The emerald green sea slug Elysia chlorotica, native to the salt marshes of the North American Atlantic coast, blurs the classic distinction between animal and plant biology. As a juvenile, the soft-bodied opisthobranch feeds voraciously on the filamentous intertidal alga Vaucheria litorea. Rather than fully digesting its food, Elysia chlorotica extracts functional chloroplasts from the algal cells and sequesters them inside specialized diverticula lining its extensive digestive tract.
This remarkable phenomenon, known as kleptoplasty, turns the translucent sea slug a brilliant green and enables it to fix carbon dioxide using sunlight alone for up to nine months without consuming additional organic biomass. However, maintaining functional chloroplasts presents a significant evolutionary puzzle. Chloroplasts require continuous synthesis of proteins to repair light-induced damage to photosystem II, but the chloroplast genome contains only a small fraction of the genes necessary for these repair mechanisms.
In plants and algae, the vast majority of chloroplast maintenance genes reside in the host cell's nuclear DNA. Comparative genomic analysis revealed that Elysia chlorotica acquired key nuclear algal genes, such as psbO—which encodes a vital manganese-stabilizing protein in the photosynthetic apparatus—through functional horizontal gene transfer (HGT). The animal captures these algal genes, integrates them into its own nuclear genome, and translocates the resulting proteins into the borrowed organelles.
This rare instance of multicellular, cross-kingdom gene transfer allows the sea slug to maintain its functional 'solar panels' across its adult lifespan. Elysia chlorotica demonstrates how horizontal gene transfer can drive extreme physiological adaptations, challenging traditional models of metazoan metabolism and organelle independence.