This salamander embryo hosts living algae inside its own cells

Spotted salamander eggs carry a green alga, Oophila amblystomatis, that doesn't just surround the embryo — it invades the embryo's own tissues and individual cells, trading oxygen for nitrogen waste. It's the only confirmed case of a photosynthetic organism living inside vertebrate cells, found nowhere else in the animal kingdom.

— Ryan Kerney et al., Intracellular Invasion of Green Algae in a Salamander Host — Proceedings of the National Academy of Sciences, 2011