Tardigrada

(invertebrate zoology) A class of microscopic, bilaterally symmetrical invertebrates in the subphylum Malacopoda; the body consists of an anterior prostomium and five segments surrounded by a soft, nonchitinous cuticle, with four pairs of ventrolateral legs.

A class of microscopic, bilaterally symmetrical invertebrates which are generally less than 1 mm in length. About 400 species are known. Commonly called water bears, bear animalcules, or urslets, they are worldwide in distribution and are found in all habitats.

The tardigrade body consists of an anterior prostomium and five segments. The mouth is located in the prostomium in a centroterminal position. A soft, nonchitinous cuticle surrounds the body and lines the fore- and hindgut. The cuticle may be smooth or sculptured and forms innervated cephalic appendages and spines on the trunk and legs. Four pairs of ventrolateral legs arise from the trunk and terminate in claws or other modified structures. The digestive tract is tubular and more or less lobed due to the presence of diverticular dilations. In the heterotardigrades, separate anal and genital openings occur, while in the eutardigrades there is a single anogenital opening, the cloaca. The sexes are separate and the gonads are unpaired dorsal sacs with paired gonoducts in the male and in theory also in the female. Food storage cells float in the spacious body cavity, the coelom, which lacks a parietal or visceral peritoneum in the adult. Circulatory and respiratory structures are lacking. These animals exhibit the phenomenon known as cell constancy. The number of epidermal cells is the same in all species of a genus.

Tardigrades lay eggs and development is direct. Embryonic development lasts 3–40 days, varying according to the species and surrounding temperature. During their active life of 18 months, tardigrades molt about 12 times. They are unable to feed in the 5–10 days of molting; the buccal cavity requires at least 5 days for renewal.

Tardigrada live as active forms, without encystment, only when surrounded by a pellicle of water. They are mainly herbivorous, and feed by piercing the wall of plant cells with their stylets. When the surrounding medium dries up, most tardigrades continue to live as inactive or anabiotic barrel-shaped structures called cysts without any protective cover. Desiccation begins when there is a loss of oxygen from the water. The animal responds by contraction and loss of body water. Dried eggs also survive. Moistened animals usually revive, but anabiosis and revival cannot be repeated indefinitely.

Most species are widely distributed. Dissemination may be by wind, birds, and certain terrestrial animals which transport tardigrade eggs and barrels. The tropics have few species, and Scutechiniscidae are rare on the Antarctic continent. Most tardigrades are terrestrial. They are found among lichens, liverworts, densely growing soft-leaved mosses, and also in rather hard-leaved Pottiacea and Grimmiacea. See also Cell constancy; Eutardigrada; Heterotardigrada; Polychaeta.