This is something our guest lecturer touched on...and I thought it was awesome!
Some sea slugs evolved a protective mechanism to “make up” for the lack of a protective shell. One type of sea slug uses chloroplasts as camouflage and energy! It feeds by slicing or puncturing algal cells and sucking out the cell contents. All the cell contents are discarded except for the chloroplasts, which are then engulfed phagocytotically into the digestive cells. Since their digestive tract is just one cell layer beneath the epidermis, the sea slugs blend into the green algal bed and capture light energy to fuel photoautotrophic carbon dioxide fixation. These sea-slugs can potentially live off of these carbon products for months in the absence of an algal food source!
The paper I read on this calls the relationship between these algal chloroplasts and molluscs symbiotic. There is debate on whether this qualifies as symbiosis because the symbiont—the chloroplast—is not a free-living organism, just an isolated organelle. What do you guys think?
Here is the link to the paper: