The ability to incorporate functional plastids by the sea slug Elysia viridis is governed by its food source

Functional kleptoplasty in sacoglossan sea slugs is among the most curious photosynthetic associations known. One member of these marine molluscs, Elysia viridis, is known to incorporate plastids from a variety of different algae food sources, but with apparently different outcomes and differences in the time span of the retention of functional kleptoplasts. While it was previously shown that kleptoplasts that stem from Codium tomentosum are kept functional for several weeks (long-term retention, LtR), those that stem from Bryopsis hypnoides or Cladophora rupestris are thought to be of limited use regarding photosynthetic capacity (short-term retention, StR). This is important, because it touches upon the popular yet controversial question of how important photosynthesis is for the thriving of these slugs. The aim of the present study was to determine to what degree the plastid source determines retention time. We, therefore, compared E. viridis feeding on either Cladophora sp. or B. hypnoides. We show that kleptoplasts of B. hypnoides incorporate ¹⁴CO₂, but with rapidly declining efficiency throughout the first week of starvation, while the plastids of Cladophora sp. are, surprisingly, not incorporated to begin with. The radulae of the different samples showed adjustment to the food source, and when feeding on Cladophora sp., E. viridis survived under laboratory conditions under both starvation and non-starvation conditions. Our results demonstrate that (i) the ability to incorporate plastids by E. viridis differs between the food sources B. hypnoides and Cladophora sp., and (ii) photosynthetic active kleptoplasts are not an inevitable requirement for survival.