Tardigrades are an ancient group of animals. The earliest fossil evidence of tardigrades dates to over 500 million years ago. There are around 1,200 species of tardigrades that have been discovered, and new species are still being discovered. There are two main lineages, eutardigrades, which live mainly on land, and heterotardigrades, some of which live on land while others live in marine environments. Marine tardigrades are not known to be desiccation tolerant, while this is common in eutardigrades and some terrestrial heterotardigrades.

Tardigrades are renowned for their ability to survive a number of extreme stresses, such as drying, freezing, irradiation, and even the vacuum of outer space, but this might not actually be the weirdest thing about them. In 2016, Dr. Frank Smith discovered that tardigrades are essentially “walking heads,” with bodies made up of segments that are homologous to the head segments of their cousins, the arthropods (e.g., insects and crustaceans).

Tardigrades can be found almost anywhere, from Antarctica to your own backyard. It is thought that tardigrades get to all these different places by being swept up by wind currents when they are dry and then deposited somewhere far away. If you have a microscope, it is actually fairly easy to find your own wild tardigrades.

Different tardigrade species reproduce in different ways. Some species have males and females that reproduce sexually, while other species do not have males--females produce eggs that do not need to be fertilized to develop.


Image: Water bear (tardigrade), Hypsibius dujardini, scanning electron micrograph by Bob Goldstein and Vicky Madden. UNC Chapel Hill, May 2008. 

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