Kleptoplasts

Kleptoplasts are functional chloroplasts that have been stolen from algae by certain non-photosynthetic organisms. The best-known examples occur in some sacoglossan sea slugs, such as Elysia chlorotica and Elysia timida. In these slugs, feeding on algae results in the uptake of chloroplasts into specialized digestive cells where the plastids remain capable of photosynthesis for extended periods, ranging from weeks to months after sequestration. The chloroplasts are maintained without full transfer of algal nuclei, and their continued function relies on host cellular factors rather than complete integration of algal genes.

Mechanisms and maintenance: The slugs sequester chloroplasts in digestive tissues where they are protected from rapid degradation. Host cells appear to provide needed metabolites and a supportive environment for photosynthetic activity, while avoiding digestion of the plastids. Kleptoplasts eventually deteriorate and must be replenished by subsequent feeding, so photosynthesis supplements rather than sustains metabolism over the long term.

Sources and diversity: Kleptoplasts have been most extensively studied in sacoglossan sea slugs that feed on green and other algae, including species such as Vaucheria litorea. The exact algal source can vary between slug species, and not all sacoglossans exhibit plastid retention.

Genetics and evolution: There is ongoing debate about horizontal gene transfer from algae to the slug genome as a mechanism for plastid maintenance. Most evidence suggests that prolonged plastid function does not require stable integration of algal nuclei, and the slug relies mainly on retained plastids and host factors rather than fully integrated algal genes.

Ecological significance: Kleptoplasty illustrates a rare form of short-term endosymbiosis and provides insight into plastid stability and host–organelle interactions. It is not considered true endosymbiotic chloroplast acquisition, since reliance on stolen plastids is temporary and reversible.