Solar-Powered Slugs Hide Wild Secrets—But They’re Vanishing

Life has certain rules and patterns. Plants, with their incredible ability to harness the sun’s energy, don’t go roaming around. They don’t need to. But animals, lacking the wondrous power of photosynthesis, do. They trot, slither, flap. They seek out plants and they eat them.

They most certainly do not photosynthesize, the animal playbook would seem to dictate. That’s a plant’s role.

But one small sea slug does not care for such rules, thank you very much.

These animals, Elysia chlorotica, which live off the U.S. East Coast, are not merely content to glide about munching algae. Instead, they steal the molecular engines that allow plants to harvest solar energy. The slugs take up these mini-machines, called chloroplasts, into their skin, which turns them emerald green.

Experiments have shown that this sea slug, which looks like a little leaf an inch or two in length, can go without eating for nine months or more, photosynthesizing with its stolen plant-parts as it basks in the sun.

“It’s unique; it’s controversial; it’s elusive; it never eats,” says Patrick Krug, a biologist at California State University, Los Angeles. “Basically your typical L.A. celebrity.”

Though other sea slugs can purloin chloroplasts and use them to catch some rays, none come close to doing it as well as Elysia chlorotica. (Related: Meet the adorable ‘sea bunny’ taking over the internet)

For this reason, these animal-plant chimeras have attracted attention from scientists, who hope that researching them could have far-reaching applications, for instance in the fields of immunology and gene therapy.

But these special slugs are becoming increasingly rare, and the small number of experts who studied them have mostly retired or have moved on to other areas.

Slugs of Mystery

Research to date on these slugs has provided few answers but many tantalizing hints, suggesting further study could unearth a wonderland of special and useful knowledge.

As yet, nobody knows how the sea slugs keep their chloroplasts running, which normally require a bevy of special proteins produced by thousands of algal genes, most of which these slugs apparently lack—though this area remains controversial.

And why don’t the chloroplasts hurt the slug? Photosynthesis produces free oxygen radicals at levels most animals shouldn’t be able to tolerate. (Related: Meet the sea’s most unusual animals)

Other mysteries abound. Why aren’t the chloroplasts destroyed in the slug gut? Why doesn't the immune system attack these foreign entitites? How do the slugs biochemically interact with them?

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Only one research group—led by Sidney Pierce, a retired researcher at the University of South Florida—has collected the slugs in the last couple years, at a single salt marsh in Martha’s Vineyard, Massachusetts. Like others who’ve studied them, he doesn’t make this information public due to the animal’s scarcity.

Karen Pelletreau, a researcher at the University of Maine who used to work extensively with the animals, has only caught the invertebrates at Martha’s Vineyard and a spot in Nova Scotia. She has searched several areas in Maine where they used to be found, to no avail.

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Slug Troubles

To find them is “difficult, very difficult,” says Pelletreau’s former advisor and colleague Mary Rumpho-Kennedy, who studied the creatures for decades but retired several years ago. “If you don’t know exactly where you’re looking and what you’re looking for, you won’t find it.”

Krug, who studies sea slugs, especially those that live on the West Coast, has looked for them near Woods Hole, Massachusetts, without luck.

He mainly studies a related genus called Alderia, which eat the same algae and like E. chlorotica live in salt marshes. These areas and their animal residents are vulnerable to sea level rise, climate changes brought by warming, and development.

“This habitat may be suffering or growing increasingly ephemeral, more than people appreciate,” Krug notes. Nobody, he says, has conducted population studies on these animals.

The green critters are also quite tough to raise in the lab. Adults need to be cared for well enough to breed, typically living less than a year. The eggs hatch into a free-swimming form that eats several different kinds of algae. Then, as young adults, they begin munching a different food, Vaucheria litorea, a slow-growing algae that’s tricky to culture and raise.

“They’ll eat it literally faster than they we could ever grow it,” says Pierce, who’s studied the animals for more than 30 years. “It’s like having teenagers in the house.”

Though two groups—the team of Rumpho and Pelletreau on the one hand, Pierce on the other—have bred the slugs and raised successive generations in the lab, it is difficult enough that wild collection is easier. They also produce prodigious levels of mucus, which complicates DNA and molecular analysis.

Moving On

However the slugs manage to maintain their chloroplasts, it must involve using algal genes or gene products in a novel, unknown way.

Studies led by Pierce suggest the slug’s genome contains genes transferred from algae, an incredible biochemical feat that could have relevance for genetic manipulation in other animals like humans. But that finding has been disputed by Rumpho and colleagues, as well as some European researchers.

Debashi Bhattacharya, a researcher at Rutgers University, led a recent study in Molecular Biology and Evolution showing that the animals express genes that tamp down their immune system when they take in the chloroplasts, and increase the activity of those genes associated with neutralizing reactive chemicals.

These patterns are reminiscent of the biochemical interactions involved in the symbiotic relationship between coral and their photosynthetic algae.

Further research into these similarities, and what implications that might have for understanding this vital symbiosis, presents just one promising area for future exploration. He, like others, hopes work will continue on Elysia chlorotica—though he doesn’t have plans to do so.

“To follow up on this, somebody would have to find a way to raise a lot of these sea slugs,” he says. “The scarcity of the animal is the problem.”