Tardigrades and their incredible peculiarities

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Tardigrades and their incredible peculiarities

Tardigrades are widespread all over the planet. There are marine, terrestrial and freshwater species. They have been observed on all continents, including Antarctica, and at all altitudes, from abyssal oceanic areas to heights of over 6,000 meters in the Himalayas.

They can be considered essentially aquatic animals, as terrestrial species also live within layers of water that may be just thick enough to house them. However, they are able to resist drying and freezing for a very long time.

Most species feed on plant cells. However, there are also predatory forms, whose food is provided by Protozoa, Rotifers, Nematodes and also by other tardigrades. Tardigrades are able to survive in conditions that would be lethal to almost all other living things.

If placed in adverse conditions such as those listed above, these animals develop a series of defense mechanisms ranging from encystidation to the suspension of any visible metabolic activity. In conditions of dehydration, they retract their legs and contract, reducing the surface to slow down the evaporation of water.

This provides time for the synthesis of protective substances. In the case of low temperatures in the liquid that fills the hemocele, controlled growth ice crystals are formed. Linear dimensions of adults can range from less than 0.1mm to 1.5mm.

Marine species are colorless or greyish-white, while terrestrial or freshwater species can be of various colors, such as orange, pink, yellow, green or black. They are eutelic organisms. The body, approximately cylindrical, consists of the head and four metamers, each of which bears a pair of legs which in many marine species are partially retractable with a telescopic mechanism.

At the extremities of the legs there are a variable number of nails or fingers, generally between 4 and 8. Rarely the nails can be reduced or missing completely. The body is lined with a thin elastic extracellular cuticle, also formed by chitin.

They can live from 3 months to 2 years, unless they go into a dormant state. Breathing is exclusively cutaneous. The small size, and therefore the high surface / volume ratio, make a respiratory system superfluous. The coelom, present in the embryos, regresses in the adult.

Inside the body there is instead a haemocele, that is a large cavity occupied by a colorless liquid in which large cells are suspended with probable function of food reserve. The liquid bathes all the organs, thus ensuring the distribution of substances in the absence of a circulatory system.