By Dr. Becker
Tardigrades, also known as water bears and moss piglets, are one of the most fascinating animals known to humankind; they’re continually surprising scientists with their unique genetic makeup and hardy nature.
Water bears can be boiled, frozen or sent to space (on the outside of a rocket). It doesn’t faze them. These animals are microscopic, but if you’ve seen any of their photos online you know they look like creatures from another world.
Water bears have short cylindrical bodies, eight legs (each with four claws at the end) and pig-like snouts with stylets (sharp pointy objects that allow them to cut into moss leaves or algae and suck out the juices).
Water Bears Are Aquatic Creatures (but They Also Live in the Desert)
Most water bears live in water or semiaquatic environments. According to Carleton College Marine Biological Laboratory:1
“All Tardigrades are considered aquatic because they need water around their bodies to permit gas exchange as well as to prevent uncontrolled desiccation.
They can most easily be found living in a film of water on lichens and mosses, as well as in sand dunes, soil, sediments, and leaf litter.”
Interestingly, though they’re considered aquatic water bears have been collected from some of the driest places on earth, such as the Sinai desert in Egypt. When their environment dries up, water bears enter a state known as dessication.
They basically shrivel up, holding on to just 3 percent of their body’s water content and slowing their metabolism to 0.01 percent of normal. They’re not active in this state, but when water hits them they will come back to life as though nothing happened.2
While active water bears lifespan is typically only a few weeks or months, it’s thought that some water bear species may survive dessication for up to 100 years.3
Water Bears Contain an Astonishing Amount of Foreign DNA
Researchers at the University of North Carolina (UNC) at Chapel Hill recently sequenced the genome of water bears, and even the scientists were surprised at the results. It turns out 17.5 percent of water bear DNA comes from other species.4
For comparison, in most animals less than 1 percent of the genome is typically from foreign DNA. Study co-author Bob Goldstein, faculty in the biology department in UNC’s College of Arts and Sciences, said in a news release:5
“We had no idea that an animal genome could be composed of so much foreign DNA … We knew many animals acquire foreign genes, but we had no idea that it happens to this degree.”
The animals have about 6,000 foreign genes, most of which come from bacteria. However, there was also DNA from plants, fungi and single-called microorganisms called Archaea.
Water Bears Demonstrate a “Web of Life”
Water bears likely attained their foreign DNA from horizontal gene transfer, in which genetic material is swapped between species (rather than inherited from parents).
Water bears now hold the title for having the most foreign DNA (at least that’s known). It’s thought the DNA is showing up in the water bears “randomly,” but that the DNA that is kept helps them survive even in harsh environments. First author Thomas Boothby explained:6
“Animals that can survive extreme stresses may be particularly prone to acquiring foreign genes — and bacterial genes might be better able to withstand stresses than animal ones.”
He further explained that the animals may change the way we think about genetic material, especially in terms of how it’s inherited.
While it’s typically assumed that DNA is passed vertically down a “tree of life” via parents, water bears show that genetic material is also passed via a “web of life.” He said in a news release:7
“ … with horizontal gene transfer becoming more widely accepted and more well known, at least in certain organisms, it is beginning to change the way we think about evolution and inheritance of genetic material and the stability of genomes.
So instead of thinking of the tree of life, we can think about the web of life and genetic material crossing from branch to branch. So it’s exciting. We are beginning to adjust our understanding of how evolution works.”
More Foreign DNA Might Help Animals Survive Extreme Environments
Water bears’ ability to survive in extreme conditions is laudable. For instance, these creatures can survive:8
Temperatures as low as -328 degrees F (-200 degrees C) Temperatures as high as 304 degrees F (151 degrees C) Changes in salinity Lack of oxygen Lack of water Levels of x-ray radiation 1,000 times the lethal human dose Some noxious chemicals Boiling alcohol Low pressure of a vacuum High pressure (up to six times the pressure of the deepest part of the ocean)
How do they do it? The UNC news release explained it this way:9
“ … when tardigrades are under conditions of extreme stress such as desiccation – or a state of extreme dryness — Boothby and Goldstein believe that the tardigrade’s DNA breaks into tiny pieces.
When the cell rehydrates, the cell’s membrane and nucleus, where the DNA resides, becomes temporarily leaky and DNA and other large molecules can pass through easily.
Tardigrades not only can repair their own damaged DNA as the cell rehydrates but also stitch in the foreign DNA in the process, creating a mosaic of genes that come from different species.”
How to Find Water Bears
If you were so inclined, you might be able to find a water bear in your own backyard and view it under a microscope (that is, if you have moss or lichen nearby). If you’re a science geek (or a water bear lover), here’s how you can find (and view) land-dwelling water bears, according to Carleton College:10
- Collect a clump of moss or lichen (dry or wet) and place in a shallow dish, such as a Petri dish.
- Soak in water (preferably rainwater or distilled water) for 3-24 hours.
- Remove and discard excess water from the dish.
- Shake or squeeze the moss/lichen clumps over another transparent dish to collect trapped water.
- Starting on a low obejctive lens, examine the water using a stereo microscope.
- Use a micropipette to transfer tardigrades to a slide, which can be observed with a higher power under a compound microscope.
The International Society of Tardigrade Hunters has even more detailed information on how to find your own wild tardigrades, including the video below, which features UNC researcher Thomas Boothby.11