Tardigrades

Tardigrades, also known as water bears or moss piglets, are tiny eight-legged animals that live wherever there is a little water. They are famous for being among the toughest animals on Earth, able to survive conditions that would kill almost anything else.

Scientific Classification & Quick Facts

Common name	Tardigrade
Scientific name	Tardigrada
Kingdom	Animalia
Type	Microscopic animal (water bear)
Diet	Algae, plant cells and tiny invertebrates
Habitat	Mosses, lichens, soil and sediments, from mountains to deep sea
Size	About 0.5 mm; up to 1.5 mm
Special feature	Survives extreme heat, cold and drying out

Discovery and Name

Tardigrades were first described in 1773 by the German pastor Johann August Ephraim Goeze, who called them “little water bears.” A few years later, in 1776, the Italian scientist Lazzaro Spallanzani gave them the name tardigrade, which means “slow stepper,” because of the way they plod along.

Tardigrades make up their own phylum, called Tardigrada. Their closest relatives are thought to be arthropods (such as insects and spiders) and velvet worms.

Description

Photo by: Dmitry Brant

Most adult tardigrades are about 0.3 to 0.5 mm long, so they are just about visible to the naked eye as a speck. The smallest are under 0.1 mm, and the largest reach around 1.5 mm.

A tardigrade has a short, plump, barrel-shaped body with four pairs of stubby legs. Each leg ends in claws or sticky discs. The first three pairs are used for walking, and the back pair points backward and is used for gripping. Their slow, lumbering walk looks a lot like a tiny bear, which is how they got their nickname.

Tardigrades have a mouth tube with two sharp, needle-like stylets and a strong, muscular throat that works like a pump. They have a simple brain, a nerve cord running along the belly and, in many species, two simple eyespots. They have no lungs or heart. Instead, oxygen passes straight in through the body surface.

Adults of each species have a fixed number of cells, often around a thousand. This is called being eutelic. Tardigrades grow by making their cells larger, not by adding more cells.

Habitat

More than 1,300 species of tardigrade have been described, and they live all over the world: from high mountains in the Himalayas to the deep sea, and from tropical rainforests to Antarctica.

Most land tardigrades live in the thin film of water on mosses and lichens. Others live in soil, leaf litter, freshwater ponds, sand dunes, beach sand and seabed mud. One species even lives on barnacles. Tardigrades can be very common: a small clump of moss can contain hundreds of them.

You can look for tardigrades yourself. Soak a little piece of moss in water for a few hours, squeeze out the water into a dish and look at it under a microscope or strong magnifying glass.

Super Survivors

When their home dries out, many tardigrades pull in their head and legs, lose almost all their body water and curl up into a dry little ball called a tun. In this state, called cryptobiosis, their body processes slow almost to a stop. When water returns, they can rehydrate and start moving again, sometimes after years.

In experiments, tardigrades in the tun state have survived:

• Being cooled to just above absolute zero, the coldest possible temperature
• Brief heating to well above the boiling point of water
• Huge pressures, many times greater than at the bottom of the deepest ocean
• Doses of radiation hundreds of times higher than would kill a human
• Exposure to the vacuum of space on a satellite in 2007

Tardigrades are not invincible, though. Active tardigrades are much more fragile, and even tuns can be killed by long exposure to heat.

Feeding

Tardigrades use their stylets to pierce plant cells, algae and moss, then suck out the juices with their pumping throat. Some species are predators and attack even smaller animals, such as rotifers, nematode worms and other tardigrades.

Reproduction

Most tardigrade species have males and females, but some species have only females, which can lay eggs that develop without fertilization. This is called parthenogenesis.

Many species lay their eggs inside their old skin when they molt, leaving the eggs safely wrapped in the shed cuticle. Others lay eggs with decorated shells directly into their surroundings. The eggs usually hatch within about two weeks. Baby tardigrades look like small adults and already have all their cells, so they grow simply by their cells getting bigger. Tardigrades molt several times during their lives.

Fossils

Because they are so small and soft, tardigrades rarely become fossils. The oldest known tardigrade fossils are preserved in amber from the time of the dinosaurs, about 90 million years ago. Scientists think their ancestors are much older, possibly going back more than 500 million years to the Cambrian period.

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