Tardigrades: resilient creatures of extreme environments

Tardigrades, also known as water bears or moss piglets, are micro-animals with remarkable resilience to extreme environments. They can be found everywhere from the depths of the ocean to the tops of mountains and have survived the harsh conditions of outer space.

Anatomy and morphology

Tardigrades have barrel-shaped bodies with four pairs of legs, each with four to eight claws. The name "tardigrade" comes from the slow, lumbering gait of these animals. They range in size from 0.02 inches to 0.2 inches when fully grown.

Reproduction

Tardigrades reproduce by laying eggs. Females deposit their eggs inside their shed cuticle, which is then covered with sperm. The eggs hatch in about 14 days, and the young tardigrades already have their full complement of adult cells.

Cryptobiosis and the tun state

Tardigrades can enter a state of cryptobiosis, where they suspend their metabolism and reduce their water content. This state, known as the "tun" state, allows them to survive for years, even in extreme environments. They can withstand temperatures ranging from absolute zero to 150 degrees Celsius, pressures six times greater than the deepest ocean trenches, and doses of radiation hundreds of times higher than lethal to humans.

Ecology and life history

Most tardigrades are phytophagous or bacteriophagous, but some are carnivorous, feeding on other smaller tardigrades. They can be found in a variety of environments, including dunes, beaches, soil, and marine and freshwater sediments. Tardigrades are most common in moist environments, but they can survive virtually anywhere as long as they retain some moisture.

Physiology

Tardigrades can reversibly suspend their metabolism and enter a state of cryptobiosis. In this state, their water content can drop to 1%, and their metabolism lowers to less than 0.01%. They can survive for years in this state, especially when dried out. When exposed to extreme cold, they dehydrate to avoid being ripped apart by expanding ice.

Tardigrades are the first known animals to survive in space. In September 2007, dehydrated tardigrades were taken into low Earth orbit on the FOTON-M3 mission carrying the BIOPAN astrobiology payload. For 10 days, groups of tardigrades were exposed to the vacuum of outer space or the vacuum and solar UV radiation.

Resistance to environmental toxins

Tardigrades are reported to undergo chemobiosis, a cryptobiotic response to high levels of environmental toxins. However, as of 2001, these laboratory results had yet to be verified.

Fossil range

Tardigrade fossils date back to the Cambrian period, and they have been found in the Cretaceous period in North America.

Research and implications

Tardigrades' ability to survive extreme conditions has implications for the possibility of extraterrestrial life and the resilience of life on Earth. Their unique disordered proteins and DNA repair mechanisms also provide insight into biochemically resilient organisms.

The tardigrade's ability to survive in space and other extreme environments is a testament to the resilience of life and suggests that life could exist in other extreme environments, such as Enceladus, a moon of Saturn.

Ethical considerations

The tardigrade's ability to survive extreme conditions, particularly radiation, raises questions about the ethics of using them in research. Researchers must ensure that they