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

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