Tardigrades: what they are, characteristics and where they live

Tardigrades, also called water bears, are microscopic invertebrates that look like they came out of a fantasy story. With their plump appearance, flattened face and eight legs armed with claws, they resemble the caterpillar from “Alice in Wonderland.” However, what really makes them fascinating is their incredible ability to survive; These little warriors can withstand extreme conditions, even the vacuum of space.

Classified within the phylum Tardigrada, tardigrades are close relatives of arthropods. Their discovery in 1773 marked the beginning of our admiration for them and their name, which means “slowly advancing”, reflects their calm way of moving.

• The size of tardigrades is between 0.3 and 0.5 mm.
• They have three states: active, anoxybiosis and cryptobiosis, which allows them to adapt to various conditions.
• They are known for their ability to survive extreme temperatures, from about -272°C (absolute zero) to about 150°C.
• They can withstand pressures of up to 40,000 kilopascals.
• They have the ability to resist ultraviolet radiation from space.
• They have a segmented and plump body, with 8 legs that allow them to move effectively.
• They have a brain and a ventral nerve cord, which provides them with a basic nervous system.
• They are covered by a hard cuticle, similar to that of some insects, which they must shed to grow.
• They use a hydrostatic skeleton filled with hemolymph to support their body.
• In a state of cryptobiosis, your metabolism can decrease to 0.01% of its normal rate. Thanks to which they can survive in a state of torpor for years, even decades.

These tiny beings are grouped into four main classes: Eutardigrada, Heterotardigrada, Mesotardigrada and Arthrotardigrada.

• Class Eutardigrada includes Hypsibius dujardini, a tardigrade that faces extreme conditions, while resting among mosses and lichens.
• Class Heterotardigrades present us with species that inhabit both fresh and marine environments, such as Echiniscus blumi, which feels at home in the embrace of land and water.
• Class Mesotardigrada, we find the elusive Thermozodium esakii, now lost to science after an earthquake, represents the fragility of life, even in the most resistant forms.
• Class Arthrotardigrada, we find Batillipes mirus, a marine species with claw-like structures.

Tardigrades can be found virtually anywhere on Earth, from the top of the Himalayas to the bottom of the sea, from frigid Antarctica to bubbling hot springs. These tiny creatures can survive extreme temperatures, ranging from -160°C to 150°C, making them extremist organisms. Something interesting to note is that tardigrades have not yet been found in the human body.

Tardigrades were observed under the microscope, and it was determined that when there is no water, a tardigrade curls up into a dry ball called a tun, slowing down its systems to the point of near death. This state of cryptobiosis allows them to survive for decades. When rehydrated, they return to life within hours. Once, tardigrades that had been in dry moss for a hundred years began to move again when moistened. Although they are hardy, they face predators such as nematodes and amoebas, reminding us that even the strongest must be careful in their environment.

Tardigrades, those intrepid inhabitants of the microcosm, are omnivores, which makes them little gourmets of their environment. With astonishing technique, they use needle-like structures in their mouths to pierce the cell walls of plants, algae and fungi, sucking out the nutritional fluids they contain. Imagine the patience and skill required to extract the nectar of life found within those tiny cells.

However, their diet is not limited to just vegetables. Some more daring species dare to hunt small living beings in motion, such as nematodes, protozoa and even other smaller tardigrades. This predatory facet adds a touch of excitement to their existence, reminding us that, although small, they are capable of interacting with their environment in surprising ways. Every food they consume not only gives them energy, but also allows them to survive in a world full of challenges and adversities.

Tardigrade reproduction is a fascinating process that shows their incredible adaptability. Some species can alternate between sexual and asexual reproduction, depending on environmental conditions. In sexual reproduction, males release their sperm into the female’s body cavity, where fertilization occurs. This results in new tardigrades emerging from the eggs, ready to take on the world.

The amazing thing is that some species are hermaphrodites, meaning they have both male and female reproductive organs. This allows them to self-fertilize, an advantage in extreme environments where finding a mate can be nearly impossible.

Additionally, in asexual reproduction, parthenogenesis allows females to produce offspring from unfertilized eggs, creating clones of themselves.

This reproductive ingenuity is a testament to their resilience. Even after molting, a female can reabsorb her own eggs if no males are present, thus ensuring the continuity of her lineage.

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