Tardigrades are not mammals. These microscopic invertebrates, often referred to as “water bears” or “moss piglets,” belong to their own unique phylum called Tardigrada. Though both are animals, their fundamental biological characteristics place them in vastly different categories.
What Are Tardigrades?
Tardigrades are tiny invertebrates, typically ranging from 0.1mm to 1mm in length, making them visible only under a microscope. They are classified within the phylum Tardigrada. Their appearance is characterized by a plump, segmented body with eight short, unjointed legs, each ending in four to eight claws.
These creatures are found globally in a wide array of moist and aquatic environments, from the deep sea and sand dunes to mosses and lichens. They lack a circulatory or respiratory system; instead, oxygen diffuses directly through their integument. Their feeding involves a probe-like mouthpart called a stylet to pierce cells and suck out contents, primarily from plants and algae, though some are predatory.
Unique Adaptations of Tardigrades
Tardigrades are renowned for their extraordinary survival capabilities, which largely stem from a reversible state called cryptobiosis. In this state, their metabolism can slow to 0.01% of its normal rate, allowing them to endure conditions that would be lethal to most other life forms. This remarkable ability is divided into several types based on the environmental trigger.
Anhydrobiosis allows tardigrades to survive extreme desiccation by losing up to 97% of their body water. During this process, they curl into a compact, barrel-shaped structure known as a “tun,” minimizing their surface area and protecting their internal structures. They achieve this by producing unique proteins, such as tardigrade-specific intrinsically disordered proteins (TDPs), which form a gel-like substance within their cells, preventing damage and maintaining cellular integrity during water loss.
Beyond anhydrobiosis, tardigrades can also undergo cryobiosis in response to freezing temperatures, osmobiosis when faced with high osmotic pressure or salinity, and anoxybiosis due to a lack of oxygen. These adaptations enable them to withstand a broad spectrum of extremes, including temperatures from near absolute zero to above boiling water, high levels of radiation, and even the vacuum of space.
Distinguishing Features of Mammals
Mammals are a class of vertebrates with distinct characteristics. They have hair or fur covering their bodies, which helps with insulation and sensory functions. Females also possess mammary glands that produce milk to nourish their young after birth.
Mammals are also endothermic, meaning they can internally regulate and maintain a constant body temperature regardless of external conditions, often referred to as warm-blooded. Their circulatory system includes a four-chambered heart, which efficiently separates oxygenated and deoxygenated blood. Many mammals give birth to live young, rather than laying eggs, and possess a more complex brain compared to other animal groups. These characteristics highlight the significant biological differences between mammals and tardigrades.