If you go down to the woods today... well what can you find? The obvious things are trees, flowers, the sound of birdsong, perhaps a brief glimpse of a deer’s tail at is disappears into the undergrowth. Yet if you stop and look carefully there is another world, literally right under your feet. The ground, tree trunks, rock and logs are often clothed with cushions of moss and tufts of lichens, which are often passed by without a second glance. If time is taken to study mosses and lichens more closely, they are seen to be home to a wide variety of almost microscopic animals.

There are the worm-like nematodes; eight legged mites, relatives of the spiders; collembolans, primitive insects known as the springtails, with a terminal spine that can catapult them away from danger. Perhaps the most remarkable group of animals inhabiting this microscopic jungle are the tardigrades. The name tardigrada means slow step, but they are more often called water bears because of their slow, lumbering gait.

Water bears are semi-terrestrial animals which live in the surface-water film found on mosses and lichens, in litter, or between grains of soil. They have been found close to the North and South Pole, high in the Himalayan Mountains, and in desert oases close to the equator. Some purely freshwater species are found in river and lake sediments or crawling over the surface of water weed. Marine tardigrades may be found in sandy beaches, on intertidal rocks and underwater sediments down to depths greater than 4,000 m.

Hypsibius evelinae, a member of the Eutardigrada

Tardigrades are bilaterally symmetrical animals, usually between 150 and 300µm long. The largest species may reach lengths up to 1,200µm, while the smallest marine species barely attain 100µm. They have four pairs of stubby legs which usually end in claws. At least one species has no claws at all, and in some marine tardigrades the legs end in flattened disks or elongated paddles. The body is covered by a cuticle which contains chitin. The cuticle may be sculptured to show incredibly complex patterns, which are useful when identifying species.

It is easier to mention the two orders of tardigrade separately. The more primitive Heterotardigrada have a definite head from which arises a number of cephalic appendages. The dorsal cuticle may be smooth, or form a series of sculptured plates. These plates may have dorsal and lateral spines or filaments. The legs may end in elongated paddles, or discs, but generally there are four single claws.

The Eutardigrada are generally cigar shaped without an obvious neck region. The body shows a slight segmentation, restricted to the cuticle. The cuticle sculpture is limited to granulation, swellings, reticulation or short spines. Tardigrades have eight legs which suggests that they are related to the arachnids. However, they have a distinct combination of characteristics which has resulted in them being placed in a phylum of their own. For example, the cuticle is covered in mucus, and there is a ladder like ventral nervous system as in the annelids; the cuticle contains chitin as in insects; the feeding apparatus is similar to many nematodes. Many of the internal organs consist of a fixed number of cells; growth is due to enlargement of cells rather than by an increase in cell number.

Many species feed on detritus, or they graze on algae and bacterial films. There are several, particularly the larger species, which are carnivorous, preying on nematodes, rotifers and other tardigrades. The mouth opens into a cuticle lined buccal tube which leads to a muscular triradially symmetrical pharyngeal bulb. On either side of the buccal tube is a stylet, which can be everted through the mouth and is able to pierce cell or body walls. The food can then be sucked back into the pharyngeal bulb.

Many tardigrade species are parthenogenetic, the populations are almost completely female, with males being found infrequently if at all. The females lay fertile eggs and this enables a rapid increase in numbers to occur under favourable conditions. Measurements of tardigrade populations have recorded over 12,000 per m2 in soil, and over 2 million per m2 in mosses. Eggs are often laid in a cast-off adult cuticle. They hatch in between three and forty days. Juveniles look similar to adults, but they may lack certain adult characters. Growth takes place as a series of instars punctuated by moults. There may be up to twelve moults during the lifespan, which is thought to be about a year. Animals become sexually mature after the third moult. Semi-terrestrial tardigrades are widely distributed and in many areas they are subject to infrequent hostile changes in environmental conditions such as freezing temperatures and exposure which leads to the animals being dried out. To counter this they have developed a remarkable survival mechanism - cryptobiosis. At the onset of unfavourable conditions the tardigrade contracts into a barrel shape, the tun. Up to 97% of the body water is removed and the cuticle becomes impervious to moisture. In the cryptobiotic state there is no detectable metabolic activity and individuals have been known to survive up to 120 years like this. In this stage tardigrades become resistant to extremes of conditions that would be fatal to normal animals, and would not normally be encountered. They can survive a range of temperatures between 150oC to almost absolute zero (-273oC), and can tolerate electromagnetic radiation far in excess of that fatal to humans. Yet with the onset of normal conditions they regain their active life within an hour or so.

Tardigrades were discovered in 1776; 300 years on there are over 530 species known worldwide.

In Britain 74 species have been found and while their distribution in Scotland is fairly well known, corresponding species lists for England are few and irregular. There are published records for Cambridgeshire, Kent, Shropshire, Sussex, Warwickshire and Yorkshire. As yet there are no Suffolk records.