Sacoglossa is a rather small taxon of marine slugs with about 300 described species, yet it is quite fascinating scientists for decades. This is mainly because of the ability of certain species to incorporate photosynthetically active plastids of their algae prey, a phenomenon known as functional kleptoplasty. With the stolen plastids, these slugs endure weeks (short-term retention) or months (long-term retention) of starvation, though contribution of the plastids to the survival and factors enhancing plastid longevity are unknown. Likewise, contrasting hypotheses on evolution of functional kleptoplasty exist and the phylogenetic relationship of Sacoglossa taxa is still under debate. We analyzed the phylogenetic relationship of 105 sacoglossan species to address the question of the origin of functional kleptoplasty. Based on our phylogenetic analysis and the ancestral character state reconstruction, we conclude that functional short-term retention most likely originated two times and long-term retention at least five times. Previous suggestions that functional longterm kleptoplasty is established with specific plastids are supported by our food analyses in Elysia clarki that finally harbors only plastids of certain algae species over a prolonged starvation period.

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