Attenborough's water bear

One of nineteen new species of 'moss piglets' from South Africa

Heterotardigrada: Echiniscidae: Echiniscus

When’s the last time that tardigrades came up in conversation? I thought so.  That’s too bad.  Tardigrades are absolutely fascinating animals, from their relationships deep in evolutionary time, to physiological feats of survival that sound like the stuff of science fiction.

So far, about 1,300 species of tardigrades have been discovered.  They are considered aquatic, but in the most liberal sense of the word.  Some are found in a mere film of water, just enough to keep them from dehydrating.  Others are aquatic in the usual sense, in both fresh and salt water.  They are seemingly everywhere, on every continent, from deep seas to high mountaintops, from tropical rainforests and hot springs to the deep freeze of the Antarctic, and from 13,000 feet below sea level to 20,000 feet above it.  They are incredibly hardy, surviving extreme temperatures and pressures, radiation, deprivation of food and air, and dehydration that would kill just about any other organism. They have survived earth’s previous five mass extinction events, and my money on them when the next global-scale catastrophe happens. In 2007, the European Space Agency’s unofficial tardigrade torture unit exposed them to the vacuum and cosmic rays of outer space, and solar UV radiation a thousand times higher than on earth, and most passed the test.  Some can withstand at least several minutes exposure to extreme cold (minus 460 degrees F.) and heat up to 300 degrees F.  When exposed to unfavorable conditions, they are capable of entering a resting state in which their metabolism slows to 0.01% of its normal rate, and the water content of their bodies is reduced to 1% of normal.  They can survive in this dehydrated state for several years, some impressively longer.  Tardigrades have been reanimated from dried moss specimens in herbaria collected as much as thirty years earlier!

Water bears are related to velvet worms (the onychophorans) and arthropods, the massively diverse group including lobsters, spiders and insects.  The oldest fossil of a modern-looking tardigrade is from amber about 90 million years old, but it is suspected that the lineage is a great deal older, probably diverging in the Cambrian over 500 million years ago.

Tardigrades go by a number of common names, including water bears and moss piglets.  As these names suggest, most are chubby little animals with fat, stumpy “legs,” each with four to eight claws.  They have a head, three body segments (each with a pair of legs), and a caudal segment bearing a fourth pair of legs.  They have a slow, lumbering gate that has reminded many of the way a bear walks.  Most are less than half a millimeter in length, with the largest measuring in at 1.2 millimeters, or so.  Obviously, these cute little creatures must be studied under a microscope to be truly appreciated.  They were first discovered by the German biologist Johann August Ephraim Goeze in 1773, and given the name Tardigrada, meaning “slow stepper”, by the Italian Lazzaro Spallanzani in 1777.

They have external fertilization and lay eggs.  Baby water bears hatch with the full number of cells found in the adult.  Their growth takes place by increasing size of individual cells, instead of by cell division.  During their life, they may molt up to a dozen times.

Most tardigrades feed on bacteria or plant material, with some eating smaller tardigrades.  They generally live from a few months to a couple of years, but that doesn’t take into account dormant states. 

One of the most recently discovered tardigrades is Echiniscus attenboroughi, named for the master of nature story-telling, Sir David Attenborough.  It is one of nineteen new species named from an extensive survey of echiniscid water bears in woodlands, savannahs, scrubs, semi-deserts, and mountain ranges of South Africa.  New species were reported in a beautifully illustrated taxonomic monograph, setting a new benchmark of excellence for treatments of the African fauna.

The new species belongs to the class Heterotardigrada which have cephalic appendages and legs with four separate but similar claws.  To date, about 450 species of heterotardigrades have been discovered.

As this impressive study demonstrates, our knowledge of tardigrades is poor and there remain many species and impressive things to be discovered about them.  So, at the next cocktail party, when conversation lulls, why not ask if they’ve heard about Echiniscus attenboroughi… you may find yourself standing alone, but who knows, you could be the life of the party

Scanning electron micrographs of habitus and dorsal sculpturing of Echiniscus attenboroughi: (A) dorsal; (B) lateral; (C) scapular plate; (D) segmental plate.  Scale bars in microns.  From Gasiorek et al (2022), with permission from author.

Acknowledgment

I thank Dr. Piotr Gasiorek of the Department of Invertebrate Evolution, Institute of Zoology and Biomedical Research, Jagiellonian University in Krakow, Poland, for permission to reproduce the images of Echiniscus attenboroughi.

Further Reading

Gasiorek, P., Voncina, K., Bochnak, M., Surmacz, B., Morek, W. and L. Michalczyk (2022) Echiniscidae (Heterotardigrada) of South Africa. Zootaxa 5156: 1-238.