Water bears or tartigrades are about 500 species of tiny aquatic invertebrate animals in the phylum Tartigrada, including about 90 species in North America. Water bears have a very widespread distribution, occurring in moist habitats from the Arctic to the Antarctic and on mountains as high as 19,680 ft (6,000 m).
Water bears have roughly cylindrical dark-colored bodies with four body segments and four pairs of stumpy legs each tipped with tiny claws. The awkward, pawing locomotion of these animals was thought to vaguely resemble that of slow-moving bears and hence the common name of these creatures which was given in the nineteenth century. The mouthparts of water bears are adapted for piercing, and they have a muscular pharynx for sucking the juices of mosses, liverworts, and algae. Water bears do not have active circulatory or respiratory systems; materials move about within their tiny body by simple processes such as diffusion, while respiratory gases diffuse across the surface of the body.
Water bears are typically found in moist films on mosses, lichens, angiosperm plants with a rosette growth form, and plant litter. Water bears can utilize this micro-aquatic habitat because they are only 0.002-0.05 in (0.05-1.2 mm) long. Water bears are highly tolerant of the desiccation that frequently afflicts these sorts of habitats because they have an ability to encapsulate into a shrivelled, spherical state until moist conditions again return. Water bears may also be collected from debris or mud in more typical shallow-water habitats, but they are seldom abundant in ponds or lakes. Some species occur in sandy and pebbly marine habitats above the zone of most vigorous wave action.
Water bears have separate sexes, but males are quite uncommon. Usually males are only relatively abundant during the winter and spring. Some species only produce females, reproducing by a process known as parthenogenesis which does not require sex. There are four to 12 post-hatching developmental stages, each beginning with a molt of the cuticle.
Water bears are interesting little creatures which have fascinated zoologists ever since the discovery of microscopy first made these animals visible several centuries ago.
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