Introduction

Tardigrades are very small, water-dwelling, segmented animals with eight legs. They are one of the most complex of polyextremophiles, which are creatures that can survive a variety of physically or geochemically extreme conditions that would be detrimental to most life on earth. There have been over 400 species of Tardigrades discovered since Goetz first discovered them in 1773. Ancestors of tardigrades most likely appeared in the Cambrian Explosion 540 million years ago, which marked the rise of life. They are speculated to have evolved in the ocean, before moving to freshwater and other terrestrial habitats. There have even been tardigrades found as they are in their present form encased in cretaceous amber. This shows that they existed for at least 100 million years as they do today.

Tardigrades are described as looking like bears, hence their nickname ‘water bears’. They have barrel-shaped bodies consisting of a head and four body segments with a pair of poorly articulated legs each. Each leg has four to eight claws on them each. Their body is covered with a cuticle containing chitin, proteins and lipids. They have no need for respiratory organs as gaseous exchange can take place over its entire body. An average tardigrade grows to about 1 millimetre when fully grown and can be viewed easily through a low powered microscope. An important feature of the tardigrade is the stylets in its tubular mouth. These stylets allow the tardigrades to pierce plant cells or animal body walls to release bodily fluids or cell contents. These stylets are lost whenever the tardigrade moults, and a new pair is secreted through a pair glands on either side of the tardigrades mouth.

Sources:
http://sun.iwu.edu/~tardisdp/tardigrade_facsts.html, Karen Lindahl and Professor Susie Balser in affiliationwith Illinois Wesleyan University, last accessed: 23/1/13
http://www.astrobio.net/exclusive/261/extreme-animals, Helen Matsos, last accessed: 23/1/13