Introduction
You might’ve encountered them on the internet, in your favorite sci-fi movie or TV show, on the news or even in real life! Tardigrades, also called water bears or moss-piglets, are without a doubt the international super stars of the micro world. They’re mostly famous because they evolved and adapted to survive extreme environmental conditions that are generally a death sentence to other animals. Among these adaptations we find resisting dehydration by forming a quiescent tun, resisting sub-zero temperatures down to -196 °C in their hydrated form and temperatures from -273°C to 100°C in their dehydrated form. They’re also able to survive enormous amount of radiations, high pressures, low oxygen concentrations and even exposure to the vacuum of space. It has been shown that water bears are among the most radiation-tolerant animals on this planet.
All of these adaptations allowed Tardigrades to inhabit every micro environment on Earth; from Arctic to Antarctic, deserts to tundra, mountains and forests, grasslands and valleys, ponds and lakes, they can even be found in the deep sea and most likely in your own garden! Since they usually measure between 50 microns to 1,2 millimeters, using a microscope to observe those from your garden would be necessary.
While some moss-piglets are terrestrial, and others live underwater, they’re mostly considered as aquatic animals since they require a fine film of water surrounding their body to be active and thus, to be able to grow, feed and reproduce.
Even though these microscopic multicellular animals are extremely resilient when dormant, they also can be pretty vulnerable creatures. They’ve got several predators including insect larvae, other tardigrades, nematode worms, rotifers and oligochaete worms, which is a group of worms that includes earth worms. However, the roles can be reversed as water bears also prey on rotifers and nematodes. Although most species prefer to feed on plant material such as filamentous or unicellular algae and different types of mosses, some other species prefer bacteria, fungi, detritus and small unicellular animals. Water bears can also be victims of fungal parasitic infections, which mostly occurs in moist habitats and when cultured in laboratories.
Anatomy and physiology
As shown in the videos, water bears have a head, a segmented body plan with four pairs of legs and generally four claws per leg. However, some species can possess up to 13 claws per leg! They also come in all types of shapes and colours; marine and freshwater species are mostly transparent or white while terrestrial species can be found in white, yellow, green, red, orange, brown and black. These colours are usually due to pigmentation of the epidermis and cuticle but can also be a result of food that has been previously ingested, like this bright green colour appearing after a generous algae meal.
Even if Tardigrades are invertebrates, they possess a lot of organ systems that more evolved animals, like us, possess as well. Their nervous system consists of a small brain with three lobes, which occupies the majority of the head region, and several neural ganglia along the body. Most tardigrades have a pair of primitive eyes near the brain acting as photoreceptors with pigment granules, which are used to detect environmental light sources. Their muscular system is complex and consists of several fibers; dorsal and ventral longitudinal muscles and transverse muscles that are used for different activity such as walking, eating and laying eggs.
They have a digestive system consisting of a telescopic piglet mouth, a buccal tube and stylets that are V-shaped, which water bears use to pierce animal or plant material and proceed to suck the fluid that is being released, after what they chew the cells content with their muscular pharynx. They also possess a stomach, some intestines, a rectum and an anus.
Despite being really small, moss-piglets are composed of about 1000 cells which will get bigger as the individual matures. Although, being as small as them also means that specialized systems such as respiratory and circulatory systems are not needed. Instead of having lungs to breathe, respiration occurs through the cuticle and oxygen circulation is made possible by the movement of fluids and storage cells in the body cavity. Storage cells look like small bubbles moving around the body and play a major role in nutrient transport and storage of lipids, sugars and pigments. Similar to humans and other mammals, water bears are able to store energy and use it when necessary. For example, stored energy can be used for reproduction purposes, like when development and maturation of eggs has come.
Like Arthropods, an important group of invertebrate animals that includes insects and crustaceans, Tardigrades possess a protective cuticle, or exoskeleton, made of chitin that molts when individuals get bigger. This cuticle is secreted by the epidermis of water bears and will shed multiple times in their life in order for cells to become bigger and bigger and thus, for tardigrades to develop and become mature.
Reproduction
Tardigrades either reproduce sexually or asexually but in both cases, they need to conceive eggs.
Some eggs need to be fertilized, while some don’t and there are even eggs that can be fertilized by the same individual that produced them, such as in hermaphrodite species. Although, in the species from these videos, which is Hypsibius dujardini, populations of tardigrades are mainly females that reproduce asexually by parthenogenesis, which means that baby tardigrades develop from unfertilized eggs.
In the majority of species, there are males and females that mate together for eggs or oocytes to be fertilized, a bit like us! When reproducing, fertilization can either be internal, whereas the male directly deposits its sperm inside the female’s reproductive tracts or it can be external; male tardigrades have to deposit their sperm into the female’s old cuticle containing eggs to fertilize them.
Where to find them?
If you love to go on special quests to collect samples, and wish to find some wild Tardigrades, these tricks are for you!
The easiest way to find some Tardigrades is by collecting, by hand, mosses growing on various substrates. You can find mosses on tree barks, rocks, soil, dead wood, house rooftops and walls. I’ve personally found my first wild water bear from a sidewalk moss sample I took near my apartment! You can also collect lichens growing on tree barks if you find some.
When you’ve got your hands on fresh or dry moss, try to remove the majority of the soil underneath and put it in a small container. You’ll then need to submerge the moss with tap water and put it in the refrigerator for a minimum of 24 hours, so Tardigrades can recover from their dormant, dehydrated state.
After 24 hours of underwater rest time, you’ll have to put your sample on top of a 1mm mesh sieve with another small container underneath (I suggest a small Petri dish) and proceed to cut the moss with your hands or scissors in couple of fragments. If you don’t have any sieves, do not panic, you can cut your moss directly in its original container. You’ll now need to squeeze the water out of the moss into the new container. Make sure there’s no water left in the moss!
You’re now ready to put your Petri dish or other small container under the microscope to see if there’s any small eight-legged creatures walking around. You’ll be able to spot one at first with your 4x or 10x objective, depending on the size and species of the individual. There are also big chances you find some Rotifers, which are microscopic multicellular organisms, along the way.
How to view them?
If you possess a stereo microscope at home or have access to one, it could come in handy to examine your Petri dish at first to see if you can spot our favourite eight-legged friend. After finding one, you can pipet it with a small quantity of water on a glass slide and gently put a cover slip on top. It’s important not to put too little water or else you might squish your Tardigrade to death. Trust me on this one, it’s better to start gradually and put a bit too much water than not enough. You can then observe your water bear either with the 10x, 20x or 40x objective of a compound microscope. If you wish to give more contrast to your images, you can buy some 3D-printed oblique and darkfield filters on Ebay! If you love darkfield images and wish to have a more refined darkfield illumination, I’d recommend you buy a darkfield condenser.
If you feel creative and wish to try a little DIY project to be able to see structures which aren’t always visible with brightfield or darkfield microscopy, you can buy linear polarizing films from Amazon or Ebay. All you have to do next is to cut two pieces from the polarizing sheet and put one piece on the light source and the other on top of your sample, beneath the objective. You should be able, by rotating the filters and/or turning them over, to see the V-shaped stylets glow! You might also be able to see some crystals glowing inside or out of the Tardigrade, depending of the species, the composition of the cuticle and what it previously ate.
The best way to capture images of these small critters is certainly with a microscope camera and Motic has a lot of great options you can add on your binocular or trinocular microscope. Though, if you wish to experiment before committing to a microscope camera, you can always buy an adapter to capture pictures and videos with your cellphone. There’s a lot of great options online depending of your budget and phone model.
Good luck!
References
- Aguinaldo, A. M. A., Turbeville, J. M., Linford, L. S., Rivera, M. C., Garey, J. R., Raff, R. A., & Lake, J. A. (1997). Evidence for a clade of nematodes, arthropods and other moulting animals. Nature, 387(6632), 489–493. doi:10.1038/387489a0
- Dewel, R. A., Joines, J. D., & Bond, J. J. (1985). A new chytridiomycete parasitizing the tardigrade Milnesium tardigradum. Canadian Journal of Botany, 63(9), 1525–1534. doi:10.1139/b85-211
- Gabriel, W. N., McNuff, R., Patel, S. K., Gregory, T. R., Jeck, W. R., Jones, C. D., & Goldstein, B. (2007). The tardigrade Hypsibius dujardini, a new model for studying the evolution of development. Developmental Biology, 312(2), 545–559. doi:10.1016/j.ydbio.2007.09.055
- Gross, V., Minich, I., & Mayer, G. (2017). External morphogenesis of the tardigrade Hypsibius dujardini as revealed by scanning electron microscopy. Journal of Morphology, 278(4), 563–573.doi:10.1002/jmor.20654
- Guidetti, R., Altiero, T., & Rebecchi, L. (2011). On dormancy strategies in tardigrades. Journal of Insect Physiology, 57(5), 567–576. doi:10.1016/j.jinsphys.2011.03.003
- Hansen, J. G., Kristensen, R. M., Jørgensen, A., Accogli, G., D’Addabbo, R., & Gallo, M. (2016). Postembryonic development, paedomorphosis, secondary sexual dimorphism and population structure of a new Florarctus species (Tardigrada, Heterotardigrada). Zoological Journal of the Linnean Society, 178(4), 871–877.doi:10.1111/zoj.12436
- Hashimoto, T., Horikawa, D. D., Saito, Y., Kuwahara, H., Kozuka-Hata, H., Shin-I, T., … Kunieda, T. (2016). Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein. Nature Communications, 7, 12808. doi:10.1038/ncomms12808
- Nelson, D. R., Guidetti, R., & Rebecchi, L. (2015). Phylum Tardigrada. Thorp and Covich’s Freshwater Invertebrates, 347–380. doi:10.1016/b978-0-12-385026-3.00017-6
- Schill, R. O. (Ed.). (2018). Water Bears: The Biology of Tardigrades. Zoological Monographs. doi:10.1007/978-3-319-95702-9
- Vasanthan, T., Alejaldre, L., Hider, J., Patel, S., Husain, N., Umapathisivam, B., & Stone, J. (2017). G-Equivalent Acceleration Tolerance in the Eutardigrade Species Hypsibius dujardini. Astrobiology, 17(1), 55–60. doi:10.1089/ast.2015.1439
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