Tardigrades

A tardigrade sitting on a moss leaf. (Internet photo) Close-up of the mouth. Glad they aren’t any bigger! (Internet photo)

THE WORLD BENEATH OUR FEET: Part 6 - Tardigrades

BARRY MUIR

If there are aliens already living on our planet then tardigrades are them! They are the weirdest little critters you won’t see when you are pottering in the garden. The biggest adults may reach a body length of 1.5mm, the smallest below 0.1mm. They might be small, but they’ve been around for a while, with fossil specimens from the mid-Cambrian (570 million years ago) deposits in Siberia. Some 1,200 species of tardigrades (also known as water-bears or moss-pigs) have been described. Tardigrades occur over the entire world, from the high Himalayas (above 6,000m), to the deep sea (below 4,000m), to boiling hot springs and from the polar regions to the equator. The most convenient place to fi nd tardigrades in your backyard is on lichens and mosses. Other environments are dunes, beaches, soil, and marine or freshwater sediments, where they may occur up to 25,000 animals per litre of soil. Tardigrades have barrel-shaped bodies with four pairs of stubby legs. The body has four segments (not counting the head), four pairs of legs without joints (they move using hydraulic pressure), and feet with four to eight claws each. The skin contains chitin, the same as insects, and they need to moult periodically to get bigger. They have a tubular mouth armed with “jaws” which are used to pierce plant cells, algae or small animals on which the tardigrades feed, releasing the body fl uids or cell contents which they then suck up. They have a brain and a nerve cord (like a spinal cord) that runs the length of the body. Many species possess a pair of cup-shaped pigment spots that act as “eyes”, detecting light and shade. Some species can reproduce without mating but males and females are usually present. Tardigrades lay eggs and mating occurs during the moult, with the eggs being laid inside the shed skin of the female and then covered with sperm. In most cases, the eggs are left inside the shed skin to develop. The eggs hatch after a couple of weeks and they grow by pumping up their cells in the same way fungi do, rather than by cell division like plants and most animals. Tardigrades live for three to thirty months and may moult up to 12 times. Several species regularly survive in a dehydrated state for up to ten years. While in this state their water content can drop to one per cent of normal. Their ability to remain desiccated for such a long period is largely dependent on the high levels of a special sugar which protects their membranes. As well as being extremely tolerant to dehydration, tardigrades can survive being heated for a few minutes to 151C or being chilled for days at -200C. Tardigrades can survive the vacuum of space and solar radiation combined for at least 10 days as found in Space Shuttle Endeavour experiments. Tardigrades can also withstand lethal doses of gamma radiation up to 5000Gy while fi ve to 10Gy could kill a human. Tough little fellows!