Crawl away from the light! Assessing behavioral and physiological photoprotective mechanisms in tropical solar-powered sea slugs exposed to natural light intensities

Photosynthesizers face a trade-off regarding light: they need enough to maintain high photosynthetic rates, yet excess leads to oxidative stress. Despite this, light and its detrimental effects are chronically underestimated. Solar-powered sea slugs (Sacoglossa: Gastropoda) provide the ideal lens with which to study this trade-off, since they steal chloroplasts from algae but do not inherit photoacclimation and photorepair capacities. We collected three slug species in Curaçao during March and December 2022, comparing the amount of light they received in nature to their optimal light intensities for photosynthesis, and their preferred light intensities. We then investigated behavioral and physiological photoprotection mechanisms to determine if and how they limit light. Finally, we examined oxidative activity under optimal and excess light. All three species were naturally exposed to more light (> 1000 µmol m−2 s−1) than is optimal or preferred. Elysia crispata (kleptoplast retention for > 3 months) is fully exposed to light in nature but reduces the light reaching its kleptoplasts via parapodial shading. Elysia velutinus retains kleptoplasts for ~ 2 weeks and hides in its macroalgal food, limiting light exposure. Both species displayed low amounts of oxidative activity under optimal light, which increased slightly under excess light. Elysia ornata retained chloroplasts for ~ 3 days, lacked observable photoprotection and always displayed high levels of oxidative activity, potentially explaining its limited capacity for kleptoplast retention. Furthermore, both E. velutinus and E. ornata display strong light-avoidance behaviors. This study clearly demonstrates links between high light intensities, photoprotection, and oxidative stress, highlighting the need for future studies that examine aquatic photosynthesizers under natural lighting.