"Solar-powered" Sea Slugs

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.