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.

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