Kleptoplasts Are Continuously Digested During Feeding In The Plastid-Bearing Sea Slug Elysia Viridis

Photosymbiosis is a widespread form of mutualistic symbiosis found among many metazoan lineages (Melo Clavijo et al., 2018) and is particularly beneficial for the animal host. The assimilates translocated by the symbionts to the host can meet or even supersede the host's nutritional demands, and this is particularly important for reef-building corals (Stanley & Lipps, 2011). A rather unusual form of photosymbiosis is found among some sacoglossan sea slugs (Trench, 1975; Rumpho, Summer & Manhart, 2000) and in two rhabdocoel flatworms (Van Steenkiste et al., 2019). In these taxa, only chloroplasts, ingested from algal prey, occur intracellularly as some sort of symbiont. In some Sacoglossa species, these prey-derived chloroplasts (referred to as kleptoplasts) remain photosynthetically active for periods ranging from weeks to months (Evertsen et al., 2007; Händeler et al., 2009; Christa et al., 2015). Due to the unique nature of this photosymbiosis, the term functional kleptoplasty was introduced (Rumpho et al., 2011). Even though functional kleptoplasty has been intensively investigated in Sacoglossa, it is still not clear to what extent assimilates can be translocated by kleptoplasts to host cells and to what degree this nutritional support is beneficial for the host (Wägele & Martin, 2014; Cartaxana et al., 2017; Rauch et al., 2017).