Dear Valérie,
Yes this does look like a phyllidiid but actually it is a juvenile sea cucumber. It's name is Bohadschia graeffei [or perhaps Pearsonothuria graeffei], and when juvenile it has this colourful phase, but as it grows it becomes a typical boring brownish sea cucmber. Have a look at the messages attached to the  Mimicry Fact Sheet for more discussion on the animal.
Best wishes,
Bill Rudman

Thanks Mary Jane,
This flatworm, which I think is Pseudoceros scriptus Newman & Cannon, 1998 is found throughout the Indo-West Pacific and is most probably a mimic of many of the Chromodoris quadricolor colour group. In some cases the blue has darker specks all over, and the black lines are broken, making it look very like Chromodoris michaeli. Mimicry is a fascinating area for conjecture - and I have done a bit in my time - but unfortunately it is very difficult to show scientifically that there is any significance to two animals looking almost identical. Despite that, I would like to think that this flatworm and the chromodorids are not just similarly coloured by chance.
Best wishes
Bill Rudman

Thanks Bob,
Bill Rudman

Dear Rosalind,
Thanks for keeping in touch. Name changes like this are good to know as often only the experts in a field know about while the rest of us continue to use out-dated names. I presume the same often happens when non-experts write about nudibranchs. A reference to the name change would be nice to include
Thanks
Bill Rudman

Dear Marina,
Thanks for these wonderful photos of the holothurian Bohadschia graeffei. To those with little knowledge of holothurians or sea cucumbers, the lower left photo shows the tube feet (yellow) very well. These are the same organs which enable it, and its close relatives, the starfish and sea urchins, to 'walk'. The lower right photo shows the circlet of oral feeding tentacles around the mouth which enables it to collect the detritus and organic matter it eats, from the substrate.

Now I have finished the anatomy lesson here is a comment on mimicry and your bservation that there were no 'models' - that is blue and yellow phyllidiids around. Mimicry is quite difficult to prove, and I am sure some similar colour patterns have evolved by chance, or the predators who are supposedly fooled by the trickery can't see the colours. But with that caution, there are certainly many examples of remarkable mimicry. I don't know whether the juvenile holothurian is mimicking phylidiids, but if it is then the mimicry would have evolved over a long time period. It is certainly n necessary for model and mimic to be around together at all times,
Best wishes,
Bill Rudman

Dear Ros,
Wolfgang Seifarth's photo shows just how remarkable this mimicry is.
Best wishes,
Bill Rudman

Thanks Erwin,
It certainly has similarities to P. madangensis but 'mimicry' has a special biological meaning and it is very difficult to demonstrate that 2 animals that look alike are the result of some long term evolutionary selection process. Even the idea that the juvenile holothurian in Wolfgang's wonderful photo is mimicking a species of Phyllidia is based on intuition rather than evidence.
Cheers,
Bill Rudman

Dear Joe,
Have a look at the following two pages - Defensive colouration and Mimicry. If you read the messages at the bottom of each page, and my comments, you will should find the information you require. By chance I have just posted a couple of messages today about a couple of amazing examples of the mimicry you are asking about. See the pages on Aldisa williamsi and Aldisa erwinkoehleri. I don't know where in the world you are from, so I can't suggest a particularly relevant example, but the red-spotted chromodorids of southeastern Australia are quite a spectacular example.
Best wishes,
Bill Rudman

Dear Adam,
Have a look at the following two pages - Defensive colouration and Mimicry. If you read the messages and my comments you will should find the information you require. For the flatworm's point of view have a look at Wolfgang Seifarth's Marine Flatworms of the World at http://www.rzuser.uni-heidelberg.de/~bu6/index.html.
Best wishes,
Bill Rudman

Dear Mark,
I can't think of any nudibranch it resembles. If anyone has an idea about which nudibranch, if any, this fish resembles can they let me know please?

Best wishes,
Bill Rudman.

Dear Wolfgang,
Thanks for the best wishes and thanks for the wonderful photo.

You are right in surmising that this is a juvenile holothurian. Even more interesting is that this species, Bohadschia graeffei, changes colour from juvenile to adult. It starts life as a brightly coloured mimic of very distasteful phyllidiid nudibranchs, such as Phyllidia varicosa and P. coelestis, but when it grows larger than phyllidiids grow, it gradually changes to a camouflaged mottled white and brown colour more typical of other species of the genus Bohadschia.

There are some photos of the intermediate and adult stages in Gosliner, Behrens & Williams (1996) Coral reef Animals of the Indo-Pacific.

Bill Rudman.

Dear Christine,
Mimicry is a good example of evolution at work. It is difficult not to give human attributes to animals when explaining how evolution works but it is important not to give the impression that the process is 'guided towards perfection'. Evolution is directionless. To say they 'tried' a number of patterns suggests they were aiming at a goal which is quite the wrong way to approach it.

If we go back to first principles - as first stated by Charles Darwin - to be 'fit' or 'survivable' species must occur in large numbers and show variability. If, or rather when, environmental conditions change, there is a good likelihood that some members of a very variable species will be able survive in the new conditions and so pass on their selection of the original genes to the next generation.

In the case of Phyllidia and the flatworm, let us presume that Phyllidia is the 'model'. Phyllidia has extremely toxic and distasteful secretions which deter fish from eating it. Fish learn to avoid the Phyllidia-shape and colour and so Phyllidia are somewhat protected from predation. In this case, any animal that looks like Phyllidia will gain some protection from predation by 'tricking' predatory fish into thinking they are a Phyllidia. If a flatworm species has a vaguely similar shape and colour to Phyllidia then those individuals that look most like a Phyllidia are most likely to escape fish attack. Gradually the surviving flatworms of each generation will become more and more like Phyllidia simply because only those with the genes to look like Phyllidia will survive. At no time will the flatworms 'know' they are improving their chances of survival, or even that there are lots of Phyllidia in their environment.

In more complicated situations there can be two or three mimic species and sometimes some or all will be distasteful. In other cases there can be many species, all of which are distasteful, as I describe for the red-spotted chromodorids of southeastern Australia.

Concerning flatworm 'tentacles'. Yes they are sensory and as with the nudibranch rhinophore can detect chemical 'smells' in the water.

Best wishes,
Bill Rudman.

Thanks Mike.

Dear Mike,
Thanks for the pics. By chance while I was looking through Dave Behrens Pacific Coast Nudibranchs to check on the spelling of 'ghiselini' I came across his picture of Polycera alabe which seems to be remarkably similar in colour to the other two species. It also seems to have a similar distribution.

It seems that Navanax inermis is quite variable in colour. In the other photo I have posted it has a striped pattern.

Do you know if Navanax and H. ghiselini have similar distributions? if your suggestion is that Navanax is mimicking the nasty tasting H. ghiselini it would be nice if their distributions overlapped and N. inermis only had the spotted pattern where it coincided with H. ghiselini - but I guess that would just be wishful thinking on my part.

Thanks again,
Bill Rudman.

Thanks Mike,
By coincidence Erwin Koehler has just sent me photos of this distinctive colour form of Chromodoris geometrica.
Bill Rudman.

Thanks Mike,
They are amazing. I've rearranged the photo you sent me. If I'm right, the top photo is of the flatworm, the bottom of the phyllidiid? Let me know if they've fooled me.
Best wishes,
Bill Rudman

Wayne, The two nudibranchs in Steve's photo are quite unrelated. On the left is Chromodoris geometrica and on the right Phyllidiella pustulosa. During his study on phyllidiids, David Brunckhorst discovered a number of invertebrates which were very similar in shape and colour to phyllidiids. In 1994, Leslie Newman and Lester Cannon (Queensland Museum) described one of these - a remarkable flatworm, Pseudoceros imatatus. The chromodorid Chromodoris geometrica also has a close resemblance to Phyllidiella pustulosa. Phyllidiella pustulosa and its "look-alikes" are probably an example of what we call "mimicry". In most general terms "mimicry" is a process where one or more species evolves to resemble another species (the model) for some biological "gain".

There are many cases amongst butterflies where the colour pattern of a highly distasteful, often poisonous, butterfly is copied by other, quite edible butterflies. Because they have identical or very similar colour patterns, the edible butterflies gain considerable protection from feeding birds who have learnt from experience to leave the distasteful species alone.

In other more complicated examples, groups of unrelated species, sometimes all distasteful, have evolved a similar colour pattern for mutual protection.

In the case of Phyllidiella pustulosa, Chromodoris geometrica and Pseudoceros imatatus we have no direct evidence that these three are mimics and that they are part of a mutual protection system. However there is considerable circumstantial evidence to suggest they are. All phyllidiids contain extremely noxious chemicals which they can exude as a milky white secretion. Phyllidiella pustulosa is also one of the few tropical reef nudibranchs which displays itself obviously during the day, apparently immune to fish attack.

Both flatworms and chromodorids produce distasteful chemicals, but it seems most probable that Chromodoris geometrica and Pseudoceros imatatus are relying on their similarity to Phyllidiella pustulosa for protection.

Phyllidiella is much more common than the other two species, and both the Chromodoris and the Pseudoceros are unique amongst their closest relatives in having pustulose bodies, suggesting that pustules have evolved in these two species for a special purpose. If this suggestion is correct then Phyllidiella pustulosa is in this case the "model" and the other two are "mimics".

A much more complicated example can be found amongst the red-spotted chromodorids of southeastern Australia which are mentioned under Chromodoris splendida.
Bill Rudman