Tardigrades are microscopic invertebrates belonging to a distinct phylum. Often called water bears or moss piglets, these tiny creatures have captivated the public due to their unusual appearance and extreme survival capabilities. They are found across the globe, from mountain tops and deep seas to tropical rainforests and polar regions, existing in the thin films of water around mosses and lichens.
The Invertebrate Classification of Tardigrades
Tardigrades are classified as invertebrates because they lack a vertebral column, the defining feature of vertebrates. They belong to their own taxonomic group, the phylum Tardigrada, which contains over 1,500 known species. This phylum is part of the larger superphylum Ecdysozoa, a group of animals that shed their outer cuticle as they grow.
Evolutionary evidence suggests tardigrades share ancestry with arthropods (insects and crustaceans) and onychophorans (velvet worms), grouping them into the clade Panarthropoda. The tardigrade body plan diverged significantly, separating them from other invertebrate groups like nematodes or annelids.
Anatomy and Distinctive Body Plan
Tardigrades are tiny animals, typically measuring 0.05 to 0.5 millimeters in length, though some reach 1.3 millimeters. Their bodies are bilaterally symmetrical and segmented, featuring a head, three trunk segments, and a posterior segment. A flexible, chitinous cuticle covers the body and must be periodically shed as the animal grows.
A defining feature is the presence of eight short, stout, unjointed legs. The first three pairs are used for crawling, and the last pair faces backward. These legs end in claws or suckers, which they use to grip surfaces. The mouth contains sharp, retractable stylets, used to pierce the cell walls of food sources like moss cells, algae, or bacteria before sucking out the contents.
Mechanisms for Surviving Extreme Conditions
Tardigrades are known for their ability to enter cryptobiosis, a dormant state where metabolic activity slows to nearly undetectable levels. This state allows survival in normally lethal conditions, including extreme temperatures, high radiation, and prolonged desiccation.
When facing a lack of water, a specific form called anhydrobiosis occurs. The tardigrade retracts its limbs and rolls into a dehydrated, barrel-shaped form known as a “tun”.
In this tun state, the animal can endure temperatures ranging from near absolute zero (-272°C) up to 150°C. Scientists have identified that tardigrades produce intrinsically disordered proteins, such as CAHS and LEA proteins. These proteins form a protective, glass-like matrix within the cells to safeguard internal structures.
A separate mechanism accounts for their resistance to high doses of ionizing radiation. This protection is achieved by a unique nuclear protein known as Dsup, or “damage suppressor”. The Dsup protein works by binding to the organism’s DNA, specifically to the nucleosomes that package the genetic material. This binding acts as a shield, protecting the chromosomal DNA from damage caused by harmful hydroxyl radicals generated by radiation exposure.