Phylum Heterotardigrada, Eutardigrada, and Mesotardigrada. Class Heterotardigrada includes order Arthrotardigrada, which are mostly marine, as well as order Echiniscoidea, which are terrestrial. Class Eutardigrada includes the primarily terrestrial or freshwater order Parachela and the unarmored, terrestrial order Apochela. Class Mesotardigrada only includes one order, Thermozodia; their existence is questionable, due to the destruction of the type locality of its single species, Thermozodium esakii, which had been found in a Japanese hot spring, and the loss of the original types themselves. There are a variety of morphological characters used to distinguish classes of tardigrades, including presence or absence of cephalic papillae, a cloaca, or malpighian tubules. They are known from all intertidal and subtidal zones, at temperatures ranging from 149°C to -272°C, and from the Arctic to the Antarctic. They can survive extreme environmental conditions, including anoxic conditions, vacuums, and ionizing radiation. (Brusca and Brusca, 2003; Horikawa, 2008; Nelson, 2002; Ruppert, et al., 2004; Zhang, 2011)is comprised of over 1,000 species placed into three classes:
These animals are found in semi-aquatic habitats, such as water films and leaf litter, as well as deep and shallow freshwater and marine habitats. They are commonly associated with bryophytes (mosses). Their ability to enter anabiotic dormancy enables them to survive extreme environmental conditions. Active specimens have been collected from marine intertidal and subtidal zones at depths of up to 4,690 m, and from lakes up to 150 m deep. They have also been collected in the Himalaya Mountains at altitudes over 6,000 m. Although as a group, tardigrades are broadly cosmopolitan, some species are found only under specific environmental conditions. (Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)
Tardigrades have both smooth and cross-striated muscles, their body walls are made of muscle bands extending between subcuticular attachment points. These bands are typically comprised of a single, or a few muscle cells. Tardigrades move by using their legs, controlled by independent sets of muscles, in a stepping motion, although at least one marine species is known to swim, expanding its cuticle to resemble the bell of a jellyfish. Their nervous systems are metamerous, with a large, dorsal, lobed cerebral ganglion connected to a subesophageal ganglion, subsequently attached to a pair of ventral nerve cords extending posteriorly and connecting a chain of four ganglia, which control their four pairs of legs. Their bodies are covered with sensory bristles or spines, similar to setae, most thickly in their anterior and ventral regions, and their bodies often terminate with long sensory cirri, often known as clava, which are likely chemoreceptive. Two eyespots, made up of five cells, one of which is light sensitive and pigmented, are often, though not always, present. (Brusca and Brusca, 2003; Nelson, 2002; Ruppert, et al., 2004)
These animals have greatly reduced coeloms, so their body cavities function mainly as hemocoels. They have a pair of oral stylets, mouth placement depends on diet: detritivorous tardigrades have ventral mouths while carnivorous and omnivorous tardigrades have terminal mouths. The mouth is connected to a muscular sucking pharynx flanked by a pair of salivary glands. In some species, there are also chitinous rods in the pharynx, which provide support and possibly a masticating action. The pharynx empties into the esophagus, which is connected to a large midgut, where digestion and absorption take place. The midgut opens into a short hindgut, leading to a terminal anus. In freshwater tardigrades, there are three or four large glands, comprised of three to nine cells each, known as malpighian tubules. The exact function of these structures is unknown but may be related to osmoregulation. Tardigrades have no blood vessels or other structures for gas exchange, relying on their body walls and cavities for this function. (Brusca and Brusca, 2003; Nelson, 2002; Ruppert, et al., 2004)
In egg-laying species, females lay anywhere from 1 to 30 fertilized eggs at a time. In aquatic species, eggs are laid in the female’s shed cuticle or glued to a nearby object; if laid within a cuticle, the eggs are most often smooth, while those laid outside the cuticle often have exterior decoration. Eggs of terrestrial species have thick shells that protect the developing embryos from potential desiccation. It is possible for embryos, as well as adults, to enter diapause or cryptobiosis at any point during development, dictated by environmental conditions. Embryonic development is direct and rapid, with holoblastic cleavage. Development may be completed within 14 days, although it may be delayed for as many as 90 days. Young use their stylets to break out of their shells. Upon hatching, they often have fewer claws, spines, and cirri than adults, and lack coloration. These animals are eutelic, having a fixed number of cells in their bodies from birth; as the animal grows, cells grow in size rather than increasing in number. Growth occurs through a series of molts; a molt usually takes 5 to 10 days to complete, occurring more rapidly in early stages of growth, and may be associated with egg laying or defecation. While molting, tardigrades shed their entire cuticles, including their gut linings, and cannot feed. (Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)
Fertilization may be direct, with the male depositing sperm into the female's seminal receptacle or body cavity, or indirect. Indirect fertilization occurs when the male deposits sperm under the female’s cuticle; when she molts, she lays fertilized eggs into her shed cuticle. Courtship behavior has been observed in a few species of tardigrades: a male will stroke a female with his cirri, stimulating her to lay eggs on a grain of sand, and then he spreads his sperm over the eggs. In some species, there are no males known, others have dwarf males. (Bertolani, 2001; Brusca and Brusca, 2003)
Halobiotus crispae) living in particularly extreme conditions exhibit cyclomorphosis, with different morphs in the winter and summer seasons. Typically winter forms hibernate and are sexually immature and gregarious; individuals gather to protect themselves against cold temperatures. In those populations, all individuals become sexually mature at the same time, in the beginning of the summer. (Bertolani, 2001; Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)reproduce year-round, assuming conditions are favorable. Females lay multiple clutches throughout their life, while males either reproduce once, or several times, depending on the species. If environmental conditions are unfavorable, tardigrades may enter a period of cryptobiosis or diapause. Although sexual reproduction is common, some are parthenogenic or hermaphroditic. Development may be completed in as few as 14 days, although it may be delayed to 30 to 90 days and, in some species, temperature may be a key factor in time to hatching and size at hatching. Some marine tardigrades (such as
Excluding cryptobiotic periods, Echiniscoides) alternates between active and inactive stages ever six months; this may increase their lifespan by decades. (Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)generally live 3 to 30 months. The aging process may be halted during cryptobiosis. At least one marine genus (
In some environments, Milnesium tardigradum and Ramazzottius oberhaeuseri, in particular) may carry a symphoriont protozoan species (Pyxidium tardigradum). Many tardigrade species living in mossy environments carry fungal parasites. (Drechsler, 1951; Glime, 2010; Sanchez-Moreno, et al., 2008; Thorp and Covich, 2010)may be a primary consumer of nematodes, greatly affecting population sizes. Some species (
Beyond scientific research and a small hobbyist following, there are no positive effects of (Brusca and Brusca, 2003)on humans.
There are no known adverse effects of (Brusca and Brusca, 2003)on humans.
As a cosmopolitan phylum, there is little concern that (Glime, 2010)will become endangered, and currently, there are no conservation initiatives focused on any specific tardigrade species. There is evidence, however, that pollution may adversely affect their populations, as poor air quality, acid rain, and concentrations of heavy metals in bryophyte habitats have led to decreases in some populations.
Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
lives on Antarctica, the southernmost continent which sits astride the southern pole.
the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).
living in landscapes dominated by human agriculture.
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.
areas with salty water, usually in coastal marshes and estuaries.
an animal that mainly eats meat
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
active at dawn and dusk
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
an animal that mainly eats decomposed plants and/or animals
a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.
At about the time a female gives birth (e.g. in most kangaroo species), she also becomes receptive and mates. Embryos produced at this mating develop only as far as a hollow ball of cells (the blastocyst) and then become quiescent, entering a state of suspended animation or embryonic diapause. The hormonal signal (prolactin) which blocks further development of the blastocyst is produced in response to the sucking stimulus from the young in the pouch. When sucking decreases as the young begins to eat other food and to leave the pouch, or if the young is lost from the pouch, the quiescent blastocyst resumes development, the embryo is born, and the cycle begins again. (Macdonald 1984)
an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.
parental care is carried out by females
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
mainly lives in water that is not salty.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.
Found in northern North America and northern Europe or Asia.
fertilization takes place within the female's body