Near the top of the tree of life—alongside the vertebrates, the mollusks, and the rest of the animal kingdom—is a mysterious phylum called Tardigrada. It contains the tardigrades, also known as water bears or moss piglets. They look something like plump caterpillars, with four pairs of clawed legs and a tubular mouth. Most species measure just a half-millimeter in length, but they are some of creation’s toughest animals. Indeed, God has quite an imagination.
The German zoologist and pastor Johann August Ephraim Goeze first described the tardigrade in 1773, naming it kleiner Wasserbär, or “little water bear,” for the way it ambled like a bear. Another naturalist, an Italian priest named Lazzaro Spallanzani, gave the phylum its current name in 1776: Tardigrada is Latin for “slow stepper.” There are over a thousand known species, and each is eutelic, meaning that its body shares an exact number of cells with all adults of the same species.
Tardigrades prefer moist environments like moss and pond bottoms, where they forage for algae, plants, and small invertebrates. Like flatworms, however, they can be found in nearly every habitat on the planet: the Himalayas, boiling hot springs, mud volcanoes, and Antarctic ice. They’re also the first animals to survive conditions not of this planet; in 2007, tardigrades were taken into orbit and exposed to the sun’s radiation for 10 days in the vacuum of space.
Other extremes tardigrades have been known to survive include: 30 years at – 4°F and several minutes at – 458°F (just above absolute zero); 6,000 atmospheres, or 6 times the pressure in the ocean’s deepest trench; and 1,000 times the level of radiation lethal to other animals. Scientists believe that tardigrades could survive cataclysmic events that would wipe out all other life on earth, including large asteroid impacts and supernovae.
How do they do it? Scientists aren’t completely sure, but tardigrades have been found to employ cryptobiosis, a state in which all their vital functions are suspended. In such a state, tardigrades can dehydrate to 3% water, and their metabolisms can slow to 0.01% of their normal rate. A special protein replaces most of the water in their cells and protects against radiation. In this inactive form, tardigrades are called tuns, and they can remain that way for up to five years.
Because of their size and ubiquity, tardigrades make excellent subjects for study under a microscope. The Science Education Resource Center at Carleton College provides the following instructions for finding and viewing tardigrades:
- 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 objective lens, examine the water using a stereo microscope.
- [Optional] Use a micropipette to transfer tardigrades to a slide, which can be observed with a higher power under a compound microscope.
Photo credit: Diane Nelson, National Park Service