The tardigrade Hypsibius dujardini
Tardigrade, or water bear, is any of the various very small, segmented invertebrates comprising the phylum Tardigrada, characterized by bilateral symmetry, four pairs of unjointed legs, and a eutelic body (fixed number of body cells in mature adults of any one species). There are more than 700 known species (Ramel 2008).
Water bears are able to survive in extreme environments that would kill almost any other animal. They can survive temperatures close to absolute zero (Bertolani et al. 2004), temperatures as high as 151°C (303°F), one thousand times more radiation than any other animal (Horikawa 2006), nearly a decade without water, and can also survive in a vacuum like that found in space.
Tardigrades are small, bilaterally symmetrical, segmented animals, similar and probably related to the arthropods. The biggest adults may reach a body length of 1.5 millimeters and the smallest below 0.1 millimeters. Echiniscoides sigimunmde is the largest known tardigrade species and is found in European and Asian habitats (Ramel 2008). Freshly hatched larvae may be smaller than 0.05 millimeters.
Tardigrades have a body with four segments (not counting the head). They have eight legs, but they are not jointed as in arthropods. The feet have claws or toes. The cuticle contains chitin and is molted.
Tardigrades have a ventral nervous system with one ganglion per segment, and a multilobed brain. The body cavity is partially a coelom, with a true coelom near the gonads (coelomic pouch), but most of the body cavity is a hemocoel rather than a coelom. Tardigrades lack circulatory and respiratory systems (Ramel 2008). Their digestive system is a straight through gut with an anus (Ramel 2008). The pharynx is of a triradiate, muscular, sucking kind, armed with stylets.
Tardigrades are gonochoristic (either male or female), although in some species only females have been found, leading to the presumption that these species are parthenogenetic. Males and females are usually present, each with a single gonad. Tardigrades are oviparous.
Tardigrades are eutelic. Eutelic organisms have a fixed number of cells when they reach maturity, the exact number being constant for any one species. Development proceeds by cell division until maturity; further growth occurs via cell enlargement only. Some tardigrade species have as many as about 40,000 cells in each adult's body, others have far fewer (Seki and Toyoshima 1998; Kinchin 1994).
Tardigrades occur over the entire world, from the high Himalayas (above 6,000 meters), to the deep sea (below 4,000 meters) and from the polar regions to the equator. Most live in moist environments, often in environments subject to frequent drying and re-wetting (Ramel 2008). They are found on lichens and mosses, and in dunes, beaches, soil, and marine or freshwater sediments, where they may occur quite frequently (up to 25,000 animals per liter). Tardigrades often can be found by soaking a piece of moss in spring water (Goldsteing and Blaxter 2002).
Most tardigrades are phytophagous or bacteriophagous, but some are predatory (Lindahl 1999), such as Milnesium tardigradum and Macrobiotus hufelandii (Morgan 1977). Those feeding on plant material may feed on mosses and algae, while those that are carnivorous may feed on nematodes and rotifers (Ramel 2008).
Tardigrades were first described by Johann August Ephraim Goeze in 1773, and dubbed Kleiner Wasserbär, meaning "little water bear." The name Tardigrada, which means "slow walker," was given by an Italian scientist, Spallanzani, in 1777. However, it may be that Anton van Leeuwenhok was actually the first to see tardigrades, when on September 3, 1702, he performed an experiment using dried dust from the gutter on the roof of his house (Ramel 2008). Leeuwenhok added previously boiled water to this dust and was amazed to see living organisms come into being. It was in repeating this experiment, in 1777, that Spallanzani saw tardigrades, naming them from the Greek for slow and walk (Ramel 2008).
Tardigrades are the most hardy animals known. Scientists have reported their existence in hot springs, on top of the Himalayas, under layers of solid ice, and in ocean sediments. They are the only animals known that can survive being observed in a scanning electron microscope, which involves bombarding them with electrons while in a vacuum (Ramel 2008).
Tardigrades are one of the few groups of species that are capable of reversibly suspending their metabolism and going into a state of cryptobiosis. Several species regularly survive in a dehydrated state for nearly ten years. Depending on the environment, they may enter this state via anhydrobiosis (extreme desiccation), cryobiosis (decreased temperature), osmobiosis (in response to increased solute concentration in environment), or anoxybiosis (in situations lacking oxygen). Horikawa et al. (2006) report that almost all terrestrial tardigrades are able to enter an ametabolic state induced by dehydration (anhydrobiosis). While in this state, their metabolism lowers to less than 0.01 percent of what is normal and their water content can drop to one percent of normal. Their ability to remain desiccated for such a long period is largely dependent on the high levels of the non-reducing sugar trehalose, which protects their membranes.
While many species survive by converting themselves into this "tun" (pulling their legs in to give their body a cylindrical shape and then shutting down their metabolism), other species do not form a tun to survive extreme conditions, including deep sea species that survive pressures as great as 6,000 atmospheres (Ramel 2008).
Tardigrades have been known to withstand the following extremes:
Recent experiments conducted by Cai and Zabder have also shown that these water bears can undergo chemobiosis—a cryptobiotic response to high levels of environmental toxins. However, their results have yet to be verified (Franceschi 1948; Jönsson and Bertolani 2001).
Recent DNA and RNA sequencing data indicate that tardigrades are the sister group to the arthropods and Onychophora. These groups have been traditionally thought of as close relatives of the annelids, but newer schemes consider them Ecdysozoa, together with the roundworms (Nematoda) and several smaller phyla. The Ecdysozoa-concept resolves the problem of the nematode-like pharynx as well as some data from 18S-rRNA and HOX (homeobox) gene data, which indicate a relation to roundworms.
The minute sizes of tardigrades and their membranous integuments make their fossilization both difficult to detect and highly unlikely. The only known fossil specimens comprise some from mid-Cambrian deposits in Siberia and a few rare specimens from Cretaceous amber (Grimaldi and Engel 2005).
The Siberian tardigrades differ from living tardigrades in several ways. They have three pairs of legs rather than four; they have a simplified head morphology; and they have no posterior head appendages. It is considered that they probably represent a stem group of living tardigrades (Grimaldi and Engel 2005).
The rare specimens in Cretaceous amber comprise Milnesium swolenskyi, from New Jersey, the oldest, whose claws and mouthparts are indistinguishable from the living M. tartigradum; and two specimens from western Canada, some 15–20 million years younger than M. swolenskyi. Of the two latter, one has been given its own genus and family, Beorn leggi (the genus named by Cooper after the character Beorn from The Hobbit by J.R.R. Tolkien and the species named after his student, William M. Legg); however, it bears a strong resemblance to many living specimens in the family Hipsiblidae (Grimaldi and Engel 2005; Cooper 1964).
Aysheaia from the middle Cambrian Burgess shale might be related to tardigrades.
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