Tardigrades, often called water bears or moss piglets, are fascinating micro-animals known for their remarkable resilience in diverse environments. These eight-legged creatures have captivated scientific interest due to their unique ability to survive extreme conditions, from intense radiation to the vacuum of space. A common question arises regarding their visibility: can these intriguing organisms be observed without specialized equipment? This exploration delves into their physical scale and the methods required for their observation.
Invisible to the Unaided Eye
Tardigrades are generally not visible to the naked human eye. Their minuscule size makes direct observation challenging, as most species range from 0.1 to 0.5 millimeters (mm) in length, though some larger individuals can reach up to 1.2 mm. A typical tardigrade is comparable in size to a grain of sand or even a period at the end of a sentence.
The human eye’s ability to resolve distinct objects is limited. Even a very large tardigrade would only appear as a tiny speck, making it impossible to identify any distinct features. The average human eye can resolve objects down to about 0.1 millimeters. For a clear view of these translucent, wiggling creatures, magnification is necessary.
Tools for Observation
Observing tardigrades requires a microscope. Both compound and stereo (or dissecting) microscopes are suitable for this purpose. A stereo microscope, offering magnifications from 20x to 30x, is useful for initially scanning samples and locating the slow-moving creatures. For a more detailed view of their characteristic eight legs, segmented bodies, and feeding apparatus, a compound microscope is ideal.
Compound microscopes require a total magnification of at least 40x to clearly see tardigrades. Magnifications ranging from 100x to 400x are used to observe their internal structures and behavior. To prepare a sample, a small amount of moss or sediment containing potential tardigrades is placed on a microscope slide with a drop of water, then covered with a coverslip. This wet mount allows the tardigrades to move freely and be easily viewed under transmitted light.
Where to Find Them
Tardigrades are remarkably widespread and can be found in various moist environments across the globe. They are frequently discovered in the water film coating mosses and lichens, which are common on trees, rocks, and even rooftops. Other habitats include leaf litter, freshwater sediments in ponds and lakes, and damp soil. Their ability to enter a dehydrated state, known as a “tun,” allows them to survive in environments that periodically dry out.
Collecting samples is a straightforward process. One effective method involves scraping moss or lichen from a surface and soaking it in non-tap water for several hours, or even overnight, to allow any dormant tardigrades to rehydrate and become active. After soaking, the water can be gently squeezed from the moss into a petri dish or a clear container. Since tardigrades are denser than water, they tend to sink, making it helpful to collect samples from the bottom sediment of the container for viewing.