Solar-powered sea slugs


UPPER: The sacoglossan Placida cf. dendritica showing the green network of ducts which contain the green chloroplasts from its algal food.
LOWER: The aeolid nudibranch Pteraeolidia ianthina which "farms" colonies of brown single-celled algae (zooxanthellae) in its body.
PHOTOS: Bill Rudman.

Two quite different groups of sea slugs have evolved ways of using the ability of plants to convert the sun's energy into sugars and other nutrients. In simple terms they have become "solar powered".
The herbivorous sacoglossans are suctorial feeders removing the cell sap from the algae on which they feed. In most, the cell contents are simply digested by the slug. Some species however have evolved branches of their gut which ramify throughout the body wall and contain plastids, which are the photosynthesising "factories" from the algae, alive and operating. In many cases these plastids are chloroplasts, but sacoglossans that feed on red and brown algae are also reported to keep the plastids from these algae alive as well. As I show elsewhere in the Forum, one species, Elysia cf furvacauda changes diet and plastid at least three times during its life history.
In nudibranchs, which are all carnivorous, many have evolved similar ways of keeping whole single-celled plants (zooxanthellae) alive in their bodies. In most cases the zoxanthellae are obtained from their food, often cnidarians, which already have symbiotic zooxanthellae in their bodies. This symbiosis as evolved many times within the nudibranchs with examples in many quite unrelated families and orders. Have a look at the following species for further information: Pteraeolidia ianthina, Phyllodesmium longicirrum, Phyllodesmium briareum, Phyllodesmium crypticum, Berghia verrucicornis, Spurilla australis, Pinufius rebus - zooxanthellae symbiosis, A. ransoni,A. harrietae, Melibe megaceras, Elysia cf. furvacauda, Elysia cf. pilosa, Plakobranchus ocellatus, Elysia crispata, Elysia chlorotica.

For further information:
Zooxanthellae Symbiosis References.
Zooxanthellae - what are they?
Zooxanthellae - in Cnidarians
Zooxanthellae - in nudibranchs
Chloroplast Symbiosis References.
Aspects of Coral feeding
Chloroplast symbiosis Research
Sacoglossan Feeding
Feeding on Palythoa

Authorship details
Rudman, W.B., 1998 (October 11) Solar-powered sea slugs. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Related messages


October 8, 2008
From: Mihir Pathak


I am a PhD student in Mechanical Engineering at the Georgia Institute of Technology. My focus area is studying the thermal science and bio-mechanics of natural phenomena. I have recently, learned quite a bit about the "solar powered sea slug " and found this forum to be very helpful in getting some questions answered. Here are my questions:

1. How do they move to light? What physically/chemically happens when they are exposed to phototaxis? Also, what is the photosynthesis process in the particular algae that they eat?

2. How do these slugs store their energy? What properties enable them to do so?

3. What is an effective way to measure the O2 and CO2 levels when the slug is exposed and not exposed to light?

4. What other measurements will the biologists and chemists in this forum find useful that an engineer can do? Essentially, I would like to create a mimic-ed device that can do something similar. Any collaborators?

5. I will be getting some slugs into my lab soon. Any advice on how to build their tank? What kinds of apparatus would I need? Lamps, heaters, water filters, bubblers, live coral?, other things? How do I keep them alive? How do I keep the algae alive?

Any sort of assistance on these questions would be great. Thank you for letting me post on this forum. I look forward to hearing back from you. Thanks.


Mihir Pathak
Mechanical Engineering
Georgia Institute of Technology

Pathak, M.G., 2008 (Oct 8) Tank/Phototaxis/Energy/Experiments. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Mihir,

As you will see on the solar power Page, there are two major groups of solar-powered slugs. One group, the sacoglossans, are essentially herbivores who remove intact plastids from the plants and keep them alive and functioning in their own bodies. The second group are essentially carnivores, or related to carnivores, and they nurture single-celled plants [zooxanthellae] in their bodies. In most cases they have 'stolen' the zooxanthellae from their original cnidarian hosts [such as sea anemones or soft corals].

Quite different procedures are needed to keep these two types of animal - and their food - in aquaria. As far as I know only the first group have been successfully kept and studied in the lab. If you go to the Elysia chlorotica and Elysia clarki Fact Sheets and look at the attached messages you will find addresses of a number of researchers working in this field.

Concerning the second group, Ingo Burghardt [] may be able to offer some suggestions,

Best wishes,
Bill Rudman

Rudman, W.B., 2008 (Oct 8). Comment on Tank/Phototaxis/Energy/Experiments by Mihir Pathak. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Solar-powered Sea Slugs - tertiary endosymbiosis?

October 28, 2002
From: Sam Hsieh

Dear Bill
Even the largest chloroplast genomes account for less than 25% of the gene products needed for plastid function. How can an isolated organelle, normally dependent upon the genes residing in its own nucleus for most of the proteins making up its photosynthetic machinery, remain physically stable and function for months in a foreign cell? Are we seeing tertiary endosymbiosis in action?
Sam Hsieh
2nd Year Student
University of British Columbia

Hsieh, S., 2002 (Oct 28) Solar-powered Sea Slugs - tertiary endosymbiosis?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Sam,
When I suddenly get 5 or 6 messages all asking much the same question, I think it is fair to suspect that a teacher has asked a class to answer a question. So this answer is for all of your class mates as well.

The ability of some animals, such as solar-powered sea slugs to remove functioning plastids from plants and keep them alive in their own bodies [sacoglossans] or to keep whole plant cells alive in their bodies [nudibranchs], is fascinating for many reasons and is fertile ground for opisthobranch workers, physiologists, botanists, geneticists etc. I think it will be many years before we can say just how the symbiosis works. I think Kerry Clark coined the term kleptoplasty, or at least popularised it, for the phenomenon of 'stealing' plastids. It is from the same Ancient Greek word which gives us the word kleptomania - [compulsive stealing] an affliction which seems to infect American filmstars with monotonous regularity.

And now to your question about whether this is a tertiary symbiosis. I guess we have to define what a plastid is and what its origin is. This is, I am afraid, getting a little outside my field of expertise. What I can say is that you should have a look at some of Lynn Margulis's publications. I have a very thumbed copy of her 1981 book Symbiosis in Cell Evolution but I am sure you can find more up to date editions to have a look at. She clearly stated the hypothesis that eucaryotic cells evolved from bacteral ancestors by a series of symbioses. Many cell organelles are considered to be symbiotic organisms which 'invaded' protoorganisms in the early stages of the evolution of life on this planet. Plastids, like mitochondria, have their own genome, and at cell division act as though they are symbionts. I don't know if we gain much in our understanding of their biology by trying to number their 'grade' of symbiosis.

As I said above, their are two types of 'solar powered slugs. If we first consider the sacoglossans. In plants, the plastids could be considered primary symbionts. When they are removed by sacoglossans to their own cells, the plastids still occupy the same position in relation to the cell, as a primary symbiont. However if we look at the plastid in the sacoglossan we could say that this is its second primary symbiosis.

If we look at the solar-powered nudibranchs, the situation is a bit more complex. They remove whole single-celled plants [zooxanthellae] from the primary host (usually a cnidarian) and re-use them in their own tissue. In this case the plastid is a symbiont of the zooxanthella which is the symbiont of an animal. I guess you could call this a secondary symbiosis but it all becomes quite confusing if you want to record that it has been moved from its first host to a second host. In fact I once described the removal of zooxanthellae from cnidarians to nudibranchs as a 'secondary symbiosis' but I was describing the transfer of the zooxanthella from its primary host to a secondary host. You, on the other hand, are trying to number how many steps we can go back until we get to the first symbiont. I think its all a bit confusing and doesn't really enlighten us very much.

• Margulis, L. 1981. Symbiosis in Cell Evolution W.H.Freeman & Co.: San Francisco. 1-419.

I hope my answer is not too confusing,
Best wishes,
Bill Rudman

Rudman, W.B., 2002 (Oct 28). Comment on Solar-powered Sea Slugs - tertiary endosymbiosis? by Sam Hsieh. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

'Solar-powered' sea slugs

September 4, 2001
From: Caroline R. Cripe

Sea slugs receive food from the algae that lives within their skin. What benefits, if any, do you think algae receive from sea slugs
Caroline R. Cripe

Cripe, C.R., 2001 (Sep 4) 'Solar-powered' sea slugs. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Caroline,
I guess you have found the Solar-powered Sea Slug Page where there is a lot of introductory information.

It's a bit hard to answer your question about 'what benefits' the algae receives from its relationship with sea slugs. Asking about benefits suggests that there is an accountant sitting there balancing expenditure against income. I'm afraid nature doesn't really work that way. These systems have evolved over millions of years and don't necessarily utilise the most cost efficient methods of operation. The important thing is that the whole system works as a single unit and the organisms involved survive to reproduce and pass on the system to the next generation of participants.

We are not even sure that the one-celled algae (zooxanthellae) can survive outside the slug. The zooxanthellae are related to free-living algae called dinoflagellates, but I don't think there is any research that has shown that they can complete their life-cycle as free living plants. Asking about the benefits of living in the slugs suggests there is an alternative, when in fact there probably is not. The symbiosis between zooxanthellae and animal is now an integral part of the life of both the plant and the animal. We might feel inclined to say that the plant benefits from a stable protected environment in the animal, but that doesn't mean very much because the plant probably hasn't any alternative.

You must also realise that in the case of the sacoglossan sea slugs it is not a whole plant that is kept alive in the slug's tissues, but just the chloroplasts, which are the organelles found in green plant cells which photosynthesise. In the case of chloroplast symbiosis, the chloroplast is either a 'slave' of the plant or a 'slave' of the sea slug, without one or the other it will die.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Sep 4). Comment on 'Solar-powered' sea slugs by Caroline R. Cripe. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

sea slugs - plants or animals?

March 23, 2001
From: Tom Mackillop

To who it concerns
I am in yr.11 at school and in our biology class we have some dispute over the kingdom into which sea slugs fall into, animal or plant. I would appriciate hearing from anyone who could set us on the right track
Tom Mackillop

Mackillop, T., 2001 (Mar 23) sea slugs - plants or animals?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Tom,
One of the main differences between plants and animals is that plants produce their own food by chemical reactions while animals obtain their nutrients by eating existing organic matter. Sea Slugs are a specialised group of snails (Phylum Mollusca), and are definitely animals. Some are herbivores (plant-eaters) while others are carnivores, often eating only a very specialised group of animals.

There are two fascinating groups of sea slugs, which I have called 'Solar Powered' because they have become very plant-like in their behaviour. Have a look at the Solar Powered Slugs Page for some background information on these animals. Perhaps they are what made you wonder whether they are in fact plants?

Have a look also at the messages below yours on this page as there is more information with each message. If you click on any underlined word it will take you to another relevant page.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Mar 23). Comment on sea slugs - plants or animals? by Tom Mackillop. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Elysia and chloroplast symbiosis

August 4, 2000
From: Daniel Barshis

To Dr. Rudman,
My name is Daniel Barshis and I am currently a Senior year undergraduate at the Evergreen State College [Washington, USA].  The opportunity for advance study in the marine sciences here is fairly limited and I am doing some preparatory searching for a possible marine diving volunteer position this coming winter.  I am contacting you because I am also extremely interested in the evolutionary biology of photosynthetic marine invertebrates, particularly Elysia sp.  I have done much research into the natural history of and current research being done on Elysia sp.  I was wondering if you had any suggestions of places to go or other people to contact about possibly working with them on a research project during January to March of 2001.  Any information or leads you can think of would be incredibly helpful.  I would like to take my interests out of the library and into the water.  I have an open water certification and have done some recreational reef diving in the tropics.
Thank you for your help,

Barshis, D. , 2000 (Aug 4) Elysia and chloroplast symbiosis. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Daniel sent this request to me personally and with his permission I have posted it on the Forum. If anyone would like an assistant or could suggest someone he should contact, perhaps they could email Daniel directly.
Bill Rudman.

Rudman, W.B., 2000 (Aug 4). Comment on Elysia and chloroplast symbiosis by Daniel Barshis . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Review of sacoglossan - plastid symbiosis

May 17, 2000
From: Liz Summer

Hi Bill,

I would like to advertise a review on photosynthetic sea slugs that Mary, Jim, and I just had published in the journal Plant Physiology titled-

Mary E. Rumpho, Elizabeth J. Summer, and James R. Manhart (2000) Solar-Powered Sea Slugs. Mollusc/Algal Chloroplast Symbiosis. Plant Physiology, 123: 29-38.

We even got the cover picture (which demonstrates the broad thinking of the American Society of Plant Physiologists) which can be seen at - click on the image to enlarge. Unfortunately, you have to have a password to access the paper on-line. Most university libraries carry the journal. The paper is written from a chloroplast's point of view and discusses why these associations defy our normal understanding of chloroplast biology as well as some wild speculations on mechanisms and evolutionary significance.

Liz Summer

Summer, E., 2000 (May 17) Review of sacoglossan - plastid symbiosis. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Liz,
Congratulations to you and your colleagues on this publication. It is nice to have a Sea Slug as the 'covergirl' on any journal and it is certainly a bonus to have it on a plant journal!

This is a very useful review of our knowledge of the physiology and evolution of the slug - chloroplast symbiosis. Although, as you say, it is from the chloroplast's point of view, it will definitely be required reading for anyone wishing to understand the significance of the symbiosis.

It also has a very interesting summary of the life history of Elysia chlorotica, and also has some useful recent references which I will need to add to my list of references on the topic.

Bill Rudman.

Rudman, W.B., 2000 (May 17). Comment on Review of sacoglossan - plastid symbiosis by Liz Summer. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Symbiosis among Phyllidia?

January 8, 2000
From: Mark Schoenbeck

Do any species of the genus Phyllidia form symbioses with algae? If not, which is the closest relative of P. pustulosa that does form a symbiosis?

Mark Schoenbeck

Schoenbeck, M., 2000 (Jan 8) Symbiosis among Phyllidia?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Mark,
To my knowledge there are no dorid nudibranchs with an algal symbiosis. The closest relatives - and they are not very close - would be the aeolids and arminoideans I discuss on the "Solar-powered sea slugs" Page.

If you know of any dorids, or have suspicions about possible symbionts, I would like to hear about them.

Bill Rudman.

Rudman, W.B., 2000 (Jan 8). Comment on Symbiosis among Phyllidia? by Mark Schoenbeck. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

The solar-powered 'Ruffled Sea Slug'

December 5, 1999
From: Molly E. Hagan

I am looking for information concerning a species referred to as a "ruffled sea slug". I know that it has the unusual ability to apply the byproducts of plants to its body to grow. Is this just one kind of slug, or is it a branch?

Thank you for any information you might have.

Hagan, M.E., 1999 (Dec 5) The solar-powered 'Ruffled Sea Slug' . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Molly,
I guess the 'Ruffled Sea Slug' is Tridachia crispata, which is found in the Caribbean. It is quite similar in shape to species such as Elysia ornata to which it is closely related. Tridachia and Elysia are sacoglossans, a group of herbivorous sea slugs, which suck the cell contents from the algae they feed on. Some have developed the means to keep the photosynthetic plastids from the plant tissue alive in their bodies where they are able to photosynthesise and provide extra nutrients for the animal. There are a number of families of nudibranch sea slugs which do something similar, keeping whole one-celled plant alive in their bodies. Have a look at the page on Solar-powered sea slugs for further information, and be sure to look at the messages and answers below yours on this page.

Best wishes,
Bill Rudman.

PS: Unfortunately I don't have a photo of Tridachia. If anyone out there can oblige I would be grateful.

Rudman, W.B., 1999 (Dec 5). Comment on The solar-powered 'Ruffled Sea Slug' by Molly E. Hagan. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Sea Slugs & symbiotic algae

August 16, 1999
From: Jennifer Whittington

Sea slugs receive food from the algae that live within their skin. What benefits if any do you think algae receive from the sea slugs?

I am a first year student in high school Biology.


Whittington, J., 1999 (Aug 16) Sea Slugs & symbiotic algae. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Jennifer,
If you look at the page on 'solar-powered' Sea Slugs you will see that there are two quite different processes at work.

The herbivorous sacoglossan Sea Slugs, suck the cell contents from the sea weeds they feed on. From this cell sap, they keep alive and functioning the plastids (those parts of the plant cell which convert the sun's energy into sugars). The conversion of the sun's light energy into food for the plant is called photosynthesis. In green plants the plastids are green, and are called chloroplasts. Most sacoglossans are coloured by the plant pigments they retain in their bodies.

Amongst the nudibranch Sea Slugs, which are all carnivores, a number of different families have evolved ways of keeping microscopic single-celled plants alive in their bodies. These single-celled plants are called zooxanthellae, and although they have free-living relatives in the plankton, they are adapted to living within the tissues of animals. The most spectacular zooxanthellae are the species which live in the tissues of coral animals. Without their symbiotic zooxanthellae, the tiny coral polyps would be unable to produce the calcium carbonate skeleton, which is the building material for the great coral reefs of the world.

Now to your question about what benefit the plant gets from the association. I guess your question refers to the Sea Slugs with zooxanthellae. I'm afraid applying cost-benefit analysis questions, which are the joy of accountants, probably doesn't have much meaning in the natural world. In the real world the only reward is survival and the ability to produce a new generation to carry on your genes. I guess being a microscopic free-living cell, floating around in the plankton, has its risks. There are many filter feeders waiting to eat you and you are at the mercy of the currents and tides. Despite this, free-living phytoplankton are clearly a very successful life form. From all accounts, living in an animal which has evolved special anatomical features and behaviour patterns, for your comfort, has its benefits, much like a plant being cared for in a greenhouse.
Ove Hoegh-Guldberg's studies on Pteraeolidia ianthina showed that zooxanthellae within its body, breed very rapidly, and at the same time produce nutrients, far in excess of their own requirements. This suggests that the zooxanthellae are living in a very healthy, protected environment.

The zooxanthellae are specially adapted for this symbiotic life and although we are not 100% sure, it seems they do not have the ability to live free. There is therefore not much point in listing the good and bad aspects of this life in some sort of balance sheet. This is the only life possible to them, they do not have the alternative of a free-living existence.

Best wishes,
Bill Rudman.

1999 (Aug 16). Comment on Sea Slugs & symbiotic algae by Jennifer Whittington. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Information on solar animals?

March 10, 1999
From: Derek Carmona


My name is Derek Carmona and I have been studying the idea of animals using chloroplasts for four years now. My question to you is if you know of any scientists who have made any advances on the subject and if so could you send me any information you can?

Derek Warren Carmona

Carmona, D., 1999 (Mar 10) Information on solar animals?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Derek,
I am not sure what level you are at in your studies so its a little hard to know at what level to answer your question. If you are at university, I would suggest you look at some of the references I have just posted on chloroplast symbiosis. Also look at the information at the top of this page above your message, and also at some of the correspondence you will find below your message. You will also find some information on the Flatworm Page about Convoluta roscoffensis which is a flatworm with symbiotic chloroplasts.

Best wishes,
Bill Rudman.

Solar-powered sacoglossans in Norway?

March 9, 1999
From: Jussi Evertsen

Dear Bill.
Thank you very much for your help. I am also very curious about if any of the Elysia species found here in Norwegian waters might show "solar power" affinities. Do you have any clue to which longtitude this solar power affinities work? Is this simply a tropical trait?

Your sincerely

Jussi Evertsen
Trondhjem Biological Station
Departement of Natural History
7034 Trondheim

Evertsen, J., 1999 (Mar 9) Solar-powered sacoglossans in Norway?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Jussi,

There are sacoglossans in quite northern latitudes with symbiotic chloroplasts. For example Elysia chlorotica in Nova Scotia and Elysia viridis in England. Also the photosynthetic flatworm Convoluta roscoffensis lives in the North Atlantic.

I have posted a list of publications on chloroplast symbiosis which may be useful. Any updates would be welcome.

One active research worker in the field, who could give you up to date advice is Cynthia Trowbridge whose address is:

Dr. Cynthia D. Trowbridge
Research Assistant Professor,
Oregon State University
Department of Zoology/Hatfield Marine Science Center
Newport, OR 97365
fax 541-867-0105

Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Mar 9). Comment on Solar-powered sacoglossans in Norway? by Jussi Evertsen. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Photosynthetic Animals

December 16, 1998
From: Michael Rhodin

Dear Dr. Bill Rudman,
My name is Michael Rhodin and I am a freshman at Trinity College in Hartford CT. I am doing research on the possibility of introducing a working chloroplast into an animal cell to make it photosynthetic. I have looked at the sea slug Elysia chlorotica and seen that it eats a type of algae called Vaucheria litorea, and its cells incorporate the chloroplasts into the slug's cells. I have also found that there are several genes found in the Elysia chlorotica which match the genes of the Vaucheria litorea, genes which work in relation to the chloroplasts. I was wondering if it would be possible through recombinant DNA to remove these genes and insert them into another creature's cells such as a fish, invertebrate, or reptile. Then I was wondering if that organism ate the Vaucheria litorea, would it take up and maintain the chloroplasts? Has this been attempted? Do you think it could work? Thank you for your time, this really is a big help, and a matter of great interest to me. I am so glad that I have found your site.
Michael Rhodin

Rhodin, M., 1998 (Dec 16) Photosynthetic Animals. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Michael,
I am not an expert in plastid physiology so I have sent copies of your message to a few colleagues who hopefully are more able to answer your question intelligently.

There are however a number of general points.
You say there are a number of genes in the Elysia which match the genes in the Vaucheria it feeds on. The obvious question to ask is how did you ensure that the Vaucheria-like genes were not contaminants from its food?

The other point is what got me interested in these symbioses in the first place, and that is the morphological adaptations that the "host" has undergone to become an efficient plastid or zooxanthellae "farmer". All these animals are related to animals which don't have symbiotic relationships and we can see how in certain phylogenetic lines of animals species have modified their anatomy, (finely branched gut, branches into the epithelial layer, flattened cerata, transparent bodies etc) all to better enable them to keep their plant tissues in an optimum environment for photosynthesising. A good example is shown in the aeolid genus Phyllodesmium where in one genus we can see major morphological changes which are correlated with the varying ability between species to grow zooxanthellae in their bodies (Rudman, 1991 - see references at top of page).

All things being equal, if gene transfer is the key - and possible, - then I suspect the experiment would only work in animals that had by chance a morphology which would provide an hospitable "nursery" for plant tissue. For example, the skin of most vertebrates, I would think, would block out sunlight and so prevent photosynthesis.

If you have any photos of Elysia chlorotica either on or off its food, or some photos of the plastids, it would be nice to put them up on the Sea Slug Forum.

With luck, one of the people I sent copies of your message to, will reply.
Best wishes,
Bill Rudman.

Rudman, W.B., 1998 (Dec 16). Comment on Photosynthetic Animals by Michael Rhodin. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

"Solar-powered" Sea Slugs

November 29, 1998
From: Tammy

I am currently doing a research paper and would love to include these wonderful solor powered sea slugs. However my question is fairly simple, is the relationship between the sea slug and the photosynthetic algae either:
mutualistic, which i cannot see
commensal, which may be possible as the algae is being provided a home.
or parasitic as the sea slug is eating the algae and then harvesting the algae in its gut.

I really need an answer to my question fairly quickly.

If anyone has any thought please feel free to give me a shout:)


University student

Dear Tammy,

I know teachers often like to pigeon-hole nature into convenient categories but I'm afraid the relationships that organisms form with one another forms a continuum which defies our best attempts at dividing into suitable categories. In the Middle Ages, theologians used to discuss in great seriousness how many angels could fit on to the head of a pin. I suspect attempts at categorising relationships between organisms is of similar value.

In this case, the plant involved is a dinoflagellate algae belonging to the genus Symbiodinium or Gymnodinium. These dinoflagellates are known as "zooxanthellae" and are commonly found living in the tissues of cnidarians. Without them the polyps of the tropical hard corals would be unable to build the huge coral reefs of tropical waters. Other invertebrates also harbour zooxanthellae, including the Giant Clams (Tridacna spp). The relationship is usually described as an "endosymbiosis" and as I said above, in the case of reef-forming corals, seems to be essential for the coral's well-being. The increased incidence of coral-reef "bleaching", where the coral colonies over large areas eject their zooxanthellae, is causing grave concern amongst reef ecologists.

There is also debate over how many species of zooxanthellae there are. Some say there are only a few species, others that each 'host' has its own species of zooxanthellae especially adapted for that 'host'. The zooxanthellae in nudibranchs are particularly interesting in that debate because they are zooxanthellae which have been removed from their initial host, a cnidarian, and transplanted to a second host, a mollusc, in a completely unrelated phylum. If a zooxanthella can survive in two quite distinct phyla then it would tend to support the argument that there are a few widely distributed species of zooxanthellae.

The other point to realise is that the ability to "house" and "farm" zooxanthellae in nudibranchs has evolved independently a number of times and not all "hosts" are equally proficient in doing so. Some species seem to do no more than temporarily retain zooxanthellae until the zooxanthellae die. In these cases they are probably only useful as colour camouflage as they help to colour the slug and so camouflage it on its similarly coloured cnidarian food. From such simple beginnings we can find all stages to the "ultimate" stage where zooxanthellae are successfully farmed and bred within the nudibranch's body and the nudibranch gains a significant proportion of its nutrient requirement from the zooxanthellae.

If you are thinking of sea slugs as a whole rather then just nudibranchs, then a few words about the sacoglossans. They have a remarkably similar story. Instead of removing whole plants from their food animals, like the aeolid nudibranchs, they are herbivores which remove the plastids from the plants they feed on intact. Again I guess this is a "symbiosis" though the plastids are hardly a potentially "free-living" partner.

Plastids are considered cell organelles. If you have time a fascinating book to look at is:
Margulis, L. (1981). Symbiosis in Cell Evolution. Life and its Environment on the early Earth. W.H.Freeman & Co: San Francisco.
In it she discusses her ideas about the evolution of life and how plastids and other cell organelles such as mitochondria possibly evolved as endosymbiotic organisms in early protozoa.

Hope this of some help,
Best wishes,
Bill Rudman.

Rudman, W.B., 1998 (Nov 29). Comment on "Solar-powered" Sea Slugs by Tammy. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Nudibranchs in Symbiosis with Zooxanthellae

November 9, 1998
From: J.E. Austin

3 November 1998
Dear Dr. Rudman:
First, I want to thank-you for responding to my last questions and praise the exciting images located on the slug site. I have spent all night reading these pages. As I mentioned, I am an undergrad from Florida State University in the US and am involved in a Research Experience for Undergraduates. On Wednesday, I will join a deep-water boat cruise off of the south coast of Bermuda. I hope to dip net Sargassum and find the Sargassum nudibranch, so I'll keep you posted on my findings. :)
I have been watching Hypselodoris nudibranchs that were gathered outside the Bermuda Biological Station. They sit in my flow-through tank and have laid ribbons with red eggs. Some of them have little purple nodules beneath the side flaps of tissue. They are beautiful and hungry, I think.
I've been thinking of algal symbiosis again as I'm reviewing literature on chemical communication between algal endosymbionts and cnidarians. The mechanisms for mutual responsiveness have yet to enumerated. It seems- given the widespread independent evolution of symbiosis with algal cells- we'd have more answers. But instead, I'd like to please ask some more questions. :)

Concerning: Kempf, S.C. 1984. Symbiosis between the zooxanthellae Symbiodinium microadriaticum and four species of nudibranchs. Biol. Bull. 166: 110-126.

1) Do you know whether further Hawaiian species of Melibe have been described and whether a phylogeny exists for the Genus?
2) With the intent of raising nudibranch eggs, can one just keep them in a bowl with an aerator? How would one know if they've hatched, what to feed them?
If I wanted to do a time series to capture structure over an age range, can I just preserve them in SW 10% Formalin, then slide mount samples?
3) Is there a general use stain for sea slug tissue? I'm leaning toward neurons.
4) The paper indicates that the aeolid Berghia major has an oral veil. And I know that Melibe species do, has this morphological attribute been derived independently in other nudibranchs? Have any further observations on Melibe pilosa feeding been made since the Kempf paper; another food source beyond just captured crustaceans in the oral veil, i.e. some algal cell-containing prey?

Concerning: Rudman, W.B. 1991. Further studies on the taxonomy and biology of the octocoral-feeding genus Phyllodesmium, Ehrenberg, 1831 (Nudibranchia: Aeolidoidea). J. Moll. Stud. 57: 167-203.

1) As Phyllodesmium can autotomize ceras that contain antifeedant chemicals, have the actual compounds responsible been isolated or characterized? How would the anti-feedant nature be characterized- fed to fish?
2) The paper mentions that Chromodorids can concentrate anti-feedants from consumed sponges. Has this been assayed in Hypselodoris zebra? This orange, blue-striped dorid found in Bermuda feeds on the purple sponge Dysidea etherea?. I have 6 specimens and would like to try something, any suggestions?
3) The paper makes reference to Rudman 1984, but I could not find the reference at the end and was wondering if you have that citation?

Thank-you for your time,
J.E. Austin

Austin, J.E., 1998 (Nov 9) Nudibranchs in Symbiosis with Zooxanthellae. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear "j.e.",
Your obviously have been doing some thinking and reading! Sorry I can't answer all your questions at once, but in the hope someone out there has something to say on any of your points I am posting your questions unanswered. .. and will get to them in the next few days.

I hope you find some some animals in the Sargassum. If you have the facilities to photograph anything you find, including Hypselodoris zebra, I would love to post them on the Slug Forum. If you can't provide scans that is no problem. Just send photos or slides to me at:
The Australian Museum,
6 College St
Sydney, NSW 2000

I will scan them here and return the photos to you.

Best wishes,
Bill Rudman.

Your "solar-powered" sea slugs.

October 11, 1998
From: Amanda Lindsey

Dear Dr Bill Rudman:
My name is Amanda Lindsey and I am currently a junior in high school and am enthralled with science.  Science fair is coming around the corner and my topic and hypothesis will interest you. 
Topic: Can invertebrates undergo photosynthesis?
Hypothesis:  By injecting slugs with a chlorophyll solution, they shall meet all the necessary requirements to undergo photosynthesis and sustain life.
While searching on the internet for information on slugs and their anatomy, behavior, environment, etc., I stumbled upon your nudibranchs and their adaptation for ingesting a whole microscopic plant intact, allowing for them to have a symbiotic relationship and practically never feed themselves.  I wonder if this natural occurrence is similar to my "artificial" method of injection of chlorophyll.  Please
respond with your thoughts on my project.  Also, any information that you have on slugs, land slugs that is, would be greatly appreciated.  Some other web sites and research "helps" would also be received with warmth and gratitude.  Thanks for your time.
                                        Amanda Lindsey

Lindsey, A., 1998 (Oct 11) Your "solar-powered" sea slugs.. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Amanda,
Your question inspired me to put some more examples of "solar-powered" sea slugs on the Slug Forum. Have a look at the examples I have mentioned above on this page.

I'm afraid your proposed experiment is doomed to failure I would think. The process of photosynthesis involves a series of complicated chemical reactions. Chlorophyll is involved, at least in green plants. It is a molecule which traps light. For anything to then happen needs the combined activity of the other molecules and membranes that make up the chloroplast or plastid within the plant cell. Have a look at a text book on photosynthesis to see the wonderful series of chemical reactions that need to occur before the conversion of light to sugars and starches takes place.

That is why the sea slugs can only act like plants by either farming small plants in their bodies (as in the case of the nudibranchs with zooxanthellae), or by keeping the plastids, which are the little photosynthetic factories in plant cells alive, (in the case of the sacoglossans).

I'm sorry if this is not good news for your experiment but I hope it will give you some ideas.

Best wishes,
Bill Rudman

Rudman, W.B., 1998 (Oct 11). Comment on Your "solar-powered" sea slugs. by Amanda Lindsey. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Zooxanthellae in nudibranchs

October 11, 1998
From: J.E.Austin

23/09/98 20:59
Dear Dr. Bill Rudman:
I have been doing some research on evolutionary relationships between
pacific northwest nudibranchs (USA). In the context of a neurobiology course at the Friday Harbor Labs (associated with the University of Washington), I have been examining immunohistochemical staining of brain regions as a method for generating phylogenetic characters for perhaps
correlating morphology with function. Would you please recommend a couple of good papers for getting a better handle on the basic taxonomic tree for Opisthobranchs? I have read some work by Schmaekel based on neural and reproductive morphology but as well, am wondering if there is corrobrating
molecular work.
On a second note, I am currently studying chemical communication between sea anemones (Aiptasia pallida, specifically) and their endosymbiotic Symbiodinium algae. Some of the work is basically looking for what Host Factor (free amino acid cocktail, ect.) controls or stimulates
photosynthate release from host to algae. With your work on nudibranchs with their own algae gardens, do you know if it is known what nudibranchs
use to signal photosynthate release by the algal cells? Are the algae concentrated in special vacuoles? Then in your photograph (from the website) of Aeolidiella foulisi, what brown sea anemone species is pictured with it?
Thank-you so much for your time. I am an undergraduate in an NSF-funded research program at the Bermuda Biological Station working with Hank Trapido-Rosenthal.
J. Austin
p.s. I realize that it's a shot in the dark, but do you by chance know Terry Gosliner's email address (California Academy of Sciences)?
Austin (J.E.)
Bermuda Biological Station for Research,Inc.
Ferry Reach
St. George, GE01

Austin, J.E., 1998 (Oct 11) Zooxanthellae in nudibranchs. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

You ask quite a few questions so I'll answer them in order.
1. A good source of information on mollusc phylogeny. The most comprehensive and up to date work would be:
Beesley,P.L., Ross,G.J.B. & Wells,A (eds) 1998. Mollusca: The Southern Synthesis. Fauna of Australia Vol 5.. CSIRO Publishing : Melbourne. Part A pp1-563. Part b pp 564-1234.
I must declare I was involved in writing parts of it but it is generally accepted as the best around at the moment. photosynthate release. Have a look at

Hoegh-Guldberg,I.O. & Hinde, R.,1986. Proceedings of the Royal Society of London, Series B, 228:493-509.

Hoegh-Guldberg,I.O., Hinde, R. & Muscatine,L., 1986. Proceedings of the Royal Society of London, Series B, 228:511-521.

3. Are the zooxanthellae in special vacuoles? They seem to be in modified ducts of the digestive gland in most but not all the species. sometimes they sem yo be loose in the ducts and in other species they seem to be in subepithelial cells. I have described the position of the zooxanthellae in the various papers I cite at the top of this Solar-powered page.

4. The anemone that Aeolidiella foulisi is feeding on in the photo is Anthothoe albocincta.

5. And lastly, yes I can give you Terry Gosliner's email address. It is

Good luck with your research and please let us know of any interesting discoveries you make. .... Bill Rudman.

Rudman, W.B., 1998 (Oct 11). Comment on Zooxanthellae in nudibranchs by J.E.Austin. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from