Red light promotes sea slug growth while blue light increases longevity of stolen chloroplasts.

In Metazoa, long-term sequestration of photosynthetically active algal chloroplasts - kleptoplasts - is a distinctive physiological trait of sacoglossan sea slugs. In this study, we examined the effects of the light spectrum on the body weight and photosynthetic activity of the sea slug Elysia crispata. Laboratory-reared specimens were exposed to equivalent irradiances of white, red, or blue light during feeding and starvation experiments. Sea slugs fed under red light increased their weight at a faster pace than specimens fed under white or blue light. At the end of the feeding trial, chlorophyll a concentration - a proxy for kleptoplast abundance - was significantly higher under blue light. Progressive weight loss was observed in starved sea slugs under all light spectra, but an initial transient weight increase was observed under red light. Photosystem II activity and photosynthetic electron transport rate declined in starvation, with the decrease being fastest under red light and slowest under blue light. Results suggest a trade-off between increased photosynthetic rates and kleptoplast degradation, leading to increased growth (under red light) and enhanced kleptoplast longevity, possibly due to reduced photooxidative damage (under blue light). We hypothesise that the higher degree of parapodia closure observed under blue light shields kleptoplasts from light-induced damage, effectively extending their lifespan.