The slug feeds on the alga Vaucheria litorea, puncturing a filament and drawing out the contents. Most of what it swallows is digested, but the chloroplasts are retained intact in the cells lining its extensively branched digestive gland, which spreads through the flattened body. This is kleptoplasty, and it is what gives the animal its green colour and leaf shape.
The retained chloroplasts continue to photosynthesise for a remarkably long time. Individuals have been kept alive for many months in light without further feeding. This is puzzling, because a chloroplast normally depends on nuclear genes from its host alga to replace damaged proteins, and the slug does not carry an algal nucleus.
How the plastids persist is the live controversy. Early work proposed that algal genes had transferred into the slug's own genome, but genome sequencing has not confirmed a functional transfer of the required genes, and current explanations lean toward unusually robust plastids and slug-produced proteins that stabilise them. The photosynthesis is real; the mechanism sustaining it is not settled.