Tardigrades (Phylum Tardigrada, Superphylum Ecdysozoa) Polyextremophiles

Tardigrade Biology Facts:
  • Common Names - water bears or moss piglets.
  • Phylum Tardigrada, part of the superphylum Ecdysozoa.
  • More than 1000 species have been identified.
  • They are microscopic, water-dwelling, segmented animals with 8 legs (see image).
  • Adults may grow to length 1.5 mm.
  • Smallest adults can be below 0.1 mm.
  • Larvae on hatching can be smaller than 0.05 mm.
  • Locations - Himalayas to the deep sea (below 4,000 m) as well as equator to polar regions.
  • Can be found on lichens & mosses.
  • Most tardigrades are phytophagous (plant eaters), or bacteriophagous (bacteria eaters).
Tardigrade Anatomy:
  • Have a body with four segments (this does include the head).
  • Consist of 4 pairs of legs without joints.
  • Have feet with claws or toes.
  • Surface cuticle which contains chitin and is able to be moulted.
  • Contain a ventral nervous system with one ganglion per segment.
  • There is a multilobed brain.
  • Tardigrades have rhabdomeric pigment-cup eyes.
  • Have a haemocoel rather than a coelom (except around the gonad, coelomic pouch).
  • They have a sucking type pharynx.
  • Males & females are usually present, each with a single gonad.
  • Some species are, however, parthenogenetic.
Polyextremophile Capabilities:
  • Tardigrades are polyextremophiles - i.e. can survive in extreme environments that would kill almost any other animal.
  • Some tardigrades can survive temperatures of -273°C, close to absolute zero. [1]
  • Some can survive temperatures as high as 151 °C (303 °F).
  • Can survive 1,000 times more radiation than other animals such as humans. [2]
  • Have been described as going for almost a decade without water. [3]
  • Some have even reported to survive the vacuum of outer space. [4]
Tardigrade History:
  • Tardigrades were first described by Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear).
  • Name Tardigrada was given by Lazzaro Spallanzani in 1777 and means "slow walker".
  • The name water bear comes from their gait - which looks like a bear walking.
Tardigrade Phylogeny:
  • Recent sequencing data (DNA & RNA) suggest that tardigrades are related to the arthropods & Onychophora. [5]
  • Considered in some schemes as Ecdysozoa.

[1] - Becquerel P. (1950). "La suspension de la vie au dessous de 1/20 K absolu par demagnetization adiabatique de l’alun de fer dans le vide les plus eléve". C. R. hebd. Séances Acad. Sci. Paris 231: 261–263.
[2] - Radiation tolerance in the tardigrade Milnesium tardigradum
[3] - Crowe, John H.; Carpenter, John F.; Crowe, Lois M. (October 1998), "The role of vitrification in anhydrobiosis", Annual Review of Physiology 60: 73–103
[4] - K. Ingemar Jönsson, Elke Rabbow, Ralph O. Schill, Mats Harms-Ringdahl and Petra Rettberg (2008). "Tardigrades survive exposure to space in low Earth orbit". Current Biology 18 (17): R729–R731.
[5] -

Image by Goldstein lab - tardigrades (cc)
Scanning electron micrograph of an adult water bear (tardigrade), Hypsibius dujardini.
by Bob Goldstein & Vicky Madden, UNC Chapel Hill

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