A new study by a University of South Florida researcher shows that a sea slug living in the marshes and creeks along the U.S. Atlantic coast is apparently half animal, half plant. Shaped like a leaf itself, the slug Elysia chlorotica has a plant chemical-making pathway working inside an animal body. And this is the first-known evidence of an animal to have that function.

This clever slug is already known for stealing organelles and even genes from the algae it eats. Wired reports that Sidney Pierce of the University of South Florida in Tampa recently revealed this in a new study at the annual meeting of the Society for Integrative and Comparative Biology. Pierce proved that the slugs were making the chlorophyll themselves and not relying on chlorophyll reserves stolen from the algae the slugs consume.

Invertebrate zoologist John Zardus of The Citadel in Charleston, S.C., points out, “This could be a fusion of a plant and an animal — that’s just cool.”

And how did scientists make this discovery? Pierce used a radioactive tracer to show that the slugs were making the pigment and not just stealing from the algae they eat. He described finding more borrowed algal genes in the slug genome for enzymes in a chlorophyll-synthesizing pathway. 

Further, to see whether the slug could actually make new chlorophyll to resupply the chloroplasts, Pierce used slugs that hadn’t fed for at least five months and had stopped releasing any digestive waste. As Wired reports, the slugs still contained chloroplasts stripped from the algae, but any other part of the hairy algal mats should have been long digested.

Hence, the slugs were making the chlorophyll.

Zoologist John Zardus remains skeptical about the findings and would like to hear more about how the team controlled for “algae contamination.” Nonetheless, he still thinks the findings are exciting. As he told Wired, mixing the genomes of algae and animals could certainly complicate tracing out evolutionary history. Further, Zardus points out that the green sea slug “raises the possibility of branch tips touching” in the tree of life.