Tardigrade- Definition, Habitat, Morphology, Physiology, Interesting facts

Definition

Tardigrade, or familiarly known as water bears or moss piglets, is a group of animals that are segmented with eight legs and an elongated plum body.

• These are near micro-organisms that are known to survive in all kinds of destructive environments. They have even survived the exposure to the outer space.
• These animals were first discovered in 1773 by a German zoologist Johann August Ephraim Goeze.
• They were named, however, later by the Italian biologist Lazzaro Spallanzani as ‘Tardigrade’ in 1777.

Figure 1: With fluorescent dye, biologist Tagide deCarvalho beautifully illuminated the insides of a tardigrade. Image Source: Tagide deCarvalho (Smithsonian Magazine)
Figure 2: SEM-images of Milnesium tardigradum. Image Source: PLOS ONE.

Classification

Tardigrades are animals that are tiny free-living invertebrates. These are known to be close relatives of arthropods like insects and crustaceans. The following is the taxonomical classification of Tardigrade:

Kingdom: Animalia

Subkingdom: Bilateria

Infrakingdom: Protostomia

Superphylum: Ecdysozoa

Phylum: Tardigrade

The animals in this phylum are further divided into various classes, orders, and families. Further classification of species in this phylum is expressed in the form of a table below:

Habitat and Diversity

• Tardigrades are found everywhere ranging from mountain tops to deep sea and even mud volcanoes.
• These are highly resistant animals capable of surviving extreme conditions like extreme temperature, pressure, radiation, dehydration, air deprivation, and even starvation.
• These are known to survive and even reproduce in outer space and are assumed to survive even an apocalypse.
• In many cases, they undergo a death-like state called cryptobiosis for survival in extreme conditions.
• About 1300 species of tardigrades are found worldwide.
• Freshwater mosses and lichens are the preferred habitats of tardigrades.
• These are considered aquatic as they require a thin layer of water around their bodies to prevent dehydration. However, only a few of them are genuinely aquatic, and most are limnoterrestrial (requiring periods of immersion and desiccation).
• Some tardigrade, namely, Echiniscoides wyethi, might survive on the surface of crustaceans.

Anatomy and Morphology

• Tardigrades are short and plump barrel-shaped organisms with four pairs of limbs each ending in four to eight claws or suction disks.
• Their size ranges from 0.05 mm to 1.2 mm in length, and thus these are microscopic organisms.
• These are covered with a tough cuticle, which is similar to the exoskeleton of insects and other arthropods.
• Tardigrade, like other insects, shed their cuticle during growth.
• The body of a tardigrade is divided into two parts; the head region and a compact body with fused segments.
• The head of the organism is provided with a tubular mouth that is surrounded by stylets used to pierce plant cells, algae, and other living beings on which they feed.
• The stylets are lost as the organism moults, and a new pair is secreted by glands found on either side of the mouth.
• Inside the mouth is a triradiate, muscular pharynx that opens into a short tubular esophagus. It continues down to the intestine that occupies most of the length of the organism.
• All Tardigrade have a buccopharyngeal apparatus (sucking device) that with the claws is used to differentiate different species of tardigrades.
• The body of a tardigrade is divided into four segments where the first three segments, each with a pair of legs and a last caudal segment with the fourth pair.
• The first three pairs of legs of Tardigrade are directed downwards and are used for locomotion whereas the fourth pair is directed backward and is used for grasping the surface.
• All the members of the same species of tardigrades have an equal number of cells, whereas different species might have a different number of cells.
• The body cavity of a tardigrade is a hemocoel, which is filled with fluid that transports oxygen and water to different parts of the body.
• They don’t have specialized organs for respiration or circulation.
• The brain of tardigrades is bilaterally symmetrical with most having paired clusters of neurons. The brain is attached to a large ganglion below the esophagus which gives out a double ventral cord that runs throughout the body.
• The cord contains ganglions on each segment of the body from which, lateral nerves are protruded towards each limb.
• Many tardigrades possess a pair of compound cup-shaped eyes and sensory bristles throughout the head and the body.

Lifespan

• The lifespan of most Tardigrade is 2-4 months while some might live up to 2 years.
• However, under extreme conditions, tardigrades change into their dormant forms by reducing their metabolic rates and survive for years or even decades.

Life Cycle (Development) and Reproduction

• Tardigrades are oviparous, and depending on the species, they might reproduce either sexually or asexually.
• In most species, fertilization is external. The female Tardigrade (which is slightly larger than the male) sheds the cuticle and lays eggs inside the cuticle to be fertilized by the male.
• In other, however, the fertilization is external where the male deposits sperm inside the cuticle of the female
• The eggs can take from 14 to 40 days to hatch, and the young Tardigrade develops all adult parts inside the egg. Some eggs might take 90 days to hatch if the eggs are present in the desiccated condition.
• Further growth of the young ones occurs by cell division and multiple moulting. Tardigrades might moult up to 12 times in their lifetime.

Physiology and Adaptation

• Tardigrades are one of the most resilient organisms on earth. Some species of tardigrades are found to survive extremely low temperatures (up to 1K) while some have survived an extremely high temperature of 420K.
• They can survive pressure almost six times that on the deepest ocean level and ionizing radiation a hundred times more lethal than the dose lethal to humans.
• However, they are not considered extremophilic as they cannot adapt but can just endure such environments. This means that they can withstand these environments for a certain period of time, but on more prolonged exposure, they might die.
• The essential physiological activity of these organisms that allow them to endure such extreme conditions is the process called cryptobiosis.
• By this process, the organisms suspend their metabolic activities to less than 0.01% of their normal rate and reduce their water content to 1% of normal.
• This stage is termed the ‘tun’ stage where they remove almost all water, retract their heads and limbs and roll up into the shape of a lifeless ball.
• In this stage, tardigrades can survive without food and water for about 30 years, only to rehydrate and reproduce later under appropriate conditions.
• The ability of these organisms to survive for such a long period of time is thought to be due to the presence of high levels of non-reducing sugar trehalose which protects their membrane.
• Some recent studies have indicated the presence of some special proteins that serve a similar purpose to those sugars.
• The protein is termed Dsup, short for damage suppressor, which protects the DNA from being destroyed by things like radiation.

Uses/ Applications

The ability of tardigrades to survive extreme temperature opens doors for a number of advanced applications of these organisms. Some of the possible application and uses of tardigrades are :

1. Tardigrades can be used as pioneer species to inhabit new ecosystems. Once tardigrades inhabit these ecosystems, other invertebrates might also move towards them along with the predators of tardigrades.
2. The proteins of tardigrades that protect them against such conditions can be used in crops and vegetation so that they can survive severe, long-lasting droughts.
3. Similarly, the use of these proteins in medication might allow the medicines to be store under room temperature instead of having to be chilled. This process will enable the transport of various medications and vaccines to remote areas that might not have the facility of freezers.
4. Because they can withstand the conditions of outer space, tardigrades are considered as the model organisms for space research.
5. Tardigrades can be used in studies related to extreme environments and extreme physical conditions.

Interesting facts about Tardigrades

• Tardigrades are also called ‘giant heads’ because all segments of the organism except the caudal region are found to analogous to the head region of arthropods.
• Tardigrades can swim as their multiple legs propel water to reach for food.
• In 2007, thousands of tardigrades were sent to the outer space attached to the satellites. After their return, many were observed to survive while female tardigrades were found to lay eggs and produce healthy young ones.
• It is estimated that tardigrades have been around for 600 million years. They are older than dinosaurs that first appeared 230 million years ago.
• They are capable of repairing their DNA when it is damaged by radiation.
• About 17.5% of the genes of tardigrades come from non-animal life, like plants, bacteria, and viruses.
• Because tardigrades are capable of surviving in the outer space, they are assumed to survive in other planets or even the moon.
• Some tardigrades are capable of reproducing by themselves through asexual means.

Video: Tardigrades Are the Toughest Animal on Earth that can Survive Space and Volcanoes (Video By: The Dodo)

References

1. Weronika, E., & Łukasz, K. (2017). Tardigrades in Space Research – Past and Future. Origins of life and evolution of the biosphere : the journal of the International Society for the Study of the Origin of Life, 47(4), 545–553. https://doi.org/10.1007/s11084-016-9522-1
2. A Family Level Analysis of Tardigrade Phylogeny – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Higher-level-Taxonomy-of-the-Tardigrada_tbl1_225792545 [accessed 26 Jun, 2020]

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