Phototaxis of sacoglossan sea slugs with different photosynthetic abilities: a test of the 'crawling leaves' hypothesis

摘要

Several sacoglossan sea slugs utilise chloroplasts ingested from algae for photosynthesis (kleptoplasty), a unique trophic strategy unknown in other animals. Its adaptive significance, especially the behavioural adaptations involved in this phenomenon, has not been fully explored. To address this issue, the effects of kleptoplasty on phototaxis were investigated, both across and within species, using sacoglossans collected along Japanese coasts in 2012 and 2013. First, the presence of phototaxis and preferred light intensity was studied in five sacoglossans with various photosynthetic capabilities using an I-maze with a light gradient (4-330 mu mol photons m(-2) s(-1)). Each individual was allowed to move for 30 min to choose the optimal light intensity. Elysia hamatanii, E. trisinuata, and Plakobranchus ocellatus, all with high photosynthetic activity (i.e. kleptoplastic), showed positive phototaxis. Among them, E. hamatanii preferred the highest light intensity followed by P. ocellatus and E. trisinuata, and the order corresponded with the shallowness of their habitats. Conversely, Stiliger ornatus and Placida sp., with virtually no photosynthetic activity (non-kleptoplastic), showed neutral and negative phototaxis, respectively. Next, the phototaxis of E. hamatanii individuals with (fed) and without (starved) functional chloroplasts was compared to examine the effects of the presence of kleptoplasts on phototaxis within a species. Both fed and starved individuals showed positive phototaxis, but the preferred light intensity of starved individuals was lower than that of fed individuals. These results suggest that sacoglossans with functional chloroplasts exhibit positive phototaxis towards a preferred light intensity which may benefit photosynthesis efficiency.