Tardigrades occur worldwide. Members of this phylum are often referred to as "water bears" because of their bear-like appearance and slow lumbering gait, though in Latin tardigrada means "tortoise-like movement". Tardigrades were first described by German zoologist Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Italian biologist Spallanzani in 1777. Tardigrades have been relatively neglected by invertebrate zoologists and so far about 800 species have been described from marine, freshwater and terrestrial habitats.
Tardigrades are cylindrical, segmented, microscopic animals with an uncalcified cuticle and extensions of the body wall that act as legs. They range in size from 50 millimeters (mm) to 200-250 mm, but some giants can reach 1,700 mm. Their body is indistinctly divided into five segments: a head segment, three body segments each bearing a pair of legs and a caudal segment with a fourth pair of legs that are directed posteriorly. Each leg ends in a claw which may or may not have extra processes. Some tardigrades have adhesive discs on the end of their claws for a better grip as they walk along the substrate. Claw shape is an important taxonomic tool in the classification of tardigrades.
The cuticle of tardigrades is thin and uncalcified which makes it highly permeable to water. The inner layer of the cuticle contains chitin to provide some rigidity and strength. The foregut and the hindgut are lined with cuticle, so they are shed when the tardigrade moults. Freshwater tardigrades are white or colorless, while those in terrestrial habitats may be brown, yellow, orange, pink, red, green, purple or black depending on their intestinal contents, granular bodies in the hemocoel, and pigmentation in the cuticle and the hypodermis. The cuticle may be smooth, sculpted with tubercles and spines, or even divided into armored plates. Differences in cuticular structures are an important distinguishing feature between the major groups of tardigrades.
The brain of tardigrades consists of a dorsal lobed ganglion and four ventral ganglia. Some tardigrades have light-sensitive eye spots and anterior cirri for chemoreception. Most tardigrades also have sensory bristles or spines around the head and ventral regions of the body.
Tardigrades exchange gases through their cuticle. The substances taken up by diffusion are often stored in storage cells that are found in the body fluid. Tardigrades do not have a circulatory system; the fluids are moved around the body cavity by the movement of the animal. Osmoregulation is accomplished by the Malpighian tubules located at the midgut-hindgut junction.
Some tardigrades have specialized excretory glands (the eutardigrades), but most rely on other mechanisms to get rid of waste. Some simple wastes are passed through the wall of the midgut and then excreted. Other wastes are stored as excretory granules in the cuticle, so when the animal moults, it rids itself of these wastes.
Most tardigrades reproduce parthenogenetically, but a few reproduce sexually. Some species are simultaneous hermaphrodites, which are able to fertilize themselves, but sequential hermaphroditism is more common. Both males and females have only one gonad, and it lies dorsal to the intestine. Fertilization can be accomplished several ways. In many species the males deposit sperm under the cuticle before the female moults. The female subsequently moults, laying her eggs in the shed cuticle and the eggs are fertilized by the sperm. Other species reproduce by copulation. In this case, the sperm fertilizes the eggs directly in the ovary. The most intrusive reproductive method found in tardigrades is traumatic insemination. Males penetrate the cuticle of the female and deposit sperm directly in her body cavity. The sperm then migrate to the ovary where they fertilize the eggs.
Female tardigrades lay 1-30 eggs at a time. Most aquatic species either leave their eggs in their shed cuticle or glue the eggs to a submerged object, but others will carry the eggs around for a time, or deposit the eggs in the empty exoskeletons of cladocerans, ostracods, or insects. The eggs may be smooth, but many have processes, pores or reticulations. Eggs hatch in 5 to 40 days and can be found year-round in most terrestrial habitats. Like most organisms with hard outer cuticles, tardigrades need to moult their cuticle in order to grow. Each moult takes 5 to 10 days, during which time the tardigrade is unable to eat because its mouth closes. The lining of the foregut and hindgut along with the claws and stylets are shed at the same time. The size of the tardigrade usually increases with each moult until they reach a maximum size, but they can decrease in size if there is a shortage of food. Tardigrades live from 3 to 30 months, but this time can be greatly extended due to periods of latency.
Tardigrades disperse as eggs, cysts or in the anhydrobiotic state. Because of their small size, they are blown by wind, carried with rain, floodwaters or melting snow. Larger animals also carry tardigrades with them in the sand or mud that clings to their body. Tardigrades are not successful at active dispersal because they require a film of water around them at all times in the active state.
Tardigrades occur in marine, freshwater and terrestrial habitats worldwide. Although most tardigrades live in terrestrial habitats, they are all aquatic because they need a layer of water around them in order to be active. They live in a wide variety of habitats including soils, leaf litter, moss, lichens, flowering plants, algae and even in hot springs. Some tardigrades live on the legs of isopods, on the gills of mussels or as parasites on the epidermis of barnacles or holothurians. They must live in areas with sufficient aeration because they respire across their cuticle and are sensitive to low oxygen levels. In moss, tardigrades can reach densities of 2,000,000 per square meter!
Tardigrades feed on plant or animal cells, bacteria, algae, decaying plant matter and some of them are carnivorous on small invertebrates. They feed by piercing the cell wall (or cell membrane) with their stylets and using their muscular pharynx to suck out the fluids inside the cell. The foregut and hindgut are lined with extensions of the cuticle. Digestion takes place in the midgut because it has only a single layer of epithelial cells and is not lined with cuticle.
When environmental conditions start to deteriorate, tardigrades enter a latent state until conditions improve. A latent state is a state in which metabolism, growth and reproduction are reduced or cease temporarily while resistance to environmental extremes (cold, heat, drought, chemicals, ionizing radiation) increases. Tardigrades in a latent state can survive temperatures up to 272°C, high vacuums, ionizing radiation, outer space, and long periods with no oxygen. One tardigrade was revived from its latent state after 120 years! Since aging ceases in the latent state, entering a latent state can greatly increase the life span of the animal. Tardigrades have five routes which they employ to enter latency: encystment, anoxybiosis, cryobiosis, osmobiosis and anhydrobiosis.
Encystment - A latent state in which resistant cysts are produced. Freshwater tardigrades in permanent water habitats enter this state when environmental conditions deteriorate. These cysts can't withstand high temperatures, but can survive a year without food.
Anoxybiosis - A latent state that is induced by low oxygen concentrations in the surrounding water. In this state tardigrades become immobile, transparent, rigid and extended (due to water absorption because of the loss of osmotic control). Recovery occurs following the addition of oxygen to the surrounding water.
Tuns are formed in the next three types of latency. The tardigrade shrivels up and excretes a cuticular envelope around itself. Single-walled tuns contain tardigrades with metabolisms lower than those in double-walled.
Cryobiosis - If cooled gradually, tardigrades form a cold-resistant tuns which allows the animal to survive being frozen.
Osmobiosis - Tun production induced by elevated osmotic pressure and decreased water availability. For example, a freshwater tardigrade placed in salt water will form a tun. Activity resumes when the tardigrade is returned to a freshwater environment.
Anhydrobiosis - Tun production in terrestrial tardigrades that is induced by evaporative water loss. If the tardigrade dehydrates too quickly, it will not survive. Tardigrades in this state are resistant to temperature extremes, radiation and chemicals. Anhydrobiosis allows the tardigrade to remain in a latent state for up to 120 years.