On The Trails: Tardigrades — tiny and tough
- Mary F. Willson
- Apr 5
- 3 min read
Updated: Apr 5
By Mary F. Willson
These strange little critters are in their own phylum, distantly related to arthropods and round worms. There are over a thousand species of them, distributed in various habitats around the world; over 80 species are known from Alaska. So small that they are almost invisible to our unaided eyes, most of them are less than a millimeter long.
Their name means "slow walker." They have eight stubby, fluid-filled little legs with claws; the first six are for locomotion and the rear two are for clinging and anchoring. They are sometimes called "water bears," because their chunky appearance and slow movement reminded someone (somehow) of real bears, and because they usually live in water. Here is one in motion in a clump of wet moss.
Tardigrades are very tough indeed: they can stand high and low temperatures, high and low pressures, prolonged desiccation, and even exposure to outer space. They can accomplish such feats when, deprived of water, they pull in their legs and form a desiccated cyst with no metabolic activity, and they can persist in that condition for several years.
Their exterior is a cuticle (exoskeleton) made of chitin and certain structural proteins. The cuticle is their means of respiration: they absorb oxygen from water and emit carbon dioxide through the cuticle. It must be molted in order for the critter to grow.
Some tardigrade species reproduce sexually. Males find females using chemical cues and they engage in a prolonged courtship involving physical contact. Females then put their eggs into their cast-off cuticle, where males fertilize them. Other species reproduce via parthenogenesis (females producing unfertilized eggs); some put their eggs in the discarded cuticle while others place them out in the environment, sometimes on the substrate. The eggs hatch in a few days, producing juveniles that are miniature versions of the adults, able to eat immediately. They mature in about two weeks.
Tardigrades feed on all sorts of things, including organic debris. They use two needle-sharp stylets to piece the cell walls of plants, bacteria, lichens, and fungi. The released cell fluids are sucked up by a muscular pharynx and passed to the rest of the digestive system. Some species prey on other miniscule invertebrates, such as nematodes, rotifers, springtails, and even other tardigrades. They may grab small nematodes around the middle with their stylets, but I have not found out how they capture springtails, which are usually quite mobile.
In turn, they are prey for various small invertebrates: spiders, mites, round worms, springtails, amoebae, etc. I wonder if the small birds (e.g., kinglets, hummingbirds) that often pick tiny things from the vegetation might also sometimes grab tardigrades. But if they do, those tough little critters may pass right through the bird’s digestive tract.
They have a couple of eyespots that are sensitive to light and dark. At least some have hair-like cirri on the head; these are used for chemical and tactile sensing. They don’t really have a brain, but rather a nerve ganglion in the head, to which all the sensory systems connect. In a sense, they can do some elementary learning: they can be trained to respond to unusual negative stimuli that results in formation of the long-lived protective cyst. And they can learn to ignore repeated, harmless stimuli.
Where do they live? Almost anywhere on Earth that has some dampness. But they are most abundant in wet, mossy places, soil, and leaf litter. There can be hundreds of thousands of them in a square meter of soil and millions of them in a square meter of moss. The capacity to reach such abundances must affect other invertebrates in any shared habitat, but I have found (so far) no documentation of such effects.
Thanks to Bob Armstrong for photo and video of tardigrades.
• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent. For a complete archive of Mary Willson’s “On the Trails” essays, go to https://onthetrailsjuneau.wordpress.com
