Spotted salamanders are the only known vertebrates with photosynthetic cells

An Unexpected Green Egg Mass

Every spring across eastern North America, spotted salamanders leave their subterranean burrows in deciduous forests and migrate to vernal pools to breed. In these temporary woodland ponds, females deposit firm, jelly-coated egg masses on submerged twigs and vegetation. Naturalists and biologists have long noticed that as these clear clusters mature in the sunlight, they frequently turn a vivid, emerald green. This coloration comes from single-celled green algae known scientifically as Oophila amblystomatis, a name that translates literally to 'salamander egg lover.'

For more than a century, biologists classified this interaction as an ectosymbiotic partnership—a helpful arrangement taking place strictly outside the embryo's body. The thick jelly layers surrounding the eggs shield the developing salamanders from predators and dehydration, but the dense gelatinous coating also restricts oxygen diffusion from the water. Trapped within this low-oxygen environment, the embryos rely on the algae proliferating in the perivitelline fluid. The algae absorb carbon dioxide and nitrogenous wastes produced by the salamander, while their photosynthetic activity releases oxygen directly into the fluid surrounding the embryo.

Under standard conditions, eggs colonized by healthy algal blooms develop significantly faster, hatch at a more advanced stage, and experience lower embryonic mortality than those raised in the dark or in sterile water. Because vernal pools are temporary ecosystems that dry up by late summer, rapid development is a vital race against evaporation and colonizing predators. This exchange was celebrated as a textbook mutualism between an aquatic plant and an animal.