METABOLIC BOLT-ONS. “Many invertebrates have intracellular bacterial symbionts whose genes encode metabolic capabilities lacking in animals, such as the synthesis of essential amino acids, photosynthesis, or chemosynthesis. Certain marine invertebrates that feed on algae maintain algal plastids as photosynthetically active symbionts, a behavior that allows the host to use photosynthate as a food source for extended periods.”
http://www.pnas.org/content/110/9/3229.full