Tardigrade

Tardigrades are a phylum of microscopic, eight-legged animals.

They are often called water bears or moss piglets due to their plump bodies and slow movement. These creatures belong to the superphylum Ecdysozoa. They are famous for being some of the most resilient animals on Earth. Individual species can survive conditions that would kill almost any other life form. These conditions include extreme temperatures, high and low pressures, radiation, and starvation.

Their ability to endure such harsh environments makes them vital subjects for biological study.

The physical structure of a tardigrade is quite unique. Most species are about 0.5 millimeters long when fully grown. They possess a short, plump body with four pairs of hollow, unjointed legs. Each leg ends in claws or specialized sticky pads used for gripping. Instead of lungs or gills, they use diffusion for gas exchange. This means they take in oxygen directly through their cuticle, which is their outer body covering. This cuticle is made of chitin and hardened proteins. Inside, they have an open circulatory system called a haemocoel. This cavity is filled with a colorless fluid that carries nutrients.

To survive extreme dryness, tardigrades undergo a process called cryptobiosis. When their environment loses water, they pull in their legs and curl into a dried-out ball. This state is known as a tun. In this form, they suspend their metabolism almost entirely. They can survive without food or water for several years while in a tun. Scientists believe this works because of specialized proteins. These tardigrade-specific proteins create a glass-like matrix inside the cells. This matrix protects the delicate internal structures from damage during dehydration. When water returns, the tardigrade rehydrates and becomes active again.

Tardigrades have diverse ways of reproducing and growing. Most species have both males and females that copulate. However, some species are hermaphroditic and can self-fertilize. Others may use parthenogenesis, where females produce offspring without males. Females lay eggs that vary in shape and size depending on the species. Terrestrial eggs have special shells to resist drying out. Once laid, the eggs typically hatch within 14 days. The tiny hatchlings use their own stylets to break out of the egg shells.

The history of tardigrade discovery spans several centuries. The German zoologist Johann August Ephraim Goeze first described them in 1773. Later, in 1776, the Italian biologist Lazzaro Spallanzani named them Tardigrada. This name comes from the Greek word for "slow walkers." The earliest known fossils of tardigrades date back to the Cambrian period. This means they have existed for about 500 million years. Today, scientists have identified approximately 1,500 different species within the phylum.

Tardigrades are found in almost every corner of the biosphere. They live on mountaintops, in the deep sea, and in the Antarctic. Many live in damp habitats like moss, lichens, and soil. In some soil environments, there can be 300,000 tardigrades per square meter. On moss, their density can exceed 2 million per square meter. They can even survive the vacuum and radiation of outer space. In 2007, the FOTON-M3 mission tested dehydrated tardigrades in space. While full solar ultraviolet light was lethal to many, some specimens successfully reanimated upon returning to Earth.

Their incredible toughness has even made them icons in popular culture and science fiction.