Credit: Mary E. Rumpho-Kennedy

Facts box

Scientific Name:: Elysia chlorotica

Range: East coast of United States and South of Canada

Length: 20-50mm

Topics: biology, zoology, marine biology

Sunbathing among sprawling intertidal algae along the North American east coast, you will find a creature suffering an identity crisis. But, luckily, the confusion is no issue for the Eastern emerald elysia (Elysia chlorotica), a unique type of sea slug that moves like an animal but feeds like a plant.

A not-too-distant relative of the humble snail, this vibrant green sea slug only needs to eat once in its lifetime before becoming a solar-powered 'leaf that crawls'.

As a tiny red-and-brown juvenile, the sea slug - which grows up to 6cm long - feeds on Vaucheria litorea algae for at least six or seven days. But instead of breaking down all that it eats, as most animals do, it stores the algae's chloroplasts in the cells of its digestive tract.

These chloroplasts - green photosynthetic structures found in plant cells - allow the elysia to survive on nothing but sunlight and air for the rest of its year-long life.

"Elysia chlorotica uses its radular tooth to puncture a hole in the filament of its algal prey," says Marie Rumpho, a biochemist at the University of Maine, in the U.S. "Once the juvenile sea slug punctures the filament, it is able to suck all the contents out."

This plant-like animal has had biologists scratching their heads for about 200 years. The weird creature first sparked naturalists' curiosity during the 1800s when small green 'organisms' were detected inside it.

Fast forward to the 1960s, and researchers confirmed these were actually algal chloroplasts. But the question remained as to how these chloroplasts continued to photosynthesise inside the sea slug without the genes to power them.

Rumpho and her colleagues are trying to crack this mystery. They found that the slug doesn't just rob the algae of their photosynthetic machinery; it also takes their genes. But there are some other mechanisms in play that the researchers are working to identify.

By comparing the genomes of the elysia and its algal prey, Rumpho discovered the two organisms share a gene that plays a role in photosynthesis. Because this gene is specific to algae, Rumpho concluded it originated from the sea slugs' food.

Theft of the algae's chloroplasts has an adaptive advantage in the seasonal environment in which the elysia live, says Rumpho. "Where [these] sea slugs are found in nature, the animals need to live weeks, if not months, by photosynthesis when the algae is not present in the winter," she explains. "Evolving the ability to photosynthesise may sustain the sea slugs until the temperature warms up enough for the algae to grow. I believe this drove the evolution of the association."