The tardigrade, affectionately known as the water bear or moss piglet, is a microscopic invertebrate. These small animals, first described in 1773, represent one of the most durable forms of life on Earth. A common question arises from their fame: can you actually see one without a microscope? The answer to this is largely no, but the specifics of their size and the limits of human vision explain this invisibility.
Tardigrade Size and Visibility
Tardigrades are not truly invisible, but their size places them below the threshold for detection by the unaided human eye. The majority of adult tardigrade species measure between 0.3 millimeters and 0.5 millimeters in length when fully grown. While some larger species can reach up to 1.2 millimeters, this is a distinct exception rather than the rule. The human eye’s resolving power is approximately 0.1 millimeters under ideal conditions. Because most tardigrades are smaller than this visual limit, they generally appear as nothing more than a tiny, almost imperceptible speck of dust, even when moving. Observing the animal’s eight-legged, segmented body and distinctive features requires magnification, often starting with a low-power stereo microscope.
What Exactly is a Tardigrade?
Tardigrades belong to their own phylum, called Tardigrada, which places them in a distinct classification within the animal kingdom. They are micro-invertebrates with a short, plump, segmented body that is typically divided into a head and four body segments. Each of the three trunk segments and the terminal segment features a pair of stubby, unjointed legs, totaling eight legs, each usually tipped with four to eight claws. Their movement, for which they were named “slow steppers” (the literal translation of Tardigrada), is a clumsy, bear-like waddle across surfaces.
The animals possess a specialized mouth apparatus that includes a pair of sharp, needle-like stylets used to pierce the cells of their food sources. They then use a muscular, sucking pharynx to extract the internal fluids from plants, algae, or other small invertebrates. The body cavity of a tardigrade is filled with fluid, known as a hemolymph, which serves as a hydrostatic skeleton for support since they lack internal bones. They do not have a specialized circulatory or respiratory system; instead, they rely on the diffusion of oxygen directly through their body surface. These simple yet robust biological characteristics have contributed to their extensive evolutionary success, with fossil records dating back over 500 million years.
Where and How to Find Them
Tardigrades are ubiquitous, found in nearly every habitat across the globe, from the deepest ocean trenches to the highest mountain peaks. They are considered aquatic animals because they require a thin film of water around their bodies to facilitate gas exchange and prevent desiccation. They are most commonly found in moist environments. The most accessible places to find them are in terrestrial micro-habitats like mosses, lichens, and leaf litter, which retain moisture effectively.
A simple method for a layperson to collect them involves gathering a small clump of moss or lichen and placing it in a shallow dish. Soaking the sample in distilled water for three to 24 hours encourages the tardigrades to emerge from their dormant state and become active. After soaking, the water can be gently squeezed from the moss into a clean dish and examined. While a magnifying lens may reveal them as moving specks, a low-power student microscope is necessary to properly observe their characteristic shape and movement.
Their Remarkable Survival Abilities
Their secret lies in a reversible, dormant state called cryptobiosis, which they enter when their environment becomes too harsh. This survival mechanism is triggered by extreme conditions such as desiccation, freezing, or lack of oxygen.
The most common form of cryptobiosis is anhydrobiosis, where the tardigrade expels up to 97% of its internal water. It retracts its head and legs, curling into a barrel-shaped structure known as a “tun.” In this tun state, the animal’s metabolism slows down to less than 0.01% of its normal rate.
While in this desiccated form, the tardigrade can survive incredible extremes, including temperatures as low as -272°C and as high as 150°C. They can also withstand pressures six times greater than those found in the deepest ocean and exposure to high levels of ionizing radiation. This tolerance is partly due to unique protective proteins, such as the Damage Suppressor (Dsup) protein, which shields the animal’s DNA from damage. The tun state allows them to survive the vacuum of space, a feat demonstrated by exposing them to the harsh environment outside a spacecraft. Tardigrades can remain in this suspended state for decades, reviving and resuming normal life within hours of rehydration.