Photosynthetic Animals

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.