ABSTRACT: Parental size and nutritional state have been identified as important interrelated parameters determining reproductive investment in marine gastropods. However, studies on reproductive investment of sacoglossan sea slugs capable of incorporating intracellular functional chloroplasts (kleptoplasts) from their food sources are scarce. In the present work, we investigated the effects of different levels of availability of the macroalga Codium tomentosum on the reproductive investment of the mixotrophic sea slug Elysia viridis. Limited food availability decreased sea slug size (dry weight), kleptoplast abundance (chl a concentration), and photosynthetic capacity (PSII maximum quantum yield, F_v/F_m). Furthermore, sea slugs with limited access to food spawned significantly smaller egg masses and displayed a reduced number of eggs per egg mass. Intermittently fed sea slugs spawned a lower number of egg masses than continuously fed and starved sea slugs, indicating a trade-off between feeding and spawning activity under limited resources. No detectable effects of food limitation were observed on the size of individual eggs and total fatty acid content per egg. However, starved sea slugs produced eggs richer in saturated fatty acids, namely stearic acid (18:0). On the other hand, sea slugs with unlimited access to food spawned eggs richer in polyunsaturated fatty acids, such as linoleic, eicosatrienoic, and eicosapentaenoic acids (18:2 n-6, 20:3 n-3, and 20:5 n-3, respectively). In conclusion, nutritional state significantly affected resource allocation to reproductive traits in E. viridis.