'Solar-powered' sea slugs

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