either have a lot more species to define, or else we need to seriously consider 'lumping' some of the existing ones. But as your comments suggest - we need to know a lot more about them first. Thanks for your non-taxonomist perspective.
Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Oct 31). Comment on Thuridilla neona and colour patterns by Skip Pierce. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


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