Tardigrade
Tardigrades, also known as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them . In 1776, the Italian biologist Lazzaro Spallanzani named them Tardigrada, which means 'slow walkers'.
Tardigrades live in diverse regions of Earth's biospheremountaintops, the deep sea, tropical rainforests, and the Antarctic. They are among the most resilient animals known, with individual species able to survive severe conditions, such as exposure to extreme temperatures, extreme pressures, air deprivation, radiation, dehydration, and starvation – that would quickly kill most other forms of life. Tardigrades have survived exposure to outer space.
There are about 1,500 known species in the phylum Tardigrada, a part of the superphylum Ecdysozoa. The earliest known fossil is from the Cambrian, some 500 million years ago. They lack several of the Hox genes found in arthropods, and the middle region of the body corresponding to an arthropod's thorax and abdomen. Instead, most of their body is homologous to an arthropod's head.
Tardigrades are usually about long when fully grown. They are short and plump, with four pairs of legs, each ending in claws or sticky pads. Tardigrades are prevalent in mosses and lichens and can readily be collected and viewed under a low-power microscope, making them accessible to students and amateur scientists. Their clumsy crawling and their well-known ability to survive extreme conditions have brought them into science fiction and popular culture including items of clothing, statues, soft toys and crochet patterns.
Description
Body structure
Tardigrades have a short plump body with four pairs of hollow unjointed legs. Most range from in length, although the largest species may reach. The body cavity is a haemocoel, an open circulatory system, filled with a colourless fluid. The body covering is a cuticle that is replaced when the animal moults; it contains hardened proteins and chitin but is not calcified. Each leg ends in one or more claws according to the species; in some species, the claws are modified as sticky pads. In marine species, the legs are telescopic. There are no lungs, gills, or blood vessels, so tardigrades rely on diffusion through the cuticle and body cavity for gas exchange. They are made up of only about 1000 cells.Nervous system and senses
The tardigrade nervous system has a pair of ventral nerve cords with a pair of ganglia serving each pair of legs. The nerve cords end near the mouth at a pair of subpharyngeal ganglia. These are connected by paired commissures to the dorsally located cerebral ganglion or 'brain'. Also in the head are two eyespots in the brain, and several sensory cirri and pairs of hollow antenna-like clavae which may be chemoreceptors.The tardigrade Dactylobiotus dispar can be trained by classical conditioning to curl up into the defensive 'tun' state in response to a blue light associated with a small electric shock, an aversive stimulus. This demonstrates that tardigrades are capable of learning.
Locomotion
Although the body is flexible and fluid-filled, locomotion does not operate mainly hydrostatically. Instead, as in arthropods, the muscles work in antagonistic pairs that make each leg step backwards and forwards; there are also some flexors that work against hydrostatic pressure of the haemocoel. The claws help to stop the legs sliding during walking, and are used for gripping.Feeding and excretion
Tardigrades feed by sucking animal or plant cell fluids, or on detritus. A pair of stylets pierce the prey; the pharynx muscles then pump the fluids from the prey into the gut. A pair of salivary glands secrete a digestive fluid into the mouth, and produce replacement stylets each time the animal moults. Non-marine species have excretory Malpighian tubules where the intestine joins the hindgut. Some species have excretory or other glands between or at the base of the legs.Reproduction and life cycle
Most tardigrades have both male and female animals, which copulate by a variety of methods. The females lay eggs; those of Austeruseus faeroensis are spherical, 80 μm in diameter, with a knobbled surface. In other species, the eggs can be ovoid, as in Hypsibius annulatus, or may be spherical with pyramidal or bottle-shaped surface ornamentation. Some species appear to have no males, suggesting that parthenogenesis is common.Both sexes have a single gonad located above the intestine. A pair of ducts run from the testis, opening through a single gonopore in front of the anus. Females have a single oviduct opening either just above the anus or directly into the rectum, which forms a cloaca.
The male may place his sperm into the cloaca, or may penetrate the female's cuticle and place the sperm straight into her body cavity, for it to fertilise the eggs directly in the ovary. A third mechanism in species such as H. annulatus is for the male to place the sperm under the female's cuticle; when she moults, she lays eggs into the cast cuticle, where they are fertilised. Courtship occurs in some aquatic tardigrades, with the male stroking his partner with his cirri to stimulate her to lay eggs; fertilisation is then external.
Up to 30 eggs are laid, depending on the species. Terrestrial tardigrade eggs have drought-resistant shells. Aquatic species either glue their eggs to a substrate or leave them in a cast cuticle. The eggs hatch within 14 days, and the hatchlings use their stylets to open their egg shells.
Ecology and life history
Tardigrades as a group are cosmopolitan, living in many environments on land, in freshwater, and in the sea. Their eggs and resistant life-cycle stages are small and durable enough to enable long-distance transport, whether on the feet of other animals or by the wind.Individual species have more specialised distributions, many being both regional and limited to a single type of habitat, such as mountains. Some species have wide distributions: for instance, Echiniscus lineatus is pantropical. Halobiotus is restricted to cold Holarctic seas. Species such as Borealibius and Echiniscus lapponicus have a discontinuous distribution, being both polar and on tall mountains. This could be a result of long-distance transport by the wind or the remains of an ancient geographic range when the climate was colder. A small percentage of species may be cosmopolitan.
The majority of species live in damp habitats such as on lichens, liverworts, and mosses, and directly in soil and leaf litter. In freshwater and the sea, they live on and in the bottom, such as in between particles or around seaweeds. More specialised habitats include hot springs and as parasites or commensals of marine invertebrates. In soil, there can be as many as 300,000 per square metre; on mosses, they can reach a density of over 2 million per square metre.
Tardigrades are host to many microbial symbionts and parasites. In glacial environments, the bacterial genera Flavobacterium, Ferruginibacter, and Polaromonas are common in tardigrades' microbiomes. Many tardigrades are predatory; Milnesium lagniappe includes other tardigrades such as Macrobiotus acadianus among its prey. Tardigrades consume prey such as nematodes, and are themselves preyed upon by soil arthropods including mites, spiders and cantharid beetle larvae.
Except for 62 exclusively freshwater species, all non-marine tardigrades are found in terrestrial environments. Because the majority of the marine species belong to Heterotardigrada, the most ancestral class, the phylum evidently has a marine origin.
Environmental tolerance
Tardigrades are not considered universally extremophilic because they are not adapted to exploit many of the extreme conditions that their environmental tolerance has been measured in, only to endure them. This means that their chances of dying increase the longer they are exposed to these extreme environments, whereas true extremophiles thrive there.Dehydrated 'tun' state
Tardigrades are capable of suspending their metabolism, going into a state of cryptobiosis. Terrestrial and freshwater tardigrades are able to tolerate long periods when water is not available, such as when the moss or pond they are living in dries out, by drawing their legs in and forming a desiccated cyst, the cryptobiotic 'tun' state, where no metabolic activity takes place. In this state, they can go without food or water for several years. Further, in that state they become highly resistant to environmental stresses, including temperatures from as low as to as much as , lack of oxygen, vacuum, ionising radiation, and high pressure.Surviving other stresses
Marine tardigrades such as Halobiotus crispae alternate each year between an active summer morph and a hibernating winter morph that can resist freezing and low salinity, but which remains active throughout. Reproduction, however, takes place only in the summer morph.Tardigrades can survive impacts up to about, and momentary shock pressures up to about.
Exposure to space
Tardigrades have survived exposure to space. In 2007, dehydrated tardigrades were taken on the FOTON-M3 mission and exposed to vacuum, or to both vacuum and solar ultraviolet, for 10 days. Back on Earth, more than 68% of the subjects protected from ultraviolet were reanimated by rehydration, and many produced viable embryos.In contrast, hydrated samples exposed to vacuum and solar ultraviolet survived poorly, with only three subjects of Milnesium tardigradum surviving. The space vacuum did not much affect egg-laying in either R. coronifer or M. tardigradum, whereas UV radiation reduced egg-laying in M. tardigradum. In 2011, tardigrades went on the International Space Station STS-134, showing that they could survive microgravity and cosmic radiation, and should be suitable model organisms.
In 2019, a capsule containing tardigrades in a cryptobiotic state was on board the Israeli lunar lander Beresheet which crashed on the Moon.