A couple of years ago I sat in on a colleague's botany lectures & was enchanted to hear about a green sea slug - green, because it eats algae & sequesters the algal chloroplasts within its own cells. A solar-powered sea slug!
Elysia chlorotica feeds on algae but instead of digesting the entre cell contents (or passing them through its gut relatively unscathed) it retains the algal chloroplasts. Shut away within the slug's own cells, the chloroplasts can survive and function for up to 9 months, happily photosynthesising despite the absence of an algal nucleus.
This is extremely unusual because although chloroplasts (& mitochondria) do have their own genome, normally their functioning is supported by nuclear genes - in chloroplasts, up to 90% of the proteins they require for functioning are encoded by DNA in the plant cell's nucleus. (So, somewhere in the development of the endosymbiotic relationship between these organelles & the cells that 'host' them, some of those control genes have been transferred to the host nucleus.) Now it seems that Elysia also has photosynthesis-supporting genes in its own nuclear genome, presumably acquired through horizontal gene transfer (Rumpho et al. 2008). (Perhaps Darwin was even more accurate than he realised, in using the metaphor of a tangled bank for the diversity of life & the interrelationships of living things.)
The slugs gain considerable benefits from this relationship - if there are no algae to eat, Elysia can survive for months on the sugars produced by 'their' chloroplasts. And it's an obligate symbiosis - the slugs have to have the algae to survive. However, the algae & the slugs are not yet in a fully endosymbiotic relationship: each new generation of Elysia must eat algae to acquire its own internal chloroplast 'solar panels'. Untl the chloroplasts are passed on in the slug's gametes, Elysia will remain a kleptoplast - a thief & stealer of chloroplasts.
M.E.Rumpho, J.M.Worful, J.Lee, K.Kannan, M.S.Tyler, D.Bhattacharya, A.Moustafa, & J.R.Manhart (2008) Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. Proceedings of the National Academy of Sciences 105(45): 17867-17871. doi: 10.1073/pnas.0804968105
NB Mary Rumpho & her team have a whole website dedicated to Elysia chlorotica - there's far more for you to find out about this fascinating animal.