Unique life form is half plant, half animal

Species: Mesodinium chamaeleon
Habitat: seawater around Scandinavia and North America, chowing down on a new generation of slaves

Many animals transform themselves almost beyond recognition in the course of their lives. Caterpillars become butterflies and tadpoles become frogs, and if we couldn't watch them do so we might not even suspect that the two stages were the same creature.

Spectacular as these shifts are, they are only shape-shifting. A tadpole and a frog are both animals, so both must take in food from their surroundings.

Not so Mesodinium chamaeleon. This newly discovered single-celled organism is a unique mixture of animal and plant.

M. chamaeleon is a ciliate – a kind of single-celled animal covered in hundreds of tiny "hairs" called cilia. It was discovered in Nivå bay in Denmark by Øjvind Moestrup of the University of Copenhagen, also in Denmark, and his team. Other specimens have since been found off the coasts of Finland and Rhode Island.

Ciliates using their hair-like cilia to motor around rapidly in water. Most get their food by eating other organisms, rather than by synthesising the nutrients themselves. This marks them as quite animal-like.

Some Mesodinium species are different, though. They engulf other microorganisms, generally algae called cryptomonads. The two then form a partnership: the algae produce sugars by photosynthesis, while the Mesodinium protects them and carries them around.

Such hybrid organisms are animals and plants at the same time. One such species, M. rubrum, only eats red algae and is often found in the algal blooms that form the famous red tides.

These hybrids play merry hell with our attempts to classify organisms into neat groups. "The division between plants and animals is collapsing completely," Moestrup says. Instead, many microorganisms may be animal and plant at once, or switch between the two, like M. rubrum.

The new M. chamaeleon breaks yet another barrier. It is halfway between a pure animal and a hybrid.

Red and green

M. chamaeleon takes in algal cells, just like M. rubrum, but it doesn't keep them permanently. Nor does it digest them immediately, as a hungry animal-like organism might. Instead, the cells remain intact for several weeks before being broken down, during which time they keep producing sugar by photosynthesis. M. chamaeleon also changes colour depending on whether it is hosting red or green algae or both.

"It is quite unusual," says Moestrup. Other Mesodinium species either retain their captured cells for ages or digest them immediately.

The ability to take in other cells and put them to work is called endosymbiosis, and is one of the most important inventions in the history of life. Some 2 billion years ago, a single cell swallowed a bacterium and used it as an energy source. The descendants of the enslaved bacterium eventually became the mitochondria that now power all complex cells, including ours. Without endosymbiosis, there wouldn't be any multicellular life.

While the first endosymbiosis may have been a lucky chance, the process now seems to be common, at least among the more complex single-celled organisms. Some are so good at taking in cells that over the years they have switched symbionts. "It happens quite regularly," Moestrup says.

M. chamaeleon may offer a snapshot of how endosymbiosis developed: the organism is still on the road from simply eating other cells to keeping them alive within itself.