Tardigrades, commonly known as water bears or moss piglets, are not bacteria. The confusion arises because these organisms are microscopic, but they are not simple single-celled life forms. Tardigrades are complex animals representing a distinct phylum on the tree of life, separate from bacteria. They are known for their remarkable toughness, surviving environments that would kill most other creatures on Earth.
Defining the Kingdoms: Prokaryotes vs. Eukaryotes
The fundamental distinction between bacteria and tardigrades lies at the cellular level, placing them into two different biological domains. Bacteria are classified as prokaryotes; their cells are simple and lack a true, membrane-bound nucleus to house their genetic material. These organisms are typically single-celled and do not contain membrane-bound compartments, known as organelles, to perform specialized functions.
Tardigrades, however, are multicellular organisms composed of eukaryotic cells, which are structurally far more complex. Eukaryotic cells possess a distinct nucleus that encapsulates the DNA, as well as various organelles like mitochondria and the endoplasmic reticulum. This internal compartmentalization allows for a much higher level of cellular organization and complexity.
Tardigrade Classification and Anatomy
Tardigrades belong to the Kingdom Animalia and are the sole members of the Phylum Tardigrada. These micro-animals are small, with most adults measuring between 0.05 and 1.5 millimeters in length. Their bodies are short, plump, and segmented, consisting of a head and four body segments.
Each segment has a pair of short, unjointed legs, giving the animal a total of eight limbs. These legs end in claws or pads, which they use to grasp substrates, earning them the nickname “water bear.” They are covered by a tough, flexible outer layer called a cuticle, which must be periodically shed as the animal grows. This complex morphology establishes them as invertebrates, not bacteria.
Surviving the Extremes
The tardigrade’s fame stems from its ability to enter cryptobiosis, a reversible shutdown of its metabolism. When the environment becomes too harsh, such as when water dries up, the tardigrade curls into a desiccated, barrel-shaped form known as a “tun.” In this tun state, metabolic activity essentially ceases, allowing them to survive otherwise lethal conditions.
This metabolic stasis enables them to withstand extreme temperatures, ranging from near absolute zero (-273 degrees Celsius) to more than 150 degrees Celsius. They can also endure crushing pressures greater than those found in the deepest ocean trenches, as well as the vacuum of outer space. Their cells produce specialized proteins, such as damage suppressor (Dsup) proteins, which bind to and protect their DNA from high doses of ionizing radiation until they can safely rehydrate and return to life.