These ambling, eight-legged microscopic “bears of the moss” are cute, ubiquitous, all but indestructible and a model organism for education
The young woman in my office doorway is inquiring about the summer internship I am offering.
What’s a tardigrade? she asks.
Tardigrades, I reply, are microscopic, aquatic animals found just about everywhere on Earth. Terrestrial species live in the interior dampness of moss, lichen, leaf litter and soil; other species are found in fresh or salt water. They are commonly known as water bears, a name derived from their resemblance to eight-legged pandas. Some call them moss piglets and they have also been compared to pygmy rhinoceroses and armadillos. On seeing them, most people say tardigrades are the cutest invertebrate.
At one time water bears were candidates to be the main model organism for studies of development. That role is now held most prominently by the roundworm Caenorhabditis elegans, the object of study for the many distinguished researchers following in the trail opened by Nobel Prize laureate Sydney Brenner, who began working on C. elegans in 1974. Water bears offer the same virtues that have made C. elegans so valuable for developmental studies: physiological simplicity, a fast breeding cycle and a precise, highly patterned development plan. Some species may, like C. elegans, be eutelic, meaning that the organisms retain the same number of cells through their development. Tardigrades have somewhere over 1,000 cells. I and others use water bears as a model educational organism to teach a wide range of principles in life science.
Tardigrades are nearly translucent and they average about half a millimeter (500 micrometers) in length, about the size of the period at the end of this sentence. In the right light you can actually see them with the naked eye. But researchers who work with tardigrades see them as they appear through a dissecting microscope of 20- to 30-power magnification—as charismatic miniature animals.
Most tiny invertebrates dart about frantically. Tardigrades move slowly as they clamber around on bits of debris. They were first named tardigrada in Italian from the Latin meaning “slow walker.” Tardigrades walk on short, stubby legs located under their bodies, not sticking out to the sides. These stout legs propel them unhurriedly and deliberately about their habitat.
Tardigrades have five body sections, a well-defined head and four body segments, each of which has a pair of legs fitted with claws. The claws vary in different species from familiarly bearlike to strangely medieval fistfuls of hooked weaponry. The hindmost legs are attached backwards, in a configuration unlike that of any other animal. These legs are used for grasping and slow-motion acrobatics rather than for walking.
Inside these tiny beasts we find anatomy and physiology similar to that of larger animals, including a full alimentary canal and digestive system. Mouth parts and a sucking pharynx lead to an esophagus, stomach, intestine and anus. There are well-developed muscles but only a single gonad. Tardigrades have a dorsal brain atop a paired ventral nervous system. (Humans have a dorsal brain and a single dorsal nervous system.) The body cavity of tardigrades is an open hemocoel that touches every cell, allowing efficient nutrition and gas exchange with no need for circulatory or respiratory systems.
Taxonomists divide life on Earth into three domains: Bacteria, Archaea (an ancient line of bacterialike cells without nuclei that are likely closer in evolutionary terms to organisms with nucleated cells than to bacteria), and Eukarya. Eukarya is divided into four kingdoms: Protista, Plantae, Fungi and Animalia. Phylum Tardigrada is one of the 36 phyla (roughly, depending on whom one asks) within Animalia—making water bears a significantly distinctive branch on the tree of life.
Tardigrades are encased in a rugged but flexible cuticle that must be shed as the organism grows. Thus they have been placed among the phyla on the ecdysozoa line of evolution between animals such as nematodes and arthropods that also shed their cuticles to grow.
I can see in my drop-in student’s eyes that she remembers the ecdysozoan way of being from introductory biology but she does not really understand it.
Animals grow in either of two ways, by adding more cells or by making each cell larger. Tardigrades generally do the latter. If an animal has a hard cuticle or exoskeleton, it must break out of that shell in order to grow. For example, in summer in many parts of the world, one encounters the shed exoskeletons of locusts on trees everywhere.
Tardigrades are divided into two classes, Eutardigrada and Heterotardigrada. As a general rule, the members of Eutardigrada have a naked or smooth cuticle without plates, whereas the Heterotardigrada boast a cuticle armored with plates.