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.