Kleptoplastidic sea slugs are unique among metazoans in their ability to retain functional algal chloroplasts (kleptoplasts) for extended periods, sustaining photosynthesis within their own tissues. In marine environments, both light intensity and spectral composition shift markedly with depth, potentially influencing kleptoplast performance. We investigated how the light spectrum affected growth and photosynthesis in the sea slug Elysia crispata. Laboratory-reared individuals were exposed to equal irradiances of white, red and blue light during controlled feeding and starvation periods. During feeding, individuals under red light exhibited the highest weight gain, whereas those under blue and white light showed slower growth. In contrast, chlorophyll a content, used as an indicator of kleptoplast abundance, was significantly higher in sea slugs exposed to blue light by the end of the feeding phase. Starvation resulted in progressive weight loss under all treatments, although a brief initial increase in biomass occurred under red light. Photosystem II efficiency and electron transport rates declined throughout starvation, with the most rapid reduction under red light, a moderate decline under white light, and the slightest decrease under blue light. Survival did not differ significantly among light spectral treatments. These findings suggest a trade-off between increased growth associated with higher photosynthetic activity and subsequent kleptoplast degradation under red light, and improved kleptoplast stability under blue light. In conclusion, natural shifts in underwater light spectra may play a critical ecological role in shaping the longevity and functionality of kleptoplasts in E. crispata populations.