Colour in Elysia (again)

Dear Skip,
I must say I have been thinking of you all in the Caribbean / Gulf of Mexico area over recent weeks - you certainly have had more than your fair share of nasty weather.

I am interested in your comments on colour. I wish there were more bright young scientists with an interest in opisthobranchs as living animals. Colour is is a case in point. It is only a useful diagnostic [= identification character] if we understand the species, and its colour variability. This of course includes understanding which parts of its colour are a direct consequence of its genes, which parts area consequence of its food, and which parts, as you report, come and go. Until we have this information then we can only make guesses - they may be intelligent guesses, but they are still guesses. Having said that, colour, if we realise its limitations, is often a very useful character.

In the case of elysiiids, such as Elysia ornata, which has been the topic of recent discussions, it is possible that both Caribbean and IWP populations are the same species, or they may not. In any future investigation of the question, it would be useful to know if there are any constant colour differences between the populations. Perhaps in the Caribbean populations, if an orange submarginal band is present on the parapodia, it is separate from the black border, and perhaps in IWP animals the orange band, if present, is touching the black border. I don't know if that is important at this stage, but photos are one thing that enthusiastic divers can provide. What opisthobranch workers lack are large collections. When you see taxonomists who work on shelled animals start work, the first thing they can do is visit museums and look at huge shell collections from around the world. Even before starting anatomical studies they can get a 'feel' for similarities and differences in shell colour, shape and ornamentation, across the geographic range of a species. This gives them a head start in searching for anatomical differences. With opisthobranchs, the collections are much smaller and usually all we have are the decolourised preserved animals and minimal information on the living animals.

If we look at a group like the chromodorids, and particularly the C. quadricolor group which we have been discussing recently on the Forum, they represent another problem. They are almost certainly mimicking each other, so we have to work out whether the colour pattern mimicry is so good thta we can't identify species solely from the colour. I was tricked by two species in southeastern Australia which were so similar in colour that I only realised the possibility there were two species involved when I noticed that some animals when preserved became decolorised and others went dark brown. In that case there were clear anatomical differences I could use to show there were two species, but I wouldn't have known to look if sone had notgone brown in preservative. However in some species of Chromodoris, the anatomical differences are so slight that they are unsafe to use on their own. That is why at times I sound like a broken record, asking people to try and find information on their egg masses, sponge prey etc.

I guess the point is that colour can often be a very useful identification tool, but only after we have defined just what the species are by using as many characters as possible, including anatomy, food, egg mass morphology, etc, and colour. I have gone on a bit, but since you are waiting out a hurricane, I hope I have a captive audience.
Best wishes,
Bill Rudman