Re: Hypselodoris fontandraui from Cannes

June 3, 2006
From: Dominique Horst

Concerning message #16751:

Hello Bill,
I've simply did not see it !
I surely have to improve my observations to check about the important features. But, thanks to your help, I will make progress, that's for sure.

So, about the simple gills and the branched gills - would it be a feature that determine two different species ?

What can be considered an abnormal form, as compared to the standard of the species and what can be a feature that can make the distinction on two different species. Sorry if this question is stupid, but for a novice...

Many thanks in advance,

Horst, D., 2006 (Jun 3) Re: Hypselodoris fontandraui from Cannes. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Dominique,
Firstly the regular branching of the gills is very strange. Some species of chromodorids [Hypselodoris, Chromodoris, Glossodoris etc], can have some irregular branches, when they are very large, but not like this. Other dorids always have branching gills, but again, nothing like this. It is such an unusual and repetitive design that I suspect it is the result of some developmental fault - but that's just a guess on my part.

Your question about how we define species is certainly not stupid, in fact it is a very good question. In the 19th century, species were often defined on very flimsy grounds - an extra spot here, a slightly longer nose there - and basically species were what the 'expert' said they were. In the 20th century, with the rise of genetics, there were many debates over how to define species, usually centred around a population of interbreeding animals, and now with the ability to relatively easily extract and uncode DNA, I guess the aim is to distinguish species by coding their DNA.

Of course as the whole mechanism of evolution is based on natural selection - that is each individual is slightly different from all others - there is always going to be this problem of just what differences define species and what differences are just part of the natural variation found within one species.

In reality, there are too few scientists working on animal taxonomy to address all these problems for every species we find, so our decisions on what is a species is usually a compromise between what is practical and what is the ideal. In most cases we try to work with a whole set of characters. It is not sensible to separate two animals as separate species if the difference is two black spots compared with five black spots on the rhinophores. Foe example, it would be good to have a number of internal anatomical differences, a few differences in shape, and some colour differences. I might give you the wrong idea when I say on the Forum that two species are different because one has a red spot on the rhinophore and another doesn't. While this may be so, there are usually anatomical differences as well which I don't mention. What I aim to do is give you some good external characters to use. After all, if the only way an ecologist could identify his subject matter was to dissect every animal, it would leave them nothing alive to study.

In brief, there is no hard and fast rule, sometimes we can't separate two species with any great confidence just by colour pattern, but thankfully we can with most, but only after we have had a look at their internal anatomy.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Jun 3). Comment on Re: Hypselodoris fontandraui from Cannes by Dominique Horst. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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