The world is teeming with life, and we're always discovering new species—including some that stretch the limits of how we view and classify biological life forms. Here are a few that clearly don't play by the rules.
1. BDELLOID ROTIFERS
Bdelloid rotifers are microscopic superstars inside a drop of water. These tiny transparent animals—which can be found all over the world (even Antarctica!)—are masters of survival and reproduction. When water becomes scarce, they dry up like brine shrimp, surviving for years completely desiccated. When water returns, they rehydrate themselves and continue on as good as new. Rotifers are all female and reproduce asexually, laying eggs that don't need to be fertilized and are essentially clones of themselves. While they're not the only animal that doesn't need a member of the opposite sex to reproduce, they're more successful than others: Bdelloid rotifers have evolved into 450 species. How can a creature evolve if it's only producing clones? Random mutations would produce changes, but cannot explain the rotifers' 80-million-year survival and successful speciation.
The secret to rotifers' evolution is that they steal genes from other living things. DNA analysis of bdelloid rotifers shows that about 10 percent of their genes come from bacteria, fungi, and plants. How does that happen? It turns out that bdelloid rotifers are also masters at surviving ionizing radiation, which damages DNA. The creatures are able to repair their own DNA, but can incorporate new genes (from the surrounding environment or something they ate) in the repair process. Over time, the new genes are used to adapt to the environment, leading to the evolution of new rotifer species as well as incorporating the necessary genetic material to protect against parasites.
Euglena is a genus that contains hundreds of species of single-cell organisms that are not plant nor animal nor bacteria, but have features of all three. Most species of Euglena are mixotrophs that power themselves based on environmental conditions. When sunlight is present, for example, Euglena will use it to make food by photosynthesis using chloroplasts, the genes for which may have been taken from engulfed alga sometime in Euglena's evolutionary history. When there is no sunlight, Euglena ingests surrounding substances like an animal to get energy. But what's really amazing about Euglena is that its behavior can be useful to humans. A company in Japan is looking into using some species of Euglena for food and biofuel, and other species might be used to clean the environment as they eat pollutants.
3. TRICHOPLAX ADHAERENS
Among multicellular animals, the microscopic Trichoplax adhaerens is the master of minimalism. It's so simple, in fact, that for decades it was assumed that it was only a larval stage of another animal. T. adhaerens is comprised of just four types of cells, and is basically two sheets of cells with some more cells in between. It has no organs and no discernible front or back, though it does have a distinct upper and lower side—the organism uses that lower side both to eat and to adhere to surfaces. It can move either by changing shape or by using tiny cilia on its outer layers. It's perhaps not surprising T. adhaerens has an extremely simple genome, too, with 98 million base pairs, compared to over 3 billion for humans. They reproduce by splitting, by budding, or by sexual reproduction. Scientists don't know exactly how they manage the sexual reproduction; organisms have been observed degenerating into eggs, but fertilization is still a mystery.
Tardigrades, also called water bears or moss piglets, resemble eight-legged faceless bears, except they're generally a half-millimeter long. Hundreds of species of these tiny animals are found in every kind of environment on earth, but they prefer to be among moss, algae, and lichen. While ocean-based tardigrades are pretty normal, land and fresh water tardigrades are famously hard to kill. If the environment is dry, they dry up too, and go into a dormant state that they emerge from when wet conditions return, even years later. They can survive boiling or freezing temperatures. They can survive in the vacuum of space and in high pressure conditions. They can survive radiation that would kill lesser animals.
In case you want a tardigrade of your own, the International Society of Tardigrade Hunters has instructions for collecting them. A low-power microscope should suffice for observation.
5. GEOGEMMA BAROSSII
A microbe of the Archaea domain, Geogemma barossii is a microbe that likes it hot. This hyperthermophile, sometimes referred to as Strain 121, grows optimally at 220°F, but does just fine at 250°F (or 121°C, hence the name). It doesn't die until temperatures go over 266°F—one of the highest-known temperature tolerances of any living thing. The discovery of G. barossi's heat tolerance in 2003 gave pause to medical specialists when they realized that their sterilization procedures would not kill this microbe. However, Strain 121 cannot grow in the range of a human's body temperature, so it isn't considered infectious. Its normal home is thermal vents in the ocean floor.
BONUS: GROMIA SPHAERICA
The size of a grape, Gromia sphaerica is too big to be a microbe—but this single-celled organism is too cool not to include. This ancient relative of the amoeba lives at the bottom of the ocean, and was first discovered in the Arabian Sea in 2000. Adult specimens can grow to be 1.5 inches in diameter, or as small .019 inches. While a single cell that big is pretty strange, the most remarkable thing about G. sphaerica is the trails they leave behind on the sea floor. They weren't created by the organisms rolling downhill (they can actually move uphill), and they weren't created by ocean currents. Somehow, these big cells moved on their own and are heavy enough to leave a trail behind them. That raises questions about fossil trails from the Precambrian that scientists assumed were left by multicellular animals, but may have been left before multicellular life arose.