If searching for another seaweed meal seems like too much hard work, sea slugs can simply laze in the sun while the enslaved chloroplasts busily produce sugars for their new master. Inside the slug, captive chloroplasts are astonishingly robust and can remain functional for up to nine months. Then it is time for the slug to replenish its chloroplast store from another seaweed f east. Lazing around in the sun rather than searching for food certainly sounds like a good idea, but the question that intrigues me is whether photosynthesis is actually an important source of energy for slugs. A theory developed by Dr Kerry Clark at the Florida Institute of Technology as long ago as 1978 suggests that photosynthesis may be an essential back-up energy source for slugs, when food is difficult to find.
Sea slugs are notoriously fussy eaters and can have a tough time obtaining food.
They are highly specialized, sap-sucking herbivores, feeding only on a few species of seaweed, each of which is an unreliable food source. For many slugs, the seaweed they consume is sparsely distributed throughout the shallow waters in which they range. Another favourite, but labour-intensive, food of slugs becomes armour plated at certain times of the year, as the seaweed lays down a protective outer layer of calcium carbonate. To get an idea of what it is like for a slug attempting to suck sap from a calcified seaweed, imagine trying to drink from a juice carton coated with concrete without any opening - a strenuous task yielding little nutrition. Their ability to photosynthesize may help slugs survive this lean time of year.