Classification
Tardigrades are a microorganism group within the Eukarya Domain, Animalia Kingdom, and Tardigrada Phylum. They split into three mini classes: Heterotardigrada, Eutardigrada, and Mesotardigrada. Over 1300 species are known. Tardigrades are closely related to arthropods and onychophorans, and belong to the Ecdysozoa clade.
Living Conditions, Ecological Role, Habitats, Feeding Habits & Reproduction
Tardigrades survive extreme temperatures from −272 °C to 151 °C by entering a cryptobiotic “tun” state. They inhabit marine environments (interstitial and deep sea), freshwater, and limnoterrestrial habitats like mosses, lichens, and leaf litter. They’re found from mountaintops to Arctic regions and tropical rainforests, thanks to their robust integrity.
Genomics
Tardigrades endure radiation, pressure, desiccation, freezing, and oxygen deprivation—even space conditions. Genomic studies on Hypsibius dujardini and Ramazzottius varieornatus revealed stress-tolerance proteins like CAHS, SAHS, MAHS, and Dsup. A major discovery was horizontal gene transfer (HGT): initially estimated at 17.5%, later refined to 3.8–7.1%, from bacteria, archaea, fungi, and plants. These genes contribute to antioxidant defense, DNA repair, and stress adaptation. Evidence includes phylogenetic clustering, codon usage evolution, intron acquisition, and physical linkage to native genes. Their ability to survive desiccation may facilitate HGT by allowing environmental DNA uptake during recovery. This genomic plasticity makes them a key model in astrobiology and biotechnology.
Survivability
The TARDIS project (ESA FOTON-M3) tested Richtersius coronifer and Milnesium tardigradum in space. Four groups were exposed to vacuum and UV combinations for 10 days. Results: vacuum-only group had nearly 100% recovery; vacuum + UV-A group had ~80% survival and normal reproduction; vacuum + UV-B group dropped to ~10–15% survival but still reproduced; vacuum + UV-A+B group did not survive. Tardigrades’ cryptobiotic nature allows ametabolic states that resist vacuum, but UV damage—especially combined—was not reversible.
External Morphology
Tardigrades have a short, segmented trunk with a dorsal cuticle that is periodically shed. They possess four pairs of legs ending in claws or adhesive disks. Their size ranges from 50 micrometers to 1.5 mm. Feeding structures include a buccal apparatus with a tubular mouth and muscular pharyngeal bulb. Many species have stylets for piercing plant cells or small invertebrates.
Cellular Structure
Tardigrades are multicellular micro-animals with specialized cells that respond to dehydration, radiation, freezing, and osmotic shock. Their flexible cuticle, secreted by epidermal cells, regulates water exchange. During cryptobiosis, cells vitrify—forming a glass-like matrix that immobilizes components. This is driven by CAHS, SAHS, and MAHS proteins. Aquaporins regulate water flow, while LEA and HSPs refold damaged proteins. Dsup binds chromatin and shields DNA from radiation and ROS. Antioxidants like superoxide dismutase and catalase neutralize radicals. Their cellular resilience is biochemical as much as structural.
Internal Anatomy
Tardigrades have a reduced coelomic/hemocoel cavity acting as a hydrostatic skeleton. Their digestive system includes a mouth, pharynx/bulb, intestine, and anus—simple but effective for their size and lifestyle.