How are species recognised?

July 4, 1999
From: Nishina Masayoshi


I believe that you probably know that the Sea Slug ID book has been released here in Japan. I've already got it and it's all color photo book.
it's so nice and beautiful. It's originally made for diver so it's not expensive so easy to get it. However it only has Sea Slugs from around Okinawa, I believe that I can find a most of kind of Sea Slug live in this area, so this might enough, but I hope you
to release world wide version one day.

I am also not sure where to send topics to the Forum.

I have often wondered how you can tell species apart? Does it depend on color and size? Do you sometimes check DNA?

Best Regards,
Nishina Masayoshi

Masayoshi, N., 1999 (Jul 4) How are species recognised?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Nishina,
I guess the book you are talking of is Atsushi Ono's on Kerama Is. I haven't seen it yet and look forward to receiving a copy. If you have questions, animals you can't identify, observations or photos I am happy for you to send them to the Sea Slug Forum. Anything to do with sea slugs is welcome. In this way we can all help to build up a 'worldwide book' on sea slugs for everyone to consult.

You can either send messages and photos by using the SEND NEW MESSAGE button on the Messages Page or by sending me an email message with photos attached. If you have trouble scanning photos you could post them to me by airmail and I would return them to you after scanning.

Now to your question about how do we decide what is a species? 

The general philosophical question - What is a species? - will be debated whenever two biologists meet! For practical purposes we tend to define 'a species' as being a unique group of interbreeding organisms with a unique set of genetic information. Usually we consider that a species is unable to breed with any other closely related species. However at this point the debate begins!

For practical purposes we tend to define a species as a group of organisms with a unique combination of characters. These characters can include anatomical features, colour patterns, behaviour, food preference, reproductive behaviour, egg masses, larval development etc.

If you look at the messages I have just put on the Forum from Yoshi Hirano, you will see that slight colour differences, differences in penis shape, and differences in the egg mass and larval development all helped to show him that he had two quite distinct species of the aeolid genus Facelina.

Once we have found a combination of characters which make a species distinct we then need to try and find some simple 'indicator' characters so that we can identify the species easily. If the only characters we have to identify a species are internal parts of the anatomy, then its very difficult for a field naturalist to identify the species. After all we can't do ecological studies if we have to kill and dissect every specimen to identify it.

An example would be the red-spotted chromodorids in southeastern Australia. Have a look at my message where I explain why there are so many species there with very similar colour patterns. In southeastern Australia many species have red spots, so it is necessary to look at other features of the colour pattern, such as colour of gills or rhinophores to find useful characters to identify living animals in the field.

There is of course a difference between identifying a named species, and deciding a particular animal is a new species. Deciding which differences are important enough to justify considering an animal a distinct species needs a broad knowledge of related species. It also needs access to a good library so all the past descriptions of related animals can be checked so that we can see if the animal already has a name. For example, when I find an animal in Australia which I am unfamiliar with, I need to look at all the descriptions of similar animals from all parts of the Indian and Pacific Oceans, from east Africa and the Red Sea, to Japan and Hawaii, because it is possible that the species lives throughout this vast region and someone else has found and named it somewhere else.

It is also important to realise there is a lot we don't know about opisthobranchs. There are still many examples of 'species' or 'variations' which we are unable to be sure about at this stage. An example here in the Forum is a red-spotted Chromodoris from eastern Australia which I suspect is a geographic colour form of the widespread Chromodoris fidelis. It is possible that it should be considered a distinct species but at this stage there is no clear way to decide one way or the other.

Another example would be the orange-spotted species of Nembrotha from the Philippines. Until the anatomy of these three animals is studied we cannot say whether we have one, two or three species. We can't even be sure they are all species of Nembrotha.

At this stage I know of no use of DNA comparisons to determine nudibranch species. Comparisons of enzymes by gel electrophoresis has been used a few times but techniques, such as DNA analysis, are too expensive for most of us to contemplate.

I hope this is of some help,
Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Jul 4). Comment on How are species recognised? by Nishina Masayoshi. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


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