The photoacclimation state of stolen chloroplasts affects the light preferences in the photosynthetic sea slug <i>Elysia crispata</i>

Photosynthetic sacoglossan sea slugs sequester the chloroplasts of the algae they feed upon and keep these organelles functional in the cells of their ramified digestive system. Whether the stolen chloroplasts - kleptoplasts - influence animal behavioural responses towards light is uncertain. To address this matter, we: (1) determined the light preferences of the photosynthetic sea slug Elysia crispata when offered different light spectra (450, 517, 520-650 and 665 nm) and intensities (60, 180, 425 and 1400 µmol photons m-2 s-1); and (2) established whether the light intensity preferences of E. crispata were different when fed algae acclimated to low (40 µmol photons m-2 s-1) and high irradiance (425 µmol photons m-2 s-1). Sea slugs were collected from a coral reef in the Gulf of Mexico and transported to the laboratory to perform controlled experiments. During trials, sea slugs exhibited marked exploratory behaviour. However, results show that E. crispata avoids red light (665 nm) and prefers low irradiance (60 µmol photons m-2 s-1), showing that both light spectrum and intensity are relevant to their behaviour. Furthermore, sea slugs increased their selection for high irradiance after being fed algae acclimated to high light. These results support our hypothesis that the acclimation state of the acquired kleptoplasts affects sea slug behaviour towards light. Light perception and photobehaviour in photosynthetic sea slugs seem to depend not only on animal photoreceptors, but also on a communication network between the endosymbiotic chloroplasts and the animal host.