Photosynthesis has long been the domain of plants and algae. In fact, one of the defining differences between plant and animal life is in how they obtain food: plants can produce their own from sunlight through photosynthesis, animals must eat either plants or other animals theirs.
Leave it to a sea slug to throw off the balance, and provide the only clear example of an animal truly photosynthesizing.
The term ‘sea slug’ can apply to organisms in a variety of groups, but the species we’re about to meet is a member of the clade Opisthobranchia, within the same class of mollusks (phylum Mollusca) as snails and slugs (i.e. Gastropoda). These sea-dwelling invertebrates are generally soft-bodied, with shells with are reduced or altogether absent. The sides of their muscular ‘foot’ have flared out via evolution into parapodia—fleshy wing-flaps. Some species are able to oscillate these parapodia to ‘fly’ through the water.
The term ‘sea slug’ is not scientific, and thus encompasses many different types of gastropods, like this nudibranch:
The slug in question is Elysia chlorotica, a modestly-sized sea slug the same deep green as the algae one might see as scum in a fish tank at home. In fact, it looks rather like a broad, thick leaf, stolen from a rainforest and dropped into the ocean; the big giveaway that this is an animal is the two-horned head on one end. Well, that and the fact that in life it swims around, providing the fast movement that is another hallmark of animal (as oppose to plant) life.
In a clever bit of bioengineering, E. chlorotica has evolved the ability to co-opt chloroplasts from the algae it eats. Chloroplasts are the cellular machinery that allow plants (and algae, obviously) to conduct photosynthesis, creating food from sunlight and carbon dioxide. The slug stores these chloroplasts in the cells that line its gut.
Scientists had known for a while that young E. chlorortica could, after being fed algae for two weeks, survive without eating for the rest of their year-long lives. With a small up-front investment, E. chlorotica is able to meet all of its energetic needs via photosynthesis.
The catch is that chloroplasts should not be able to function outside of a photosynthetic organism. Like all cellular machinery, they require proteins coded by the DNA of the organism to stay active. Sea slugs simply don’t have the necessary genetic equipment to run their stolen chloroplasts—yet they were clearly running them just fine.
Work by Mary Rumpho of the University of Maine solved this conundrum. In sequencing the DNA of both the algae these slugs eat and the slugs themselves, she and her colleagues found that E. chlorotica was apparently stealing entire genes for photosynthesis from its food, in addition to the organelles. Even more interestingly, these genes for photosynthesis were also found in the animal’s sex cells, meaning it should be heritable. The exact mechanics of how the slug activates these genes inside its own DNA and how it acquired it in the first place remains unclear.
A video from NewScientist showing Elysia chlorotica in action:
The most recent news on this remarkable slug has only further emphasized its perfect fusion of plant and animal traits. The newest results, as put forth by Sidney K. Pierce of the University of South Florida in Tampa, show that these slugs have actually established an entire chemical-making pathway to produce the chlorophyll required to sustain the use of their contraband chloroplasts: a chemical feat usually reserved for plants and other naturally photosynthetic life-forms.
While some other organisms, like corals, have symbiotic relationships with microbes which perform photosynthesis, none of them have achieved the level of seamless fusion of plant and animal elements seen in E. chlorotica. In other, symbiotic situations, the microbes are still whole, if tucked neatly into their host’s cells; E. chlorotica only takes the necessary bits and pieces of the algae it eats and incorporates them into its own body and metabolism. Some other sea slugs have even been known to take up chloroplasts from their food, like E. chlorotica, but none maintain them for active service like this species.
The symbiotic relationship corals share with photosynthetic, unicellular algae seems less impressive in light of E. chlorotica’s impressive feats of kleptoplasty:
Some zoologists, like John John Zardus of The Citadel in Charleston, S.C., remain skeptical, but the implications of this slug are staggering. It seems to represent an unprecedented level of genome-sharing between multicelluar organisms, and it blurs the line between plant and animal.
Behold! The ultimate blend of solar power and mobility, a hybrid of plant-like self-sufficiency and animal vigor—yet too unassuming to even have a common name—it’s Elysia chlorotica.
“At a recent meeting of the American Society for Photobiology, chemist Pill-Soon Song, of Texas Tech University, reported the discovery of a blue-green, trumpet-shaped protozoan that employs photosynthesis to sustain itself. Called Stentor coeruleus, this protozoan is only 0.2 mm long and swims backward by rotating its cilia. According to the article, this is the first instance of a photosynthesizing animal.”
Click here to cancel reply.
Sorry,At this time user registration is disabled. We will open registration soon!
Don't have an account? Click Here to Signup
© Copyright GreenAnswers.com LLC