Opisthobranch defence


Moridilla brockii Bergh, 1888. Darwin Harbour, Northern Territory, Australia. May 1986. PHOTO: Bill Rudman.

In most snails, bivalves and other molluscs, the shell is the primary organ of defence. It is a “house” into which the animal can retreat from attack from predators and from harsh environmental conditions. Although the loss of the shell in sea slugs has given them the freedom to evolve spectacularly shaped bodies and brilliant colour patterns, it has come at a price. Their soft juicy bodies are a tempting available item of food for other hungry animals.

Nudibranchs, and to some extent other sea slugs, have evolved a series of strategies to protect themselves from predation. Many, but not all of these strategies, involve colour. In vertebrate animals with good eye sight, colour patterns have often evolved as signalling displays between individuals of a species or between different species. For example the great tail of the male peacock is an advertising sign to females, while eyespots in many moths and other insects are there to trick birds into believing they are dealing with a much larger prey than they though.

Nudibranchs however have very poor eyesight, in fact it is unlikely that they can sense much more than light and dark and passing shadows. Their colour patterns therefore have evolved entirely for defense against predators, mainly fish, which have well developed vision. Here are some of the main ways colour is used amongst sea slugs.

The first line of defence is to try and become invisible. Anyone who has attempted to collect nudibranchs knows how hard it is to find many species. There are many spectacular examples of camouflage. In some cases both the texture and the colour of the body match the colour and the texture of the food they eat. Examples include the nudibranchs:
Trippa osseosa (Dorididae)
Rostanga arbutus (Dorididae)
Jorunna sp. (Dorididae)
Corambe sp. 1 (Corambidae)
Favorinus japonicus (Glaucidae)
Cuthona kuiteri (Tergipedidae)
Marionia sp (Tritoniidae)

And amongst the sacoglossans :
Placida cf. dendritica (Limapontiidae)
Stiliger smaragdinus .(Limapontiidae).

A few opisthobranchs have the ability to change their colour to better match their surroundings. In one well-studied example, the bubble shell Haminoea navicula, has colour pigments in its skin contained in ramifying epithelial and subepithelial cells. By the extension and contraction of parts of these "melanophores" colour pigments can be moved or obscured so that the skin colour can change from black or dark brown to white in four or five hours. (See Edlinger,1982).

A few nudibranchs have bright colour patterns which are usually hidden but can be displayed rapidly as a means of startling and frightening off a potential predator. The classic example is the Spanish Dancer, Hexabranchus sanguineus, which only displays the brilliant red and white colour pattern on the dorsal side of it mantle when it is disturbed and beginning to swim.

Aeolid examples include:
Sakuraeolis nungunoides (Glaucidae)
Moridilla brockii (Glaucidae)

A shelled sacoglossan example is:
Lobiger viridis (Oxynoidae)

Although the startling deimatic displays are clearly warning displays, they involve an element of surprise. In one nudibranch family, the Chromodorididae, the colour patterns of many species are spectacular and obvious. Research in recent years has shown that colour in chromodorids is often used to warn potential predators that these animals are full of nasty distasteful chemicals and not worth attempting to eat. This is known as APOSEMATIC COLOURATION.

In a development of this we often find geographic areas where groups of unrelated chromodorids have evolved very similar colour patterns, so that they share the load of teaching fish to leave the colour pattern alone. One example of this MIMICRY in southeastern Australia are a group of about ten red spotted species, some of which are very difficult to tell apart. Most chromodorids have these mantle glands.

A few examples include:
Mimicry between red spotted species in south-eastern Australia.
Mimicry between nudibranchs and flatworms .
Ceratosoma brevicaudatum
Chromodoris woodwardae

Many fishes that swim in open water are dark above and light, often coloured silver, below. This adaptation results in the fish being difficult to see from above because the dark colour of the dorsal surface blends into the dark colour of the water below. It also helps to make the fish less visible from below because the light colour of the underside of the fish is less noticeable against the light shining from above. Two pelagic nudibranchs are spectacular non-fish examples of this very special form of colour camouflage.
Glaucilla marginata
Glaucus atlanticus

• Edlinger,K.,1982. Colour adaption in Haminoea navicula (Da Costa) (Mollusca - Opisthobranchia). Malacologia, 22: 593-600.
• Rudman, W.B. (1981b) The anatomy and biology of alcyonarian feeding aeolid opisthobranch molluscs and their development of symbiosis with zooxanthellae. Zoological Journal of the Linnean Society, 72: 219-262.
• Rudman, W.B. (1981c) Polyp mimicry in a new species of aeolid nudibranch mollusc. Journal of Zoology, London, 193: 421-427.
• Rudman, W.B. (1982a) The taxonomy and biology of further aeolidacean and arminacean nudibranch molluscs with symbiotic zooxanthellae. Zoological Journal of the Linnean Society 74: 147-196.
• Rudman, W.B. (1985) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris aureomarginata, C. verrieri and C. fidelis colour groups. Zoological Journal of the Linnean Society 83: 241-299.
• Rudman, W.B. (1986b) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Noumea purpurea and Chromodoris decora colour groups. Zoological Journal of the Linnean Society 86(4): 309-353.
• Rudman, W.B. (1986c) Nudibranchs: Nature's thieves. Australian Natural History, 22(1): 2-6.
• Rudman, W.B., Avern, G. (1989a) The genus Rostanga (Nudibranchia: Dorididae) in the Indo-West Pacific. Zoological Journal of the Linnean Society 96(3): 281-338.
• Rudman, W.B. (1991) Purpose in Pattern: the evolution of colour in chromodorid nudibranchs. Journal of Molluscan Studies, 57, (T.E. Thompson Memorial Issue):5-21.

Authorship details
Rudman, W.B., 1998 (October 14) Opisthobranch defence. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/defcol

Related messages

Colour in Elysia (again)

October 26, 2005
From: Skip Pierce

Hi Bill-
I'm presently sitting at home waiting for Hurricane Wilma to pass by. Tampa is going to be fine, but our collecting sites in the Keys (I'm actually supposed to be down there today) got hammered, not to mention the folks living in areas south of here (and our sites in Woods Hole are in some danger tomorrow). So while all this unpleasantness is going on, since I have electricity and an internet connection, I thought I'd divert my thoughts to all the recent comments on the Forum about slug bands and spots. Really, amongst the elysiids at least, bands and spots are not a reliable taxonomic characteristic, in my view. I have a paper in (lengthy) review that presents some microscopic evidence that the orange, red, blue and iridescent green pigments in at least some species of Elysia are contained in round, unattached cells in the hemolymph. That is, they, and therefore the color they contain, can move (or be moved-interesting question is why or by what??) around. So, in our aquaria we have the same species of slug (same specimen actually) that can have a continuous orange band at the edge of the parapodia, a broken orange band, or no orange band-or a red band- on any particular day. Similar is true for size and location of red spots, blue spots, and white spots-much of these pigments are in hemocytes too-sort of reminds me of nucleated red (hemoglobin in this case) blood cells that are present in some species of bivalves. These bands and spots are all superimposed on how long the slugs are able to sustain their symbiotic chloroplasts, and when they have last eaten. So specimens of the same Elysia species may be deep green, light green, brown, yellow or almost white (as I said, I think in an earlier message about Elysia ghosts [#14039 ]), depending upon when they last ate and what it was they ate. Now, I have no notion if this applies to non-elysiid species of course, but it would be easy for someone to check out, and maybe they should, because if it is the same as in the elysiids, it'll make determining species easier and if it's different from the elysiids, it'll give me something else to ramble on about during hurricanes.



Pierce, S. K., 2005 (Oct 26) Colour in Elysia (again). [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/15109

Dear Skip,
I must say I have been thinking of you all in the Caribbean / Gulf of Mexico area over recent weeks - you certainly have had more than your fair share of nasty weather.

I am interested in your comments on colour. I wish there were more bright young scientists with an interest in opisthobranchs as living animals. Colour is is a case in point. It is only a useful diagnostic [= identification character] if we understand the species, and its colour variability. This of course includes understanding which parts of its colour are a direct consequence of its genes, which parts area consequence of its food, and which parts, as you report, come and go. Until we have this information then we can only make guesses - they may be intelligent guesses, but they are still guesses. Having said that, colour, if we realise its limitations, is often a very useful character.

In the case of elysiiids, such as Elysia ornata, which has been the topic of recent discussions, it is possible that both Caribbean and IWP populations are the same species, or they may not. In any future investigation of the question, it would be useful to know if there are any constant colour differences between the populations. Perhaps in the Caribbean populations, if an orange submarginal band is present on the parapodia, it is separate from the black border, and perhaps in IWP animals the orange band, if present, is touching the black border. I don't know if that is important at this stage, but photos are one thing that enthusiastic divers can provide. What opisthobranch workers lack are large collections. When you see taxonomists who work on shelled animals start work, the first thing they can do is visit museums and look at huge shell collections from around the world. Even before starting anatomical studies they can get a 'feel' for similarities and differences in shell colour, shape and ornamentation, across the geographic range of a species. This gives them a head start in searching for anatomical differences. With opisthobranchs, the collections are much smaller and usually all we have are the decolourised preserved animals and minimal information on the living animals.

If we look at a group like the chromodorids, and particularly the C. quadricolor group which we have been discussing recently on the Forum, they represent another problem. They are almost certainly mimicking each other, so we have to work out whether the colour pattern mimicry is so good thta we can't identify species solely from the colour. I was tricked by two species in southeastern Australia which were so similar in colour that I only realised the possibility there were two species involved when I noticed that some animals when preserved became decolorised and others went dark brown. In that case there were clear anatomical differences I could use to show there were two species, but I wouldn't have known to look if sone had notgone brown in preservative. However in some species of Chromodoris, the anatomical differences are so slight that they are unsafe to use on their own. That is why at times I sound like a broken record, asking people to try and find information on their egg masses, sponge prey etc.

I guess the point is that colour can often be a very useful identification tool, but only after we have defined just what the species are by using as many characters as possible, including anatomy, food, egg mass morphology, etc, and colour. I have gone on a bit, but since you are waiting out a hurricane, I hope I have a captive audience.
Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Oct 26). Comment on Colour in Elysia (again) by Skip Pierce. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/15109

Colour compounds in opisthobranchs

June 22, 2002
From: Virginie Fruh

Just wondering if anyone could send me information or post a reply concerning what types of compounds are known to be active in the colouration of nudibranch mantles. I know that symbiosis with dinoflagellates is one of the sources of the colouration, but is it for all the colours, or are there carotenoids and others which are involved?


Fruh, V., 2002 (Jun 22) Colour compounds in opisthobranchs. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/7280

Dear Virginie,
I gave a very brief comment some time ago to a similar question. If anyone can send a more detailed reply to the Forum it would be a useful addition
Bill Rudman

Bright colors?

June 18, 2001
From: Alisse Cassell

Why are nudibranchs so brightly colored?
Alisse Cassell


Cassell, A., 2001 (Jun 18) Bright colors?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4611

Dear Alisse,
Have a look at the page on Opisthobranch Defence - colour. I think you will find more than enough information there.

If you are looking for background information on sea slugs, one good place to look is the General Topics Index (just click the button at the top and bottom of each page). Or use the SEARCH button and type in a word you are interested in like colour. You may have tried, but if so, I guess you typed color. I'm afraid there will be a few words like 'colour' which the Forum spells a little differently from you - sorry my SEARCH BUTTON doesn't know that North Americans spell 'colour' without the 'u'
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Jun 18). Comment on Bright colors? by Alisse Cassell. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4611

Sea Slugs changing colour

December 3, 1999
From: Diana Huynh

I am doing an experiment on sea slugs/hares and I was wondering if you can send me any information on my topic. My experiment is about if they will change their color to the color of the algae that we feed them and I need to know some types of algae that are different colors and if they are available to get in the San Diego area. I have approximately 2 weeks to do my experiment. Please if you can help me.

Diana Huynh


Huynh, D., 1999 (Dec 3) Sea Slugs changing colour. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/1634

Dear Diana,
I'm afraid from the other side of the Pacific I'm not the best person to ask about sea life in San Diego. Perhaps you should try your local museum or university.

I can make a few general points. Firstly even if you knew exactly what you were going to do I very much doubt you could do it in 2 weeks. Finding out what animals do usually takes a very long time, often with much failure on the way.

Have a look at the page on Colour, where some different ways sea slugs can change colour is described, and also look at the page on Solar-powered sea slugs where colour changes which are caused by the colour of the algae in the sea slugs is described.

Some Sea Hares change colour as they grow and change their algal food, but this is a relatively slow process. The most likely animals you could use to show how they change colour are sacoglossans, which often keep algal plastids and their pigments, in their bodies. One species we know changes colour as it changes algal food is Elysia cf. furvacauda but that is only found in southeastern Australia, and only eats certain species of algae.

Apart from not knowing which slug to choose, your other main problem will be finding out which algae the slug you choose will eat. Most are very choosy about their food.

I'm afraid this is not much help other than to demonstrate to you that finding out what animals do is a very difficult and time-consuming task, usually with lots of problems. Although you can sometimes make a flower change colour by putting different coloured water in the vase, its a little bit more difficult to demonstrate such things in living animals.

Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Dec 3). Comment on Sea Slugs changing colour by Diana Huynh. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/1634

Colour in sea slugs

May 28, 1999
From: Austin

Dear Bill,
Can sea slugs synthesize their own body color pigments or are these necessarily sequestered from ingested prey material?



Dear Austin,
Most of the information on opisthobranch colour is on one page Defence - Colour patterns. Have a look there as it links to many different ways that colour is used by sea slugs, and also the different ways they obtain their colour.

Many do obtain their colour from external sources. Many of the sacoglossans, like Stiliger smaragdinus, get their colouring from the algal pigments they ingest, and the nudibranchs which retain unicellular plants (zooxanthellae) are coloured by those plants. Others, like most species of Rostanga, including Rostanga arbutus and Rostanga bifurcata take the red pigments from their sponge food and incorporate in their skins, and apparently their eggs as well.

Despite this widespread reliance on other plant and animal pigments, the majority of opisthobranchs manufacture their own pigments. A good example of this is the chromodorids which apparently generate the pigments which make their spectacular colour patterns.

Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (May 28). Comment on Colour in sea slugs by Austin. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/891