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AMAZING TARDIGRADES
15/02/2024
You may not have heard much about tardigrades, sometimes known as water bears, but they are extraordinary and fascinating eight-legged “mini-beasts”. First noted and described more than 250 years ago, they are surprisingly ubiquitous but tiny, so we don’t notice them – you’ll need a decent microscope to get a good view of them: even the largest are barely 2mm long and tend towards transparency. An image search on the web will provide dozens of good illustrations, though, and you can see why their other common name is “moss piglet” from their shape and the habitat in which many are often found.
The drawings in this note are in the public domain, taken from probably the first thorough paper on tardigrades by M. Doyère, professor of natural history at the French Henry IV Royal College in 1840[1]. Doyère described their anatomy, physiology and reproduction in considerable detail. He recognised that they were a biologically different group of animals and in his 1840 text he recognises the classification by eminent scientist of the day, Félix Dujardin[2] and wrote (please use an online translate facility if you don’t read French):
Mais ce qui assure au travail de M. Dujardin une place importante dans l'histoire des animaux qui nous occupent, c’est qu’il leur a irrévocablement assigné leurs rapports naturels, en le craignant pas de les réunir aux Rotifères, c'est-à-dire à ceux de tous les animaux inférieurs dont il pouvait sembler qu'on dût les tenir le plus éloignés, pour en former un groupe unique auquel il donne le nom de Systolides. Après deux années presque entièrement consacrées à l'étude de ces êtres singuliers et d’une organisation jusque-là, ou si négligée ou si vainement débattue ; après avoir établi, je l'espère du moins, d'une manière définitive, la théorie à très peu près complète de tous leurs systèmes organiques; après avoir découvert dans ces organisations si éloignées de la portée -de nos sens livrés à eux-mêmes, un ensemble de faits que rien de ce qu'on avait dit jusque-là n'eût permis d'y prédire sans témérité, je n’en aurai pas retiré d'autre profit pour la classification, que d’avoir fourni de nouvelles données pour fixer sur des résultats anatomiques et physiologiques, une détermination zoologique et des rapports naturels auxquels M. Dujardin était arrivé depuis plusieurs années déjà, et qu'il avait établis avec une certitude de jugement à laquelle je suis heureux de pouvoir rendre cet hommage.
Today, tardigrades have their own Phylum, an upper biological classification ranked below a Kingdom and above a Class. Tardigrades are special and extraordinary for several reasons. Since they are very rarely found as fossils, ancient specimens of reference are those preserved in amber and so we know they have been around on our planet for about 500,000,000 years. At least a thousand species have now been classified and when I say ubiquitous above, it is because they are found in terrestrial, freshwater and marine habitats. They are common amongst moss and lichens and early-described specimens came from Parisian roofs. In fact, you can wash them out into a petri dish from a handful of moss or lichen and hunt for them with a stereo microscope at x20 or x30 magnification. They have also been found by intrepid biologists in boiling volcanic mud, hot springs, the tops of mountains and the depths of oceans.
Tardigrades are famous for being able to survive severe conditions of drought and famine by entering a cryptobiotic state of extreme inactivity, a desiccated little ball known as a “tun”. They are thus able to slow their metabolism to a hundredth of the normal level and they have been shown to survive exposure to intense radiation, outer space and to many times normal atmospheric pressure. They can withstand toxic levels of hydrogen peroxide, sugar and salt that would kill most organisms. They do this by producing free radicals which oxidise the amino acid cysteine provoking the tun state[3], reviving when conditions improve.
Betts Ecology maintain an awareness of nature in all its myriad forms, but surely tardigrades are amongst the most amazing.
© Betts Ecology
[1] Doyère, M. (1840). Mémoire sur les Tardigrades. Annales des Sciences Naturalles, Zoologia, 2(14), pp 269–362. Paris.
[2] Félix Dujardin (1801–1860) was a key figure of 19th century study of the eukaryotic organisms known as protists. He is known for recognizing the true nature of the amoeba-like foraminifera and grouping them with others in a new group called “rhizopodes”, and for disputing Ehrenberg's characterization of protists possessing many stomachs and other organs. His 1841 monograph “Histoire naturelle des Infusoires” is generally considered his major work.
https://doi.org/10.1016/j.protis.2021.125821Get rights and content.
[3] PLoS One, doi.org/mc4n.
