Tardigrades, also known as water bears or moss piglets, are eight-legged microscopic animals that constitute the phylum Tardigrada. They are one of the most resilient creatures on this planet, having stood the test of time and survived all five major mass extinctions due to their remarkable adaptability to their surroundings. They are found in almost every habitat, ranging from scorching deserts to freezing glaciers. Though they usually feed on the fluids of plant and animal cells, tardigrades can go without food and water for up to thirty years.
The phylum Tardigrada, which is part of the superphylum Ecdysozoa, includes around 1,300 known species. All ecdysozoans are characterized by their growth through ecdysis, the process of shedding their exoskeleton.
According to the World List of Tardigrada, these animals are currently categorized into two recognized classes: Eutardigrada and Heterotardigrada.
While the largest tardigrades measure around 0.079 in (2 mm), the smallest of them are barely 0.0039 in (0.1 mm) long. In contrast, tardigrade hatchlings are comparable to the size of pollen, measuring only 0.0020 in (0.05 mm).
Due to their tiny size, these animals are commonly studied under a microscope.
These animals are short, stout, and barrel-shaped. Their bodies comprise a head and four segments, with each segment possessing a pair of unjointed legs.
The first three pairs of legs point downward and are used for locomotion, while the fourth pair points backward to help grasp onto surfaces. These legs culminate in four to eight claws (also called suction disks).
A thin, uncalcified cuticle covers the entire body. It is secreted by the epidermal layer beneath it. It is made of seven layers of protein and chitin and is periodically shed during molting.
Tardigrades have a body cavity called a hemocoel, filled with hemolymph, a fluid that carries oxygen throughout their bodies. They lack distinct circulatory and respiratory systems, so the hemolymph functions as both.
Their digestive system includes a tubular mouth, a muscular sucking pharynx, a short esophagus, and an intestine that occupies most of their body length. The mouth is equipped with sharp stylets, which are used to pierce plant and animal cells but are lost during molting.
Three tubular glands are associated with the rectum, which possibly has an excretory function and is assumed to be homologous to the Malpighian tubules found in arthropods.
The nervous system consists of a bilaterally symmetrical brain located dorsally, which accounts for about 1% of the total body volume. It connects to four segmental ganglia and is linked to a large ganglion beneath the esophagus. From this ganglion, a double ventral nerve cord extends throughout the body. This cord comprises one ganglion per segment, each producing lateral nerve fibers that innervate the limbs.
Many tardigrades have simple rhabdomeric eyes, or pigment-cup ocelli, as well as multiple sensory bristles (resembling setae) distributed across their bodies.
Both male and female tardigrades have a single gonad located above the intestine. In males, two ducts lead from the testis to a single opening in front of the anus. In females, a single duct opens either above the anus or into the rectum, followed by a cloaca.
These creatures were first named Kleiner Wasserbär (meaning little water bear) by German zoologist Johann August Ephraim Goeze, referring to their bear-like gait. Later, Italian biologist Lazzaro Spallanzani coined the term Tardigradum for the phylum, which literally translates to ‘slow walkers.’
Though referred to as bears, morphological studies suggest they are more closely related to arthropods or velvet worms. However, recent genomic analyses have revealed that they are more close to nematodes in evolution.
Initially, tardigrades were classified under three classes: Eutardigrada, Heterotardigrada, and Mesotardigrada. Those with naked or smooth cuticles (lacking plates) were placed under Eutardigrada, while others with plated cuticles were grouped under Heterotardigrada. The third class, Mesotardigrada, was monotypic, comprising the species Thermozodium esakii. However, all members of this species were found dead in an earthquake, and thus, no living specimen is currently found. Ever since, due to a lack of scientific evidence, scientists have been doubtful about considering Mesotardigrada as a valid class.
Since tardigrades are extremely minute and have a membranous integument, their fossil records are scarce. Their only known fossil specimens date back to the mid-Cambrian and Cretaceous Periods and were recovered from Siberia and North America. Further research revealed that these animals probably evolved from a large ancestor, such as a lobopodian (phylum Lobopodia), like Aysheaia.
Earliest tardigrades varied from modern forms in multiple ways. For instance, they had three pairs of legs instead of four and had simpler heads. Additionally, they had columnar cuticles covering their bodies.
Although the earliest known tardigrades, recovered from North America, date back to the Cretaceous Period (around 145 to 66 million years ago), they are believed to have originated more than 500 million years ago during the Cambrian Period.
Milnesium swolenskyi, the oldest known modern tardigrade, belongs to the Late Cretaceous Period, with its remains discovered from amber in New Jersey. Beorn leggi, another fossil species, was recovered from Canadian amber dating back to the Late Campanian Age (about 72 million years ago). Also, Paradoryphoribius, the youngest known fossil tardigrade genus, was found in amber, dating back to about 16 million years ago.
These animals are cosmopolitan, inhabiting nearly every type of environment, from lakes, ponds, and meadows to hot springs, glacial holes, solid ice layers, and seas at depths of around 13,000 ft. They have even been found in the Himalayas at altitudes of nearly 20,000 ft.
They also live on mosses and lichens growing on stone walls and roofs. One species, Echiniscoides wyethi, inhabits the barnacles. Additional habitats include dunes, coastal areas, soil, leaf litter, and both marine and freshwater sediments. Only 62 species of tardigrades live exclusively in freshwater habitats.
They are omnivores that feed on algae and aquatic plants (phytophagous), bacteria (bacteriophagous), fungi, and tiny invertebrates. It is the fluid of the plant and animal cells that they primarily consume. Moreover, some tardigrades, like Tetrakentron synaptae, are parasitic on other invertebrates, like sponges.
The lifespan of these animals ranges somewhere between three months to two years, depending on the species.
Although most tardigrades reproduce sexually, some, like Milnesium tardigradum and Macrobiotus dispar, multiply asexually through parthenogenesis. They also engage in courtship before mating, with up to nine males aggregating around a female.
With a few exceptions, tardigrades fertilize externally and are oviparous or egg-laying. During molting, the female releases her eggs in the shed cuticle, which are then fertilized by sperm from the male. Usually, the females leave their eggs to develop in the cuticle but may sometimes attach them to some nearby substrate. In terrestrial species, the eggs have thick shells that prevent desiccation in dry environments.
The embryo develops rapidly through holoblastic cleavage, becoming fully mature in about 14 days, though development may be delayed by up to 90 days under certain conditions. Once fully developed, the young use their stylets to break open the eggshell. Newborns lack body coloration and generally have fewer claws compared to adults.
The young tardigrades are eutelic, which means they have a fixed number of body cells that remain constant throughout their life. Their growth only occurs through the enlargement of existing cells (hypertrophy). They continue to undergo a series of molts (up to 12), with each molt taking 5 to 10 days to complete.
They are usually preyed upon by amoebas, nematodes, snails, mites, spiders, insects (including their larvae), and other tardigrades.
In extreme environmental conditions, tardigrades enter a state of cryptobiosis or anabiosis, drastically reducing their metabolism to less than 0.01% of the normal rate. They also decrease their water content to around 1% of their usual level. These adaptations allow them to survive without food or water for over 30 years. When conditions become favorable, they rehydrate themselves and continue to forage and reproduce. When in a cryptobiotic state, tardigrades are called ‘tuns.’ Anhydrobiosis is the most common form of cryptobiosis in tardigrades, where they survive extreme dehydration by contracting into smaller forms and producing a non-reducing disaccharide sugar, trehalose, that helps to recover internal water levels and protects the cells from desiccation.
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