SOLAR-POWERED SEA SLUGS

Imagine being able to turn on solar-powered cells whenever food became limiting in your environment to exploit the sun’s energy to produce chemical energy.  Further imagine the advantage of being mobile and camouflaged with a green, rippling leaf appearance in a sea filled with predators in search of soft-bodied creatures.  The sacoglossan mollusc Elysia chlorotica (Gould) possesses all of these traits.  It is a shell-less, green “walking leaf” that will feed on algae when they are available, stealing the chloroplasts, and using them for solar-power when food is scarce. 

These sea slugs feed by slicing or puncturing siphonaceous algal cells and sucking out the cell contents.  All of the contents, including the algal nucleus, are discarded except the chloroplasts which are engulfed phagocytotically into the digestive cells.  (The figure to the right shows the "stolen" chloroplasts, or kleptoplasts, within the cells of the slug's digestive tract.)  

By distributing the “photosynthetic factories“ throughout their extensively branched digestive system just one cell layer beneath the epidermis, the sea slugs not only blend into the green algal bed (the figure to the right shows the sea slug on strands of the alga Vaucheria litorea ), they also capture light energy to fuel photoautotrophic CO2 fixation.  In some cases, the resulting carbon products can totally sustain the sea slugs for several months in the absence of an algal food source and serve as precursors for synthesis of chemical defense compounds and the copious mucus which bathes and protects the sea slugs.