The smallest bears in the world have almost superhero abilities. Actually, they are not bears: water bears is the popular name of tardigrades. They are virtually indestructible invertebrates: they can survive decades without water or food, to extreme temperatures and they have even survived into outer space. Meet the animal that seems to come from another planet and learn to observe them in your home if you have a microscope.
WHAT IS A TARDIGRADE?
Tardigrades or water bears, are a group of invertebrates 0.05-1.5 mm long that preferably live in damp places. They are especially abundant in the film of moisture covering mosses and ferns, although there are oceanic and freshwater species, so we can consider they live anywhere in the world. Even a few meters away from you, in the gap between tile and tile. In one gram of moss they have find up to 22,000 individuals. They are found in Antarctica under layers of 5 meters of ice, in warm deserts, hot springs, in mountains 6,000 meters high and abyssal ocean depths: they are extremophiles. It is estimated that over 1,000 species exist.
Its popular name refers to their appearance, and the scientific name to their slow movements. Their bodies are divided into five segments: cephalic, with its tube-shaped mouth (proboscis) with two internal stilettos and sometimes simple eyes (ommatidia) and sensory hairs, and the remaining 4 segment with a pair of legs per segment. Each leg has claws for anchoring to the ground.
Look at this video of Craig Smith to see tardigrade’s movements in more detail:
With its mouth stilettos, tardigrades perforate plants and absorbe the products of photosynthesis, but they can also feed absorbing the cellular content of other microscopic organisms such as bacteria, algae, rotifers, nematodes… Some are predators too and can eat whole microorganisms.
Their digestive system is basically the mouth and a pharynx with powerful muscles to make sucking motions that opens directly into the intestine and anus. Some species defecate only when they shed.
They have no circulatory or respiratory system: gas exchange is made directly by the body surface. They are covered by a rigid cuticle which can be of different colors and is shed as they grow. With each moult, they lose oral stilettos, to be segregated again. They are eutelic animals: to grow they only increase the size of their cells, not their number, that remains constant throughout life
Tardigrades generally have separate sexes (are dioecious) and reproduce by eggs (are oviparous), but there are also hermaphrodites and parthenogenetic species (females reproduce without being fertilized by any male). Fertilization is external and development is direct: they don’t have larval stages.
The tardigrades are incredibly resilient animals that have survived the following conditions:
- Dehydration: they can survive for 30 years under laboratory conditions without a single drop of water. Some sources claim that resist up to 120 years or have been found in ice 2000 years old and have been able to revive, although it is likely to be an exaggeration.
- Extreme temperature: if you boil one tardigrade survives. If you put it to temperatures near the absolute zero (-273ºC), survives. Their survival rate ranges from -270ºC to 150ºC.
- Extreme pressure: they are capable of supporting from vacuum to 6,000 atmospheres, ie 6 times the pressure in the deepest point on Earth, the Mariana Trench (11,000 meters deep).
- Extreme radiation: tardigrades can withstand bombardment of radiation at a dose 1000 times the lethal to a human.
- Toxic substances: if they are immersed in ether or pure alcohol, survive.
- Outer space: tardigrades are the only animals that have survived into space without any protection. In 2007 the ESA (European Space Agency) within the TARDIS project (Tardigrades In Space) left tardigrades (Richtersius coronifer and Milnesium tardigradum) for 12 days on the surface of the Foton-M3 spacecraft and they survived the space travel. In 2011 NASA did the same placing them in the outside of the space shuttle Endeavour and the results were corroborated. They survived vacuum, cosmic rays and ultraviolet radiation 1,000 times higher than that of the Earth’s surface. The project Biokis (2011) of the Italian Space Agency (ASI) studied the impact of these trips at the molecular level.
HOW DO THEY DO THAT?
The tardigrades are able to withstand such extreme conditions because they enter cryptobiosis status when conditions are unfavorable. It is an extreme state of anabiosis (decreased metabolism). According to the conditions they endure, the cryptobiosis is classified as:
- Anhydrobiosis: in case of environmental dehydration, they enter a “barrel status” because adopt barrel shaping to reduce its surface and wrap in a layer of wax to prevent water loss through transpiration. To prevent cell death they synthesize trehalose, a sugar substitute for water, so body structure and cell membranes remain intact. They reduce the water content of their body to just 1% and then stop their metabolism almost completely (0.01% below normal).
- Cryobiosis: in low temperatures, the water of living beings crystallizes, it breaks the structure of cells and the living being die. Tardigrades use proteins to suddenly freeze water cells as small crystals, so they can avoid breakage.
- Osmobiosis: it occurs in case of increase of the salt concentration of the environment.
- Anoxybiosis: in the absence of oxygen, they enter a state of inactivity in which leave their body fully stretched, so they need water to stay perky.
Referring to exposures to radiation, which would destroy the DNA, it has been observed that tardigrades are able to repair the damaged genetic material.
These techniques have already been imitated in fields such as medicine, preserving rat hearts to “revive” them later, and open other fields of living tissue preservation and transplantation. They also open new fields in space exploration for extraterrestrial life (Astrobiology) and even in the human exploration of space to withstand long interplanetary travel, ideas for now, closer to science fiction than reality.
ARE THEY ALIENS?
The sparse fossil record, the unclear evolutionary relatedness and great resistance, led to hypothesis speculating with the possibility that tardigrades have come from outer space. It is not a crazy idea, but highly unlikely. Panspermia is the hypothesis that life, or rather, complex organic molecules, did not originate on Earth, but travelled within meteorites in the early Solar System. Indeed, amino acids (essential molecules for life) have been found in meteorites composition, so panspermia is a hypothesis that can not be ruled out yet.
But it is not the case of tardigrades: their DNA is the same as the rest of terrestrial life forms and recent phylogenetic studies relate them to onychophorans (worm-like animals), aschelminthes and arthropods. What is fascinating is that is the animal with more foreign DNA: up to 16% of its genome belongs to fungi, bacteria or archaea, obtained by a process called horizontal gene transfer. The presence of foreign genes in other animal species is usually not more than 1%. Could be this fact what has enabled them to develop this great resistance?
DO YOU WANT TO SEARCH TARDIGRADES BY YOURSELF AND OBSERVE THEM IN ACTION?
Being so common and potentially livIng almost anywhere, if you have a simple microscope, you can search and view living tardigrades by yourself:
- Grab a piece of moss of a rock or wall, it is better if it is a little dry.
- Let it dry in the sun and clean it of dirt and other large debris.
- Put it upside down in a transparent container (such as a petri dish), soak it with water and wait a few hours.
- Remove moss and look for tardigrades in the water container (put it on a black background for easier viewing). If lucky, with a magnifying glass you’ll see them moving.
- Take them with a pipette or dropper, place them on the slide and enjoy! You could see things like this:
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- Anhydrobiosis in invertebrates
- “Alien” water bears amaze scientists
- Tardigrada (Astronoo)
- La criptobiosis en el phylum Tardigrada
- El animal “más resistente de la Tierra” se prueba en el espacio
- Cover photo by Oliver Meckes