Symbiosis in photosynthetic sea slugs

Sacoglossan (or sap sucking) sea slugs are an evolutionary wonder. These small creatures live symbiotically with the algae that they eat by taking over the algae’s chloroplasts, the cell structures responsible for photosynthesis. The sea slugs extract the chloroplasts during digestion. The chloroplasts then act as a continuous source of food during periods of starvation, turning the slugs green and giving them the colloquial name ‘crawling leaves’. Despite being extracted from the algae, the chloroplasts remain fully functional, splitting water into hydrogen and oxygen (unlocking available energy in the process) and synthesising carbohydrates inside the slug’s body for as long as 9 months. This phenomenon is known as ‘kleptoplasty’, from the Greek word for “thief”.
But the familiar green hue of chlorophyll isn’t the only colour we see in these symbionts. There are also beautiful, brightly coloured blue and red spots scattered across the body of the slugs, but what they are and how they are created remains a mystery to be solved.