The land, freshwater lakes and rivers, and seas are adorned with all types of organisms. Up until now, only 2.5 million species have been identified. Upon a complete investigation of the deep seas and inaccessible areas of Earth, the species count is expected to reach 5 or even 9-10 million. Fossil records show the number of living species in the past was much higher than what it is today.

The number of taxonomically defined animal species is around 1.5 million. These are classified into 32 phyla according to their distinct features. One of these is the phylum of Tardigrada.

These cute, tiny water creatures were discovered in 1773 by Johann August Ephraim Goeze. Despite being aquatic, these animals were called "water bears" because of their legs; and because of their resemblance to pandas, they were named "Tardigrada," meaning "slow stepper," by the Italian biologist Lazzaro Spallanzani. Tardigrades also look like dwarf rhinos and armadillos. There are about 1,000 different species in the Tardigrade phylum.

Tardigrades live in habitats with variable amounts of humidity, from high mountains to deep oceans, and from polar regions to the equator. They are especially found in lakes, freshwater springs, or on stone walls, mosses, lichens, leaves, and litter.

As cute, charismatic, miniature animals, tardigrades can be seen under a microscope. The length of adults can reach 1.5 mm, while the smallest ones are under 0.1 mm; their larval length is only around 0.05 milimeters. Their body is covered with a strong but elastic material.

Animals grow in two ways: via an increase of cell numbers or the enlargement of a single cell. In Tardigrades, generally the latter is observed. These animals possess a hard external skeleton, like insects, and this structure changes as they grow.

Their bodies are composed of five sections: a distinct head in addition to four body parts, each equipped with claws. They walk using the short, blunt feet under their bodies. Their clawed legs help them cling to sand particles or plant surfaces. Their hind legs are used for snatching and slow acrobatic movements. They have a sharp mouth, called a "stylet," which enables them to consume plant cells, algae, small invertebrates, and even their own kind.

They are provided with anatomical and physiological features similar to larger animals, including a digestive track and system: a mouth, esophagus, stomach, small intestine, anus, well developed muscles, a pair of abdominal nervous systems, and a brain. The body lumen of Tardigrades are filled with a fluid that is in contact with every cell and this provides them with their necessary nutrition and gas exchanges without the need for a circulatory or respiratory system. Their respiration occurs throughout their body surfaces. Because of their physiology and ability to quickly reproduce, Tardigrades can be used as a model organism for education and research. The cell count of certain species of Tardigrades at birth never changes during their lives. While some species contain around 40,000 cells, some have fewer. Their reproduction can be sexual, but it also can occur via parthenogenesis (offspring development without the fertilization of the egg).

Tardigrades: Organisms of extreme conditions

Tardigrades are created with a resistance to a wide range of temperatures, pressures, and radiation. Therefore, they can live in environments where many living things die. They can survive a temperature of 150 C for minutes, and can also live at minus 200 C without suffering any damage for days; they can even stay alive at temperatures near absolute zero (−273 C).

Some Tardigrades can live at extreme low pressures, including situations approaching a vacuum, or at extreme high pressures, such as 600 times the normal atmospheric pressure. This pressure is six times the pressure present at the depths of the Mariana trench, the deepest part of the Earth's oceans (roughly 11,000 meters). This was discovered when Tardigrades were taken to space and exposed to different pressures. When brought back to Earth, they were still alive.

They can also survive in environments with no humidity for 10 years, and they can stay alive in places where radiation is 1,000 times more (5000 Gy to 6200 Gy) than many organisms can endure (10 Gy is fatal for humans).

Hibernation – a dead phase

How does a Tardigrade stay alive in detrimental conditions?

When they are exposed to conditions unsuitable for life, they enter a semi-dead phase called Cryptobiosis. One of the most distinct changes during this state is that their metabolic speed slows down to near zero, and they experience programmed dehydration. In very low temperatures, Tardigrades' water ratios drop from 85% to 3%. This way, damages that can occur by freezing are prevented. As is well-known, the main hazard during freezing is the cell membrane damage caused by the crystallization of cellular water.

During the dehydration stage, trehalose sugars are synthesized (this also happens when Tardigrades are faced with low temperatures). This sugar prevents possible damages to the cell membranes during freezing and water loss. This sugar is very intriguing for the pharmaceutical industry because of its potential use in the prevention of freezing-related damages in organ transplants.

Another benefit of dehydration is resistance to radiation. This is because reactive molecules generated in the cell by the effects of radiation cannot cause a reaction in a dehydrated medium; due to the low water concentration, the possibility of harmful reactions drops.

Cryptobiosis does not only take place during dehydration. It also happens during periods of low temperature (cryobiosis), high salinity (osmobiosis), and low oxygen. By being able to hibernate, Tardigrades are important for space research. Maybe during interplanetary trips, passengers could be hibernated by freezing.

Tardigrades could have other uses for medical purposes. Certain disease-causing microorganisms could be dehydrated without killing them via Cryptobiosis. This way, "weakened organisms" contained in vaccines get to be stored in a dry fashion, eliminating the need for freezers, making them easier to store and distribute. Similar technologies could also be employed for the conservation of seeds, sperms, blood, and various nutrients.