Tardigrade

Tardigrades are a phylum of eight-legged (tardigrade size ranging between 0.3 to 1.2mm) segmented micro-animals also referred to as water bears or moss piglets. Johann August Ephraim Goeze, a German zoologist, was the first one to identify them in 1773, naming them "little water bears." They were given the name Tardigrada by the Italian biologist Lazzaro Spallanzani in 1777, which indicates "slow steppers."

From mountaintops to the deep sea and mud volcanoes, as well as from tropical rainforests to the Antarctic, they've been discovered all over the biosphere.

Species of Tardigrade

Specific species of tardigrades are some of the most resilient animals recognized, enduring harsh environments including high temperatures, extreme pressures (respectively elevated/ low), air scarcity, radiation, dehydration, and malnutrition which would kill almost all other known types of existence.

Tardigrade information shows that they have undergone the trip to the farthest reaches of the universe. The phylum Tardigrada, which is part of its superphylum Ecdysozoa contains the animals which evolve by ecdysis including arthropods as well as nematodes. It has around 1,300 species.

The oldest true group members are identified from Cretaceous amber in North America, although they are basically modern forms, and thus probably have a much earlier origin, as they deviated from their closest living relatives over 500 million years ago throughout the Cambrian.

Habitat

Tardigrade information displays that they can be identified on lichens and mosses through dipping a fragment of moss in water. They can also be located in dunes and coasts, in particular, leaf litter, soil, and marine and freshwater silt, where they can be found in large numbers (up to 25,000 animals per litre). Barnacles are home to a kind of tardigrade named Echiniscoides wyethi.

Taxonomy

To help comprehend whether tardigrades contribute to other ecdysozoan lineages, scientists have performed molecular and morphological studies. There are two potential placements: tardigrades were more closely linked to Arthropoda and Onychophora. A typical finding of morphological studies is proof for the former; evidence for the others can be located in certain molecular analyses.

Recently, microRNA and articulated sequence tag studies have ruled out the latter hypothesis. The relationship of tardigrades and nematodes discovered in a variety of molecular studies tends to become a long-branch attraction artefact.

There are three potential trends of relationship inside the arthropod group (named Panarthropoda and consisting of Tardigrades, Onychophora, and Euarthropoda): tardigrades are sister to onychophora plus arthropods; onychophora is sister to tardigrades along with arthropods (the tactopoda hypothesis), and onychophora is sister to tardigrades.

According to new research, the panarthropoda group is monophyletic, and tardigrades are indeed closely related to Lobopodia (the arthropod and Onychophora lineage).

Owing to tardigrades' small size and membranous integuments, their fossilisation is hard to detect and extremely uncommon. The only identified fossils are from Siberian mid-Cambrian beds and a very unique Cretaceous amber specimen.

In some aspects, the Siberian tardigrade fossils vary among living tardigrades.

They possess three sets of legs instead of four, a simpler head morphology, and no posterior head appendages, though they have the same columnar cuticle construction as modern tardigrades. They are considered to be a stem category of living tardigrades, according to scientists.

Tardigrade Anatomy and Morphology

Morphology

Tardigrades feature four pairs of stubby legs and a barrel-shaped body. The majority tardigrade size lies between 0.3 and 0.5 mm (0.012 and 0.020 in) in length, with the largest species exceeding 1.2 mm (0.047 in). The head, three body sections with two pairs of legs each, as well as a caudal segment with a fourth set of legs make up the body. The feet possess four to eight claws each, whereas the legs have no joints.

There have been no respiratory organs, but the gaseous exchange is possible throughout the body. Three tubular glands are connected with the rectum in certain tardigrades; those might be excretory organs equivalent to arthropod Malpighian tubules, but specifics are uncertain. There are also no nephridia.

The stylets on the tardigrades' tubular mouth pierce the algae, plant cells, or small invertebrates upon whom they feed, producing body fluids or cellular components. A triradiate, sucking pharynx, muscular emerges from the mouth. Whenever the animal moults, the stylets are damaged, as well as a fresh pair is secreted from a set of glands one on each corner of the mouth.

The pharynx attaches to a small oesophagus, which further attaches to an intestine that extends the body length and is the primary site of digestion. The intestine attaches to an anus at the very end of the body through a small rectum. When certain animals moult, they only defecate once, leaving the faeces behind with the shed cuticle.

Anatomy

The cuticle is made up of chitin and protein and is shed on a daily basis. The first three sets of legs are directed downward all along sides and can be used for locomotion, whereas the fourth pair remains directed backwards on the very last section of the trunk and therefore is predominantly used for gripping the surface.

The number of cells throughout all mature tardigrades of the same genus is just the same. An adult in certain species has about as many as 40,000 cells, whereas others have much less.

The cavity of the body contains a haemocoel, however, the gonad seems to be the only location where a real coelom can be identified.

Tardigrade animal lack many Hox genes as well as a large portion of the body axis in the middle. This applies to the entire thorax as well as the abdomen in insects. But for the last set of legs, the entire body is composed of segments that are structurally similar to the head area in arthropods.

Reproduction

Even though some species appear parthenogenetic, males and females are typically found together, however, females are typically bigger and more popular. A single gonad is present, well above the intestine in each of these sexes. Males have two ducts that arise from the testes and open via a single pore in front of the anus.

Females, on the other hand, carry a single duct that opens somewhere above the anus or straight into the rectum, creating a cloaca.

Tardigrades became oviparous, which means they fertilise themselves. Mating takes place during the moult when the eggs are laid within the female's shed cuticle and afterwards coated with sperm.

Internal fertilisation occurs in some few species, with breeding taking place until the female's cuticle has completely shed. The eggs are generally left to grow within the shed cuticle, however, some species bind them to neighbouring substrates.

After just 14 days, the eggs hatch, and the young already have their complete set of adult cells. Hypertrophy (enhancement of single cells) instead of just cell division is used to achieve adult size. Tardigrades will moult up to 12 times in their lifetime.

Ecology and Life History

The majority of the tardigrade animals were bacteriophagous (bacteria eaters) or phytophagous (plant eaters) although few are carnivorous, consuming tiny tardigrade species.

Tardigrades display morphological features with a number of species that are categorised differently. Because of such a connection, biologists, therefore, have a difficult time finding confirmation within tardigrade organisms.

These species seem to be the most likely to be related to the roots of arthropods. Tardigrade fossils have been found in North America dating back to the Cretaceous period. Tardigrades became cosmopolitan animals that can be found in all parts of the globe. Tardigrade eggs and cysts have become so robust that they can be borne for lengthy ranges on the foot of other species.

All five mass extinctions have been prevented by tardigrades. It has endowed them with a number of survival skills, such as the ability to withstand conditions that would kill most such other species.

Tardigrades have a lifespan ranging from 3–4 months for certain species to 2 years for others, and that doesn't include time spent in dormant states.

Physiology

Scientists have found tardigrades throughout hot springs, on peaks of the Himalaya (6,000 m; 20,000 ft, from sea level), throughout the deep sea (-4,000 m; -13,000 ft), and in the earth's oceans from the polar regions to the equator. Many organisms thrive in cooler habitats including lakes, rivers, and meadows, whereas others thrive in stone walls and ceilings. Tardigrades thrive in wet conditions, but they can survive wherever there is enough moisture to keep them alive.

Also, maximum global mass extinction events caused by astrophysical phenomena like gamma-ray bursts or massive meteorite impacts were assumed to be survivable for tardigrades.

A few of them can endure temperatures as low as 1 K (near absolute zero), whereas others endure temperatures as high as 420 K for many minutes, pressures six times greater than any of those encountered in the deepest ocean trenches, ionising radiation with doses hundreds of thousands of times greater than those of the lethal dose for humans, as well as the vacuum of outer space.

They aren't known as extremophiles because they haven't evolved to take advantage of these conditions, just to survive them. This implies that the more they are subjected to harsh environments, the more likely they seem to die, while real extremophiles survive in physically or geochemically harsh habitats that might destroy any other species.

Evolutionary History

There are several pieces of evidence suggesting tardigrades are descended from some kind of larger ancestor, most likely a lobopodian and possibly resembling Aysheaia, that several studies place near to the tardigrade lineage's divergence. Another idea is that tactopoda is descended from a group that includes Dinocaridids and Opabinia.

Milnesium swolenskyi, a member of the evolving genus Milnesium, was discovered in Late Cretaceous(Turonian) amber in New Jersey and is believed to be about 90 million years old. Some other fossils, Beorn leggi, were found in the Late Campanian period (72 million years ago). Canadian amber was initially identified as its own family, but it was later proposed that it belonged to the Hypsibiidae. The same accumulation also yielded an indeterminate heterotardigrade.

Tardigrade Facts

Below mentioned are some of the Tardigrade facts:-

• They're relatively insignificant, but only barely.

• They are Phylum unto themselves- Tardigrades are classified as a phylum, that is yet another taxonomic rank under the kingdom. Some phyla throughout the animal kingdom would include arthropods (that mostly comprises all arachnids, insects, and crustaceans) as well as vertebrates.

• They can go for years without eating or drinking.- The uncanny endurance of tardigrades is perhaps their most well-known attribute. Although they are not invincible and their invulnerability varies, certain tardigrades may withstand a number of seemingly insurmountable situations. To cope with environmental stress, tardigrades use a mechanism known as cryptobiosis to place their metabolism on hold.