Adaptive significance of light and food for a kleptoplastic sea slug: implications for photosynthesis.

Sacoglossan sea slugs can 'steal' chloroplasts from their algal food and use them for photosynthesis (kleptoplasty). Although it has been shown that light has positive effects on survival and body size retention of some sacoglossans likely through photosynthesis, it is unknown whether light affects their fitness components such as number of offspring or offspring size. Moreover, whether the effects of light extend over the sacoglossans' lifetime has been unexplored. To assess such long-term effects of light intensity and food availability on fitness components, we conducted a 15.9-week laboratory experiment using Elysia atroviridis under a combination of two light intensities (low or high) and two food conditions (with or without food). The total number of eggs laid was greater in the presence of both strong light and food than in other conditions, suggesting positive effects of both light intensity and food availability. The shell height at hatch was also largest in the presence of strong light and food. Larval rearing experiments showed that the size difference at hatch between conditions corresponded to a 1.19-1.93 days growth and 7.9-18.1% survival increase. Thus, positive effects of light and food on the fitness components extend over the lifetime of E. atroviridis.