Around 150 years ago it was discovered that several species of Sea Slugs (in the clade Sacoglossa ) contain large numbers of chlorophyll-pigmented granules - called plastids - similar to those of plants. (Note: 'Klepto' comes the Greek for 'stolen')

It was later discovered that the sea slugs acquire the plastids from the algae which they eat.

An assumption was made that they benefit from the bio-chemiclal activities of the plastids, to the extent that some researchers have called them 'Solar-Powered'.

Direct evidence, however, that the slugs substantially benefit, has not been found.

While we now understand that these “green granules” are plastids the slugs sequester from siphonaceous algae upon which they feed, surprisingly little is really known about the molecular details that underlie this one of a kind animal-plastid symbiosis. Kleptoplasts are stored in the cytosol of epithelial cells that form the slug’s digestive tubules, and one would guess that the stolen organelles are acquired for their ability to fix carbon, but studies have never really been able to prove that.
[…]
We know that the ability to store kleptoplasts long-term has evolved only a few times independently among hundreds of sacoglossan species, but we have no idea on what basis.

Source : Polish Botanical Society, Acta Societatis Botanicorum Poloniae, 83(4):415–42

Other studies have shown that the benefit to the slugs is at best very minimal in some species, and non-existent in others. See : Proc Biol Sci. 282(1802)