Dear Marc,

Apart from your photos being another good record of the nudibranch feeding on this white species of Dysidea, it is also gives us a look at another colour form of Hypselodoris fontandraui.

If I had seen just the large animal with its broad median whitish band, I would have suspected this was H. bilineata, but in the small animal alongside we can see that the median 'white' line shows signs of widening near the gills. It certainly makes me wonder if H. bilineata is just a colour form of H. fontandraui.

Best wishes,
Bill Rudman

Dear Antoni,

Thanks very much. As I said previously [#16948 ], I am not sure if this sponge is Dysidea avara. Interestingly, the only other records we have of H. fontandraui are on this whitish sponge. In comparison, the pinkish sponge - which seems typical of Dysidea avara - is usually associated with Hypselodoris tricolor [see your message #22641].

Best wishes,
Bill Rudman

Dear Antoni,

Thanks for these photos. As you no doubt realise there still seem to be difficulties in distinguishing some of the blue species of Hypselodoris from each other, in your part of the world. Photos of mating pairs can be quite useful in determining colour pattern variability within species.

Another thing that greatly interested me in your photos was the sponge in the background which is clearly a species of Dysidea. As I am trying to build up a fuller picture of feeding specificity in this family, every new bit of information is valuable. Looking at you flickr pages I see a couple more sponge associations which would be nice to include on the Forum.

As I discuss in an earlier message [#16948] I am not sure if this whitish sponge is Dysidea avara which is usually a pinkish colour with a more compact fibrous network. Perhaps it is a form of D. fragilis? Whatever its identity, it is good to get confirmation that another species of Hypselodoris is feeding on a dysideid sponge.

Best wishes,
Bill Rudman

Dear Joao Pedro,

Although I hesitate to identify the notorious 'blue chromodorids' of your region, I am pretty sure these are Hypselodoris fontandraui. Marina's table is quite a good guide but you could be misled by her table saying H. tricolor  has one median yellow line on the mantle and H. fontandraui has a white one when in fact both species seem to vary in that character. However her description of the gill colour is correct, H. tricolor having one yellowish line on the outside while H. fontandraui has one inner yellow line and two outer lines. You can see these lines quite clearly in the middle photo, which is a close-up. It also shows the tear in the mantle which looks as though it is the result of some physical damage.

Best wishes,
Bill Rudman

Dear Pedro,

The 'blue chromodorids' from your part of the world are quite tricky so I can't be 100% sure that I am correct, but I would agree with your identification of this as H. fontandraui. The white patch of the back of the rhinophore stalk and the secondary yellow lines running at an angle from the median line down each side of the body/foot seem to be good characters differentiating this from similarly coloured species.

Best wishes,
Bill Rudman

Dear Laurent,

I am pretty sure these are all Hypselodoris fontandraui. They seem to be feeding on the dirty yellow sponge which has bundles of fibres sticking out of conical outgrowths on the colony. This species is reported to feed on Dysidea avara, which looks nothing like this. It is possibly a dying colony but I will have to check with a sponge expert.  The egg ribbon is rather tantalising but its not possible to say if these slugs are responsible for it.

Best wishes,
Bill Rudman

Dear Dominique,

I will start with your last question first, because it has an easy answer. All sea slugs are hermaphrodite, that is they have full functional male and female reproductive systems, so there can be no gender differences in colour pattern.

Concerning age and colour patterns, there are certainly obvious differences in very small juveniles in which colour pigmentation is first appearing, but once that stage is past most species can be recognised by their colour pattern, even if there are some differences between young and adult animals. In many species we just don't have enough observations to say whethre there are distinct 'adult' and 'juvenile' colour forms.  From what I can read about H. fontandraui, there seems to be some differences between juveniles and adults, with juveniles having a darker blue background colour and the median line between the rhinophores and the gills being white rather than yellow. However the many other differences you and others have illustrated don't seem to be age-related.

However if you regularly visit a place where there is a fairly constant population of the species, it would be fairly easy for you to study this very question. All you would need to do is regularly take photos of animals of varying sizes for a few months and see if there is any correlation between colour, colour pattern and length. The simplest way to record the length of your animals would be to take a small centimeter scale with you and put it alongside the animals you are photographing.  With luck you could tell us, with photographic proof, the answer to your own question.

I almost forgot your question about how long they live? Again, for most species we don't know, but from the species we do know, the general belief is that most nudibranchs live for less than one year, although some live for only a few weeks and others for four of five years. Following life cycles is quite difficult and in animals such as chromodorids, which are dependent on a particular species of sponge for food, the life cyccle of the sponge is also an important factor. Some species of sponges are short-lived and others apparently live for some years, but again our knowledge of sponge natural history is as poor as our knowledge of nudibranch natural history - the ones we know about are considered to be 'typical examples' when in fact they may be strange exceptions.

Best wishes,
Bill Rudman

Dear David,

I think these are all Hypselodoris fontandraui. If I am right, then your photos are a very valuable example of colour variability in the species, shown perhaps most notably in the change in colour from yellow to white of the dorsal midline and its tendency to become a double line in a couple of cases.

When you see a cluster of nudibranchs together like this, you can be fairly certain that the attraction is food. It may lead to mating, because they are in close proximity, but the initial trigger for the 'togetherness' is their attraction to their food sponge. This species is reported to eat Dysidea avara [see message #5338] which is usually pink, but the greyish white sponge [see close-up] which they seem to be attracted to, looks like a Dysidea and could well be Dysidea fragilis or a colour form of D. avara.

Although this may seem 'strange behaviour', it is always a good indicator of feeding. You can always check by gently lifting an animal by its 'tail'. If it is feeding it will either be attached by its everted buccal bulb at the head end, or you will see the buccal bulb partly everted from the mouth. It is always possible that you will have more than one species together on a sponge, but in my experience, tight clusters like this are inevitably single species clusters. So this is a valuable record, because apart from confirming its food sponge, it gives us a good selection of the colour variety in the species.

Best wishes,
Bill Rudman

Dear Dominique,
I think you are right with Hypselodoris fontandraui. Although these 'blue chromodorids' are all very similar in colour, if you are lucky there will a few small fatures which are 'unique' to a particular species. I am by no means an expert on these Mediterranean species but two features which seem to be unique to H. fontandraui are the white patch at the posterior base of the rhinophores, and the yellow patches attached at an angle to the median line on the mantle - which is often repeated on the side of the body. Hypselodoris picta, on the other hand, always seems to have a dark blue or dark purple rhinophore club, with a yellow edge to the rhinophore sheath.
Best wishes,
Bill Rudman

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

Dear Dominique,
Yes these two animals are H. fontandraui.  I often say that the colour pattern is often more important than the actual colour. In this case the background can vary from pale to darker blue. Distinctive things to look out for - but they are not always all present - are the irregular yellow marks attached to the median line and the yellow marks on the base of the rhinophores. 

One interesting feature in the second animal [in the photo alongside] is that each gill has small gills branching off it like a fern frond. If you compare that with the typical gills in the upper photo, you will see it is quite different. When I saw your photo I remembered seeing a similar photo once before. Have a look at Erwin Köhler's message [#5165] of another specimen of the same species with the same branched gills. It is either an unusual coincidence or perhaps something this species does as it gets larger.

Best wishes,
Bill Rudman

Dear Dave,
I am pretty sure this is Hypselodoris fontandraui, with the proviso that the Mediterranean blue chromodorids are quite difficult to distinguish from one another and I have little personal knowledge of them alive.
Best wishes,
Bill Rudman

Thanks Marina,
Bill Rudman

Dear Luis,
Thank you very much for these photos. I wish my Spanish was good enough for me to use Ortea, Valdés & García-Gómez (1996) to work out some of the key characters you use to separate these blue species.

All I know for certain is that this species is said to completely lack mantle glands. They are indeed a fascinating group.
Best wishes,
Bill Rudman

Thanks Luis,
Bill Rudman

Dear Erwin,
I can understand your doubts. Apart from the gills, it certainly looks like a species of Hypselodoris and in external colouration it fits the descriptions and illustrations in Ortea, Valdes & Garcia-Gomez very well. One other possibility would be H. picta but the radular morphology of the two species is rather different so if Gery Parent has had a look at the radula and decided it looks like H. fontandraui I am happy to accept that.

Now about the gills. Chromodorids usually have what I have called 'simple' gills, which basically means there is no branching. In a few cases irregular secondary branches can occur, but it is not usual for any species. In most other dorids, the gills are usually tripinnate which means they have secondary and tertiary branches on the gills. In this animal the gills are showing a regular pattern of secondary branching. I have no explanation for this, but think it is a developmental anomaly in this animal rather than evidence of a strange evolutionary off-shoot.
Best wishes,
Bill Rudman