Sea Slug Mucus Production Is Supported by Photosynthesis of Stolen Chloroplasts

A handful of sea slugs of the order Sacoglossa are able to steal chloroplasts-kleptoplasts-from their algal food sources and maintain them functionally for periods ranging from several weeks to a few months. In this study, we investigated the role of kleptoplast photosynthesis on mucus production by the tropical sea slug Elysia crispata. Animals reared for 5 weeks in quasi dark (5 μmol photons m^-2 s^-1) showed similar growth to those under regular light (60-90 μmol photons m^-2 s^-1), showing that kleptoplast photosynthesis was not relevant for growth when sea slugs were fed ad libitum. However, when subjected to short-term desiccation stress, animals reared under regular light produced significantly more mucus. Furthermore, the carbohydrate content of secreted mucus was significantly lower in slugs limited in the photosynthetic activity of their kleptoplasts by quasi-dark conditions. This study indicates that photosynthesis supports the synthesis of protective mucus in kleptoplast-bearing sea slugs.