Nature has produced some very odd couples. One of the most beautiful and bizarre may be the duo of green algae and baby salamanders. It’s a relationship that has fascinated scientists for more than a century. Now a new paper in the journal eLife sheds a little light on their weird and wonderful dynamic.

Mutually beneficial relationships between species are a dime a dozen, lead author John Burns of the American Museum of Natural History said in a statement, “but the relationship between this particular alga and salamander is very unusual.”

The alga Oophila amblystomatis and its pal, the spotted salamander, don’t just hang out in a living room somewhere. The beautiful, grassy green alga enters the salamander’s jelly-like egg—and then enters the cells of the salamander itself.

The in-egg cohabitation seems to benefit both parties. The alga makes oxygen for the salamander, and the salamander makes nitrogen for the alga through its waste.

© Roger Hangarter

It’s a “strange arrangement,” said co-author Ryan Kerney of Gettysburg College. But how does it work?

To find out, Kerney, Burns, and their colleagues analyzed both species’ cells at the molecular level. They looked at the RNA of algae that lived with and inside salamanders; algae that didn’t; salamanders that lived with algae; and salamanders that didn’t.

They found that, as with any intense relationship, being together changed things for both partners. For the algae, the changes weren’t all positive: those living inside salamanders showed signs of stress and difficulty adapting. This is not a huge surprise; green algae ordinarily get their energy from the Sun. Moving inside an animal’s sunless body would be a pretty big shock.

The same was not true for the salamanders. The results showed that their bodies changed to become more hospitable for the algae by suppressing their immune systems, which suggests that they may have something to gain by letting the algae in.

Learning more about these two weirdos could teach us more about the way the rest of the natural world works.

“These two fundamentally different cells are changing each other dramatically,” co-author Eunsoo Kim of AMNH said. “This might be relevant for other symbiotic systems, including human and parasitic microbe relationships.”