By Victoria Gill
Science and nature reporter, BBC News
The salamander eggs appear green because of the algae inside
Researchers have discovered a unique and rather weird example of a plant living inside an animal.
A team from Dalhousie University in Halifax, Canada, discovered that a green alga invades tiny developing salamander embryos.
This is the first documented case of a plant living in partnership, or symbiosis, with a vertebrate.
Even more strangely, the researchers think the salamanders might inherit the alga from their parents.
The study is published in Proceedings of the National Academy of Sciences.
It was already known that green algae lived inside the eggs of this species - the spotted salamander (Ambystoma maculatum), which is very common in North America.
Although these salamanders only emerge from beneath the ground to hunt and breed, they lay their translucent eggs in ponds, where they are suspended near the surface. This makes an ideal, sunny and protected environment for the algae.
Symbiosis, sometimes called mutualism, is when two different organisms live together and benefit from this intimate lifelong association
An example is the bacteria that live in our digestive tracts, taking nutrients from our food and helping regulate our immune systems
Some algae live symbiotically with fungus to form lichens. Algae have also been found to live and grow inside reef-building corals
"The eggs actually look green because the algae is inside the egg capsules," explained Dr Ryan Kerney of Dalhousie University, who led the research.
"The algae inside the egg capsules provide oxygen to the embryo and the algae gets waste from the embryo [which is rich in the nitrogen the plant needs]."
Although this relationship had been known about for more than a century, scientists did not understand how the algae got into the egg.
Viewing the salamanders under a fluorescent microscope, the researchers were able to see the algal pigments "shine" when they were illuminated with light of a certain wavelength.
Prior to this, many scientists did not think that plants could live inside the cells of vertebrates. This group of complex animals, which includes fish, birds, reptiles and mammals (like us humans) have highly specialised immune systems, which should not let these foreign organisms live in our cells.
Under the microscope: Algae could be seen glowing inside the salamander embryo
"We also found algae DNA in the reproductive organs of the adult salamanders," said Dr Kerney, "so it seems possible that it is being inherited.
"We call that vertical transmission, but there is probably a mixture of this and the algae being absorbed from the environment."
Dr Kerney added: "So many new discoveries in biology comes from research in the tropics.
"I think this shows that it's worthwhile to look at our local species. They can still surprise us."