Solar bodies - no need to eat !

The leaf shaped green sea slug Elysia chlorotica, found widely on the Eastern seaboard of the USA, has incorporated chloroplasts (the same structures responsible for photosynthesis in all plants) into its body cells which allows it to obtain its energy needs by sunbathing rather than by having to eat.

Photo by N E Curtis and R Martinez from Wired.com article 11/1/10.


Scientists have shown that after a young slugs initial feed of a particular green algae, from whence it obtains its chloroplasts, it can survive the rest of its life - about a year - without having to eat another meal. The slug, now solar-powered, no longer needs a mouth.

I first heard of this plant/animal in Frank Ryans' book Virolution in which he explores the central role of viruses in evolution and cell metabolisim. Elysia initially needs to feed on Vaucheria algae to acquire a sufficient "skin-full" of chloroplasts. The mystery though has been how the process of photosynthesis continues long after the initial chlorophyll reserves acquired from Vaucheria have been used up (photosynthesis requires continual replenishing of this light absorbing pigment).

It seems that in the evolutionary past there has been a natural transfer of genes from the algae to the slug which allows Elysia to produce the chlorophyll pigment proteins itself. These genes are found even in unhatched sea slugs which have never been exposed to algae. While bacteria and other microbes freely exchange genetic information (e.g. the fast build-up of resistance by bacteria to antibiotics), there are not many examples of genes transfering between two different kingdoms of life. Ryan writes that there is gathering evidence that the gene transfer was probably accomplished by a retrovirus that is commonly found throughout the body of the slug. Aggressive reproduction by the same virus is implicated in the quick death of the slug soon after it has laid its eggs the following spring. Ryan speculates that we are looking here at an example of "aggressive symbiosis" where the virus that had previously enabled the slugs idyllic lifestyle now switches behaviour to cull the now redundant adults after they have laid their eggs. He sees this is as a previously unnoticed evolutionary mechanisim perhaps as important as the better known mechanisims of natural selection, adaptation and genetic drift.

So, viruses help design the creatures they inhabit. Over evolutionary time they shape their own hosts. Which virus(es) designed us? Gives a new meaning to "Lord of Hosts".

A good green argument here of course for human genetic engineering. Splice chloroplast genes into our skin cells and we can all retire to the beach. Perhaps those little green Martians have already stolen a march on us here.

Youtube clip of Elysia still with a mouth :


Rumpho ME, Worful JM, Lee J, et al. "Horizontal Gene Transfer... " Nov '08 Proceed. Nat. Acad. Sciences.

Pierce SK, "Chlorophyll a manufacture in Elysia..." Jan '10 Ann. Meeting Soc. Integrative and Comparative Biology.