A Snorkeler in Nova Scotia Thought This Was a Leaf, but It Was Something Far Weirder

• Not a rotten leaf
• Ephemeral populations

It’s a black blob! It’s a rotten leaf! No, it’s a slug, and a pretty cool one at that.

Snorkelers in Nova Scotia, Canada, have found a number of Elysia chlorotica, commonly known as eastern emerald elysia—elusive slugs that steal the photosynthetic abilities of their food. Studying these creatures could carry implications for various human industries, but they’ve been difficult to study both in the wild and in the lab.

Not a rotten leaf

When snorkeler Elli Ofthenorth, admin of the Facebook group Snorkel 💙 Nova Scotia, first spotted the slug, she “just thought, oh, that’s a rotten leaf, keep going,” she told CBC. It wasn’t until her third time swimming past it that she paid closer attention and realized it wasn’t just a leaf. “I just started yelling, there’s a sea slug here!”

We can forgive Ofthenorth for mistaking an E. chlorotica for a rotten leaf. When the enigmatic slug eats the alga Vaucheria litorea, its primary food source, it starts using the plant’s chloroplasts—the organelles that photosynthesize—to create energy for itself. The chloroplasts turn the slug green, and it looks remarkably like a leaf, veins and all, which works both as camouflage and an effective photosynthesizer. The green is temporary, and the slug eventually turns gray.

“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,” Hunter Stevens, a biologist with the Canadian Parks and Wilderness Society’s Nova Scotia chapter and a member of Ofthenorth’s Facebook group, told CBC. “These slugs are essentially doing the same thing.”

What’s more, the slugs appear to consume algae only at the beginning of their lives, with the chloroplasts working their entire lifetimes. But since E. chlorotica can live for long periods of time without sunlight, researchers aren’t sure of the extent to which the slugs rely on the organelle’s photosynthesis.

Ephemeral populations

After Ofthenorth’s discovery near Halifax, Stevens also went in search of E. chlorotica and was greatly rewarded—he found hundreds of individuals, according to his social media post. “Yes, it’s a slug that photosynthesizes like a plant!” he wrote. “They also have remarkable regenerative abilities, with laboratory studies showing they can regrow their bodies even if they are decapitated.”

Understanding how E. chlorotica can go without eating, living only on sunlight, for seemingly immeasurable amounts of time could have practical applications for areas such as clean energy technology, drug treatments, wound repair, and more, according to the CBC.

But while the slugs live along North America’s east coast, from Nova Scotia all the way down to southern Florida, researchers have had a hard time getting their hands on them.

The slug’s populations are “ephemeral,” Patrick Krug, a researcher at the College of Natural & Social Sciences at California State University, Los Angeles, told the Canadian broadcaster. They seem to cycle through periods of abundance and sudden disappearance. Plus, they’re divas—they live in particular habitats and have particular eating habits, which could contribute to the difficulty of spotting them in the wild and sustaining populations of multiple generations in labs, the CBC said.

It remains to be seen whether “these beautiful emerald angels,” as Stevens described them in his post, will yield any of their enduring secrets any time soon.