Every animal needs energy to live, and they usually get it by ingesting other living things. We’ve all seen videos of predators hunting down and killing their favorite prey, or docile herbivores munching on grass, leaves, or berries. But one group of animals has evolved a strategy to harness energy that’s a bit more complex and far more clever than simply finding something and eating it. Sacoglossan sea slugs are close relatives of the more colorful and better-known group of slugs called nudibranchs.
Nudibranchs are carnivorous – they prey on sponges, cnidarians, bryozoans, and even cannibalize other members of their own species. Sacoglossans, on the other hand, are more like their hippie vegan cousins. They feed on green algae using a specialized tooth-like feeding structure common to all snails and slugs called a radula. A sacoglossan sea slug uses its radula to pierce cell walls, allowing it to suck out all those sweet, sweet algal juices.
Okay, so some sea slugs are vegan, but what’s so clever about that? Of course being vegan in and of itself isn’t so special, but there’s a unique characteristic known to many sacoglossan species that makes them more environmentally responsible than even the most dedicated vegan. It’s called kleptoplasty, a process by which the slugs steal chloroplasts from their food – and then use them! In other words, instead of digesting the photosynthetic plastids as part of an algal meal, sacoglossans manage to keep them and maintain their functionality. The upshot of this is that the slugs can then bask in the sunlight of shallow water to generate their own energy via photosynthesis, making them the only known solar-powered animals. I’ve personally observed this in Elysia crispata, commonly called the lettuce slug. Since it uses chloroplasts but can’t produce them on its own, the slug’s green color fades over the course of several weeks if not given an opportunity to feed and replenish its stock.
The species of kleptoplastic slug that has been most thoroughly studied, however, is Elysia chlorotica. In fact, a study published last year used transcriptome data to show that as many as 52 genes involved in photosynthesis have been horizontally transferred from the alga that it eats and lives on, Vaucheria litorea, into its own nuclear genome (Pierce et al. 2012). As opposed to vertical transfer, which is simply an organism passing its genes down to its offspring, horizontal transfer refers to the exchange of genes between two completely different species, often distantly related to each other.
This is interesting because the phenomenon is fairly common and well-understood in bacteria (prokaryotes), but less so in more complex organisms (eukaryotes). In addition to the algal nuclear genes that have been incorporated into the slug’s genome, the chloroplasts bring with them about 100 of their own genes that are used for photosynthesis in the slug. In order to identify these genes, the authors looked at expressed gene sequences (mRNA) from both E. chlorotica and its host alga, V. litorea, and compared them to the genomic DNA library of V. litorea.
So while humans are still struggling to find practical and efficient ways to harness the sun’s abundant energy as an alternative to fossil fuels, these little slugs have been perfecting their own solar powered generators for millions of years. Stay tuned to find out how some nudibranchs use an analogous strategy to steal defensive weapons from their cnidarian prey…
UPDATE: I originally wrote this post on a different blog in 2011, and since then some additional work has been done that calls into question whether sacoglossans really do rely on chloroplasts to produce food, or whether the stolen plastids really photosynthesize the slugs at all. Unfortunately I don’t have time to write a whole post addressing this, but you can read about the latest research in a recent Scientific American blog post by Ferris Jabr.
Pierce, S.K., X. Fang, J.A. Schwartz, X. Jiang, W. Zhao, N.E. Curtis, K.M. Kocot, B. Yang and J. Wang. 2012. Transcriptomic evidence for the expression of horizontally transfered algal nuclear genes in the photosynthetic sea slug, Elysia chlorotica. Molecular Biology and Evolution. Advanced online publication ahead of print, Feb. 21, 2012.
Rumpho, M.E., K.N. Pelletreau, A. Moustafa and D. Bhattacharya. 2011. The making of a photosynthetic animal. Journal of Experimental Biology. 214: 303-311.