A rare photosynthesizing sea slug has been found off N.S. Here’s why scientists are excited

When she made the discovery that would thrill her fellow snorkellers and excite researchers across North America, she didn’t think much of it at first.

“I just thought, oh, that’s a rotten leaf, keep going,” says Elli Ofthenorth.

The avid snorkeller passed by this “black gunk” once, twice, but it wasn’t until her third pass that something caught her eye enough to take a closer look, and she realized it was a living creature.

“I just started yelling, there’s a sea slug here!”

Ofthenorth’s mother, who was on the shore at Rainbow Haven Provincial Park near Dartmouth, N.S., lit up the snorkel group chat, and within minutes, members identified it as Elysia chlorotica, or Eastern emerald elysia.

This unassuming creature could almost pass for your garden-variety slug — the kind that decimates your lettuce every summer. That is, until its crinkly-looking back unfurls a stunning, emerald green “leaf,” complete with pale “veins” branching outward from the centre.

It’s this “leaf,” and what it does for the sea slug, that holds so much promise for research in medicine, clean energy and other fields. 

But it’s so elusive that researchers are having a hard time studying it.

E. chlorotica can photosynthesize, stealing the chloroplasts — the photosynthesizing organs — of the algae it eats, keeping them alive in its body, and using them to get energy from the sun. The sea slugs can then subsist for months at a time without consuming food.

“It’s like if I ate a whole bunch of spinach and then I just woke up this morning and I just sunbathed for an hour and then I wouldn’t need to eat for the rest of the week,” says Hunter Stevens, a biologist with the Canadian Parks and Wilderness Society’s Nova Scotia chapter. “These slugs are essentially doing the same thing.”

While the ability to photosynthesize is rare in the animal kingdom, E. chlorotica is not unique in this regard. Other sea slugs can too, but none nearly as well as E. chlorotica.

“Elysia chlorotica is sort of the reigning world champion,” says Patrick Krug, a professor of biological sciences at Cal State University, Los Angeles, who has been studying marine invertebrates for decades.

Krug says researchers don’t know just how long E. chlorotica can go without eating.

He says every other species he’s worked with in the lab gradually “bleach out” and become unable to keep the chloroplasts alive.

“Chlorotica just doesn’t,” Krug says. “I had them in my lab once and I never saw them go ungreen. They were just vividly green for as many months as I kept them, just living on light, and they never ran out.”

Potential research applications

Researchers want to find out how E. chlorotica pulls off this feat so well, and hope that uncovering its secrets can ultimately help humans.

Those insights could someday lead to advances in wound repair, energy and nutrient supplementation, drug therapies, cleaner energy technology and biophotovoltaic cells, scientists say.

Joshua Widhalm is an associate professor of horticulture and the director of the Purdue Centre for Plant Biology in Indiana. For the past six years, he’s been studying a different species of photosynthesizing sea slug — Elysia crispata, commonly called the lettuce sea slug — because it’s so hard to find and maintain E. chlorotica. 

He says unlocking the mysteries of how these creatures steal chloroplasts, incorporate them into their own cells and keep them functioning could have implications for everything from vaccines to chemotherapeutic drugs, to herbicides or pesticides that are expensive to synthesize.

Plus, since the process of photosynthesis produces oxygen, understanding how the sea slug protects itself from or gets rid of excess oxygen could advance research on human inflammatory diseases, neurodegenerative diseases, cancers or even age-related conditions, Widhalm says.

“The imagination is the limit,” he says.

Elusive and ephemeral

However, this coveted slug excels at eluding researchers.

Historically, known populations have existed in the Minas Basin area of Nova Scotia and in Martha’s Vineyard in Massachusetts — and theoretically their habitat exists all along the Eastern Seaboard of the U.S. — but recent efforts to find them have been unsuccessful.

“For so long it just seemed like nobody had seen them,” says Krug. “It was such a shot in the dark, it wasn’t even worth going to look.”

Dylan Gagler, a PhD student at the American Museum of Natural History in New York, has searched the slug’s favoured habitat off Martha’s Vineyard repeatedly this year, but without luck so far.

When Stevens’s Instagram post about the Rainbow Haven discovery popped up in Gagler’s feed, he says he was “having like a freak-out, FOMO [fear of missing out] moment of, like, I got to get up to Nova Scotia. Like, this is clearly where all the action is.”

Gagler contacted Stevens to get information about the conditions at the Rainbow Haven location, such as the air and water temperature and the depth they were found at, in order to fine-tune his own searches. He’s also exploring the permitting process to collect specimens from Nova Scotia to raise in a lab.

Though E. chlorotica has been hard to find, Krug says there have been a few sightings in recent months, including the one in Nova Scotia, as well as in the Carolinas and in Tampa Bay, Fla.

He says the populations are “ephemeral,” seeming to go through cycles of boom and bust — sometimes abundant, but then vanishing suddenly.

Requires specific habitat, food

E. chlorotica is the ultimate Goldilocks — and that could be part of the reason it’s so hard to find in the wild and so hard to maintain a multi-generational population in a lab.

They live in tidal pools that get flushed with each tide, but they need currents that are mild enough that they don’t get swept away into the open ocean. They like water to be salty, but also a little fresh, and prefer environments where the two are mixing.

They need to be able to bask in sunlight to photosynthesize, but they also like to be near their preferred food source — and, “like your typical four-year-old, they’re just extremely picky eaters,” says Krug.

Since they don’t have a real skin barrier, they are sensitive to pollution.

The sea slugs are also vulnerable to any changes in currents, so any nearby development that alters the flow of water could displace an entire population. And if they are displaced from a particular location, it can take decades for another slug to float in by chance, possibly from hundreds of kilometres away, to establish a new population, says Stevens.

“The odds of that happening are very, very low,” Stevens says. “Finding more habitats as a little slug that just crawls along the ground or that just floats through the water column, it’s really, really a shot in the dark.”

Stevens says when he learned about Ofthenorth’s discovery in Rainbow Haven Provincial Park, “my eyes just about popped out of my head.” 

He headed there a few days later, and says it took about 10 minutes of snorkelling before he spotted one small sea slug.

“And then right after that it was like, boom, boom, boom.”

Swimming back to land after braving 6 C waters on another visit on a chilly early November morning, Ofthenorth shouted back to shore that she had just seen “hundreds, if not thousands.”

Since sharing the discovery online, Stevens has had researchers contact him from both the East and West coasts of the U.S.

The fact that the recent discovery of this thriving population was made within the bounds of a provincial park in Nova Scotia underscores how important protected areas are to biodiversity, Stevens says.

“As coastal development proliferates and continues to advance, some of these populations, we may not even know about them, and they’ll disappear,” he says. “And so these slugs will probably get rarer as time goes on.”

Stevens says Ofthenorth’s discovery highlights the importance of citizen science.

“It just shows the power of curiosity and how anybody here can go into the water and there’s still that potential to find this really scientifically significant observation.”

Ofthenorth has already gone back to see the slugs a couple more times since her discovery.

Her passion for living creatures is evident in the way she talks about them. Where some might say E. chlorotica has a face only its mother could love, she refers to it as a “cutie patootie.”

She hopes that by sharing her finds online, others will see the beauty in them too.

“I think the more of us that know what’s in there, my hope is that we’ll be more excited to protect what we have here. It’s really special that we have so much ocean in Nova Scotia and so many cool and fascinating creatures under there.”

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