Tardigrades

For my project on invertebrates, I chose to study tardigrades. These are microscopic — or to be more precise — near-microscopic animals. There are well over a thousand known species of these tiny animals, which belong to the phylum Tardigrada. Most tardigrades range in length from 0.05 to 1 millimetre, though the largest species can grow to be 1.2 millimetres in length. They are also sometimes called ‘water bears’: ‘water’ because that’s where they thrive best, and ‘bear’ because of the way they move. Q31. ‘Moss piglet’ is another name for tardigrades because of the way they look when viewed from the front. They were first discovered in Germany in 1773 by Johann Goeze, who coined the name Tardigrada.

As I say, there are many different species of tardigrade — too many to describe here — but, generally speaking, the different species share similar physical traits. They have a body which is short, Q32, and also rounded — a bit like a barrel — and the body comprises four segments. Each segment has a pair of legs, at the end of which are between four and eight sharp claws. I should also say that some species don’t have any claws: what they have are discs, Q33, and these work by means of suction. They enable the tardigrade to cling to surfaces or to grip its prey. Within the body, there are no lungs, or any organs for breathing at all. Instead, oxygen and also blood are transported in a fluid that fills the cavity of the body. Q34

As far as the tardigrade’s head is concerned, the best way I can describe this is that it looks rather strange — a bit squashed even — though many of the websites I looked at described its appearance as cute, which isn’t exactly very scientific. The tardigrade’s mouth is a kind of tube Q35 that can open outwards to reveal teeth-like structures known as ‘stylets’. These are sharp enough to pierce plant or animal cells.

So, where are tardigrades found? Well, they live in every part of the world, in a variety of habitats: most commonly, on the bed of a lake, or on many kinds of plants or in very wet environments. There’s been some interesting research which has found that tardigrades are capable of surviving radiation and very high pressure, and they’re also able to withstand temperatures as low as minus –200 degrees centigrade, or highs of more than 148 degrees centigrade, Q36, which is incredibly hot.

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It has been said that tardigrades could survive long after human beings have been wiped out, even in the event of an asteroid hitting the earth. If conditions become too extreme and tardigrades are at risk of drying out, they enter a state called cryptobiosis. They form a little ball, called a tun — that’s T-U-N — by retracting their head and legs, and their metabolism drops to less than one percent of normal levels. They can stay in this state for decades, and if re-introduced to water, when they will come back to life in a matter of a few hours. While in this state of cryptobiosis, tardigrades produce a protein that protects their DNA. Q37. In 2016, scientists revived two tardigrades that had been tuns for more than 30 years. There was a report that,in 1948, a 120-year-old tun was revived, but this experiment has never been repeated. There are currently several tests taking place in space, to determine how long tardigrades might be able to survive there. Q38. I believe the record so far is 10 day.

So, erm, moving on. In terms of their diet, tardigrades consume liquids in order to survive. Although they have teeth, they don’t use these for chewing. They suck the juices from moss, or extract fluid from seaweed, Q39, but some species prey on other tardigrades, from other species or within their own. I suppose this isn’t surprising, given that tardigrades are mainly comprised of liquid and are coated with a type of gel.

Finally, I’d like to mention the conservation status of tardigrades. It is estimated that they have been in existence for approximately half a billion years and, in that time, they have survived five mass extinctions. So, it will probably come as no surprise to you, that tardigrades have not been evaluated by the International Union for Conservation of Nature and are not on any endangered list. Q40. Some researchers have described them as thriving.

Does anyone have any questions they’d like to ask?