Laboratory culturing of Elysia chlorotica reveals a shift from transient to permanent kleptoplasty

The kleptoplastic sacoglossan Elysia chlorotica shares a requisite, intracellular symbiosis with the plastids (= chloroplasts) of the Xanthophyte alga Vaucheria litorea. Although wild specimens have been used to address a range of biological questions, no studies have thoroughly characterized animal development during the initial establishment of the symbiosis under controlled laboratory conditions. Laboratory culture conditions were modified and the time required for successful metamorphosis was reduced by 40 % relative to previous work. Plastids were not initially stable within the host; “permanent kleptoplasty” was obtained only after ≥7 days of feeding on V. litorea. Feeding for shorter time periods resulted in the loss of plastids and abnormal development; this phase was characterized as “transient kleptoplasty”. Individuals in the transient state exhibited a significantly greater decrease in length compared to animals with permanent kleptoplasts after the same starvation period. To test the effect of food availability after obtaining permanent kleptoplasty, animals were subjected to various dietary regimes followed by a recovery period of constant feeding. Thirty percent of animals survived prolonged starvation (>4 weeks) after only the initial week of feeding required to establish permanent kleptoplasty. All treatments showed rapid growth when re-exposed to Vaucheria. Thus, during initial development E. chlorotica experiences enhanced fitness when Vaucheria is available for consumption. However, the animal rapidly establishes permanent kleptoplasty, which bestows flexible food requirements and resistance to food limitation, a likely advantage for E. chlorotica in salt marsh environments where Vaucheria sp. abundance is sporadic.