Dear Dom,

Thanks very much for these great photos. Yes this is Hypselodoris orsinii. One other way to identify it is from the sponge it is feeding on. Of the more than 40 species of Hypselodoris I have information on, all feed on sponges of the family Dysideidae except for two. One of these, H. bennetti, feeds on a species of Psammocinia (Fam: Irciniidae), and the other, H. orsinii, feeds on sponges of the Family Thorectidae. At present the literature suggests it feeds on both Cacospongia mollior and Scalarispongia scalaris, but as all the photos on the Forum [#7384, #11147, #7305, Fact Sheet] have been identified as Scalarispongia by Dr Bergquist, who is the world expert on the family, it makes me wonder if the literature records for Cacospongia mollior are misidentifications, as the two species are difficult to separate externally, and Scalarispongia scalaris was previously considered a species of Cacospongia.

I have not shown your photos to Dr Bergquist but I am pretty sure the sponge is Scalarispongia scalaris. One of the features of the genus is the way the secondary bundles of fibres in its skeleton are arranged in a regular ladder-like structure, which we can see in the close-up I have included. Why H. orsinii is the only species of the genus to feed on a thorectid sponge rather than a dysideid, is a mystery, as their are other species of Hypselodoris in the Mediterranean and they find dysideid sponges living their quite palatable. Perhaps H. orsinii will turn out to be unrelated to other Mediterranean species of Hypselodoris or perhaps it is an adventurous species beginning a new branch of Hypselodorisevolution.

• Cook, S. de C. & Bergquist, P.R. (2000) Two new genera and five new species of the "Cacospongia" group (Porifera, Demospongiae, Dictyoceratida). Zoosystema, 22(2): 383 - 400.
• Cook, S. D. C. & Bergquist, P. R. (2002). Family Thorectidae Bergquist, 1978. pp. 1028-1050 [In] Hooper, J.N.A. & Soest, R.W.M. Van (eds) Systema Porifera. A guide to the classification of sponges. New York, Boston, Dordrecht, London, Moscow: Kluwer Academic/Plenum Publishers Vol. 1
• Rudman, W.B. & Bergquist, P.R. (2007) A Review of feeding specificity in the sponge-feeding Chromodorididae (Nudibranchia: Mollusca). Molluscan Research, 27(2): 60-88

Best wishes,
Bill Rudman

Thanks Adam
Bill Rudman

Dear Adam,
Like Hypseldoris tricolor in your accompanying message this is another of the 'blue hypselodorids'. I am pretty sure this is Hypselodoris orsinii. Juvenile H. tricolor can look quite like this as well so I can't be 100% sure.

Are you sure of the sponge identification? Ortea et al (1996) report that it feeds on sponges of the genus Cacospongia.

• Ortea, J., Valdés, A. & and García-Gómez, J.C. (1996) Revisión de las especies atlánticas de la familia Chromodorididae (Mollusca: Nudibranchia) de grupo cromático azul. [Review of the Atlantic species of the family Chromodorididae (Mollusca: Nudibranchia) of the blue chromatic group.] Avicennia, 1996, Suppl. 1: 1-165.

Best wishes
Bill Rudman

Dear Nicolas,
The correct name for Hypselodoris coelestis is Hypselodoris orsinii. If you look at the messages in the Forum about this species you will see that in France several guides to the Mediterranean fauna call Hypselodoris orsinii either H. tricolor, which is another species, or H. coelestis which isn't a valid name. As you will see there are quite a few blue chromodorids in the Mediterranean with very similar colour patterns.
Best wishes,
Bill Rudman

Thanks Jean-Pierre,
Bill Rudman

Dear Jean-Pierre,
Thanks for this photo. You can clearly see the feeding scars on the sponge and in the closeup photo [lower] you can see that the upper animal on the left has its oral tube everted onto the sponge - a sure sign of feeding.
Best wishes,
Bill Rudman

Thanks Marina,
No one should get embarrassed about getting an identification wrong and especially when the blue chromodorids of the Mediterranean are concerned. The scientific literature is full of mistakes made by 'the experts'. When you have a group of animals which have evolved together to look like each other, we should be amazed by how well they have succeeded rather than be ashamed by how easily they can trick us.

What would be very useful would be to have photos of these species on their food sponges and laying eggs. Most have very specialised diets, eating only one or two related sponges, and most have very distinctive egg ribbons. While this information is available in the scientific literature - (although sometimes the information is confused because of misidentifications of the sponges and the nudibranchs)- photos would make it much more accessible to the general public.
Best wishes,
Bill Rudman

Thanks Alma,
Bill Rudman

Dear Marina,
I am pretty sure the uniform blue gills and rhinophores, the submarginal white/yellow mantle border, and the single median white line, are characteristic of this species, but I will wait for confirmation from a local expert.
Best wishes,
Bill Rudman

Dear Baki,
Thanks for these photos. I can understand your confusion with these 'blue chromodorids' which are still in the process of being sorted out. They represent a group of related species which have evolved to look very alike so that predators learn to leave this blue colour pattern alone. Predators avoid them because they store noxious chemicals in glands or sacs in their skin. Have a look at the Page on a group of red-spotted species in Australia which do the same thing.

So there are a number of problems to untangle. Firstly, we need to work out how many species there are and how to recognise them, preferably without having to dissect every individual to be sure! Secondly, we need to go through all the old literature and try and determine what all the old names for these species refer to. In some cases the descriptions consist of 2 or 3 lines of text saying little more than 'blue with yellow lines'. And thirdly we then need to try and untangle all the subsequent identifications and misidentifications of the names which have occurred in the last century.

Unfortunately many of the biological and ecological studies which have been published on these animals may be useless unless there is some way of checking and confirming the identifications of the animals they studied.

I am using the most recent revision of this group [Ortea, Valdés & García-Gómez, 1996 -see reference at top of page] but even that may not be the final story. In that your animal is identified as Hypselodoris orsinii and Cacospongia sp. is its reported food sponge. It is possible you have more than 2 of these blue species in your area. Apart from keeping an eye open for strange colour patterns, one valuable contribution would be to see if you can find blue chromodorids on other sponges, and where possible see if you can photograph them laying eggs. A closeup of the egg ribbon might provide a very useful piece of information.

Best wishes,
Bill Rudman