Kleptoplasty or kleptoplastidy is a symbiotic phenomenon whereby plastids from algae are sequestered by host organisms. The alga is eaten normally and partially digested, leaving the plastid intact. The plastids are maintained within the host, temporarily retaining functional photosynthesis for use by the predator. The term was coined in 1990 to describe chloroplast symbiosis.
The stability of transient plastids varies considerably across plastid-retaining species. In the dinoflagellates Gymnodinium spp. and Pfisteria piscicida, kleptoplastids are photosynthetically active for only a few days, while kleptoplastids in Dinophysis spp. can be stable for 2 months. In other dinoflagellates, kleptoplasty has been hypothesized to represent either a mechanism permitting functional flexibility, or perhaps an early evolutionary stage in the permanent acquisition of chloroplasts.
Sacoglossan sea slugs 
The only known animals that practice kleptoplasty are sea slugs in the clade Sacoglossa. Several species of Sacoglossan sea slugs capture intact, functional chloroplasts from algal food sources, retaining them within specialized cells lining the mollusc's digestive diverticula. The longest known kleptoplastic association, which can last up to ten months, is found in Elysia chlorotica, which acquires chloroplasts by eating the alga Vaucheria litorea, storing the chloroplasts in the cells that line its gut. Juvenile sea slugs establish the kleptoplastic endosymbiosis when feeding on algal cells, sucking out the cell contents, and discarding everything except the chloroplasts. The chloroplasts are phagocytosed by digestive cells, filling extensively branched digestive tubules, providing their host with the products of photosynthesis.
This very unusual ability has led to these sacoglossans being referred to as "solar-powered sea slugs".
See also 
- Karyoklepty, a process in which the nucleus of the prey cell is kept by the predator as well
- Horizontal gene transfer
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- Catherine Brahic (24 November 2008). "Solar-powered sea slug harnesses stolen plant genes". New Scientist. Retrieved 24 November 2008.
- "SymBio: Introduction-Kleptoplasty". University of Maine. Retrieved 2008-11-24.
- "Solar Powered Sea Slugs". ABC Science Online. June 2007. Retrieved 2008-11-24.