Celebrity sea slugs prove photosynthesis isn’t just for plants

Martha’s Vineyard has become the go-to location for studying a slug that challenges what distinguishes fauna from flora.


Imagine needing to eat only once in your lifetime. The meal is nothing special, but the powers you obtain from it certainly are. First, you turn from brown to bright green. You are an animal, and yet you can now derive energy from the sun through photosynthesis — like a plant. This is not the origin story of the next superhero, but instead, the catalyst for metamorphosing from the juvenile to the adult phase of life for Elysia chlorotica, a sea slug native to Martha’s Vineyard and much of the Eastern Seaboard. If this sounds like magic, consider too that the “secret ingredient” is found in an algae you’ve likely trod over recently. The really inconceivable part, however, lies within the slug itself.

Hatching into shallow marine environments in the early spring, these slugs live for only 11 months — dying shortly after laying eggs the following spring. Within this relatively short span of time, each of these one- to two-inch slugs accomplishes something that has both baffled and astonished scientists for more than 30 years.

One such scientist is Sidney Pierce. Professor emeritus of biology at the University of South Florida, Pierce is on the forefront of E. chlorotica research, and has been for more than 30 years. He credits two of his former students in an invertebrates class at the Marine Biological Laboratory in Woods Hole for introducing him to E. chlorotica. After sending the class out into the field with the assignment of finding an animal that intrigued them for further study, two students appeared in his office with a pair of brilliantly green sea slugs. Pierce had never seen E. chlorotica before, and asked, “Where the heck did you get that?”

Pierce soon became involved in research focused on the slugs’ ability to tolerate changing salinity within their habitats. A continuous stream of questions — from fellow scientists and the public alike — regarding the vibrant color of the animals eventually led Pierce to study the fascinating relationship between the slugs and the chloroplasts they ingest. His research, which we’ll get to in a moment, may one day yield insight for conducting successful gene therapy.

While those first E. chlorotica slugs came from a tidal creek in Woods Hole, one of the most prominent populations is found right here on Martha’s Vineyard. In a shallow and protected brackish pond environment fed into by the Vineyard Sound, a modest group of these leaf-shaped, shockingly green animals spend their days turning the sun’s energy into their own. The reasons behind E. chlorotica’s abundance on the Island have not been studied yet; however, population density is the primary reason that the Vineyard has become the go-to location for the scientific collection of E. chlorotica. Pierce noted that even after Hurricane Bob ravaged their environment, the slugs returned within two years. Aside from this anomalous event, Pierce noted that over the course of the past 30 years, he does not believe there has been any significant change in the size of the Martha’s Vineyard E. chlorotica population. This is not to say that the water is teeming with E. chlorotica, simply that the population appears stable, though no official ecological survey has been conducted to prove or disprove this observation.

These slugs are difficult to raise in the lab environment, and therefore must be collected in controlled numbers annually. Pierce warned that increased disturbance or collection could be detrimental to the slugs’ ongoing prosperity, saying, “I’m pretty confident we could wipe them out in a weekend. We’re very careful not to do that; we count as we collect, and we take defined numbers.” Thus it has become somewhat of a common occurrence for a handful of green slugs to be transported hundreds of miles to Pierce’s lab in Florida — and even farther, to the labs of other involved scientists on the West Coast — where they’re diligently studied for their unique plantlike abilities. To protect these slugs from harm, the whereabouts of their Vineyard residence remain shrouded in a healthy dose of mystery, quite similar to that of celebrities seeking the tranquility of the Island’s quiet nooks and crannies.

The slugs’ previously mentioned singular meal is truthfully more of a period of grazing, during which E. chlorotica eats one species of yellow-green algae called Vaucheria litorea, which grows in the bottom of salt marsh habitats. When these slugs ingest V. litorea, they strip the chloroplasts — the part of the plant cell that carries out photosynthesis — directly from the noodle-like strands, and incorporate these foreign organelles into their own digestive tract. As Pierce put it, “the animal slits open the [tube-like strands], sucks out the contents like a soda straw, and certain cells that line the walls of the digestive tract are able to take up the chloroplasts and keep them. If you shine light on the animal, it photosynthesizes — it fixes carbon and blows off oxygen.”

This is extremely rare. Animals inherently require oxygen from their environment for respiration, and yet these slugs produce it. There are presently about 100 known species of sea slugs worldwide that are capable of doing this — utilizing plants’ chloroplasts to conduct photosynthesis, termed “chloroplast symbiosis.” There’s even a word for this group of mollusks; sacoglossan, which means “sap-sucking,” for their ability to ingest the contents of algae at the cellular level.

As sacoglossan specialist Patrick Krug explained, E. chlorotica stands out from its “sap-sucking” peers for the extensive period it is able to maintain and utilize stolen chloroplasts. Krug, a biology professor at the California State University, Los Angeles, has been researching the evolutionary patterns of this type of animal for the past 25 years, and painted a clear picture of the uniqueness of these animals among its peers.

“There are some species [of photosynthesizing sea slugs] that will use parts of their bodies to cover the chlorophyll. The reason they do that is the light-harvesting machinery of the chlorophyll will burn out if you expose it to sunlight, and the slugs can’t replace that — it becomes obsolete: They can’t replace sun-damaged chlorophyll. So instead they hide the chlorophyll from the sun, and that allows them to extend the lifespan of the chlorophyll. But E. chlorotica is not like that. I’ve had them in my lab for months and months, and they’re not like that. Their entire body is green from chloroplasts. Everything else blanches after six to eight weeks — going pretty much white after two months, but chloritica does not — it’s a living, breathing salad. They’re definitely doing something different and unique, and I don’t know what it is. E. chlorotica is the only species that can really really do this, and Martha’s Vineyard is one of the [only] places you can really find them.”

So you may be asking, what makes these slugs more able to maintain chloroplasts — the capsules that contain chlorophyll — than any other known species of photosynthetic animal on earth? This is where Pierce’s research comes in. Pierce has a sound body of evidence suggesting that 50 algal genes have been integrated, and are now present in these slugs’ DNA, transferred somehow from the algal cell nucleus to the animal cell nucleus: “The critical part of maintaining these plastids inside of the animal’s cells is that the maintenance has to be there. The proteins have to be regenerated, and the chlorophyll has to be made. So that’s what we looked for, and we found it. Chloroplast proteins are being synthesized inside the animal cell.” Pierce stressed that this ability has not been found in any other species. When these results came back, Pierce said, “I told my technician she’d messed up, and said, ‘Go do it again, that can’t possibly be.’”

Sequencing the E. chlorotica genome fully will shed further light on this theory, and then the exciting exploration of the potential applications in medicine and evolution can begin in earnest.

While the implications of successful gene transfer from an animal to a plant may be immense for both medicine and science, there are still many questions to be answered. Krug is curious about the evolutionary story behind this ability. As he framed it, “It’s like asking why is Usain Bolt as fast he is — why is the best the best — and then reverse-engineering from that, why nothing else is as good as the best” on the genetic level. As he said, half-jokingly, at the end of our interview, “Maybe someday we’ll have green cows that don’t need to eat grass.” I laughed, and he continued, “Think about how much you’d save on your grocery bill if you could just sit outside for an hour and then not eat.”

In a more serious tone, Krug added, “Imagine if you could take a pill with DNA in it that could temporarily be expressed in your body to break down a tumor or jam a virus, and then break down and go away.” As with the notion of a green cow, the feasibility of this type of medical advancement relies on the scientific discovery of how these slugs integrate foreign DNA into their bodies, and if this method is transferable to other animals, like us. How special it is that the species holding these answers lives — and photosynthesizes — right in our waters.