In less than two months I will be leaving for Antarctica. Before I leave I want to provide a few journal entries with a little bit of background information about tardigrades. To start things off here is some general information and a few pictures.
This scanning electron microscope (SEM) image shows a Eutardigrade (Macrobiotus spp.). Pictures from a SEM only show surface details. Specimens are prepared for SEM in a multi-step process that involves completely drying the animal and then coating it in gold before images can be taken.
First Encounters
I don't remember how I first heard about tardigrades, but I do remember how excited I was when I first saw one lumbering around in the bottom of a petri dish. I had heard about the tardigrades' ability to survive extreme environmental conditions. At the time it seemed almost impossible that something so small could survive at all let alone survive some of the most extreme conditions imaginable. I was and still am amazed that an animal so tiny can exist while having many of the characteristics found in much larger animals. I was hooked from the first moment I saw them. Tardigrades have had their devotees since they were first discovered in the second half of the 18th century. In the 1770s Johanne August Ephraim Goeze and Lazzaro Spallanzani were among the first to ever record seeing these remarkable animals. I imagine that these pioneering microscopists were also excited to see tardigrades slowly ambling along amongst other species that often seem frantic by comparison. Goeze and Spallanzini are responsible for the common name "Water Bear" and formal name "Tardigrada" respectively.
Characteristics
Tardigrades are microscopic animals with five body segments and four pairs of claw-tipped legs. Their bodies are covered with a cuticle that must be shed (ecdysis) as they grow. They have many of the same internal body systems found in larger animals: nervous, excretory, muscular, digestive and reproductive. Their small size eliminates the need for a circulatory or respiratory system.
Digestive System
The digestive system of a tardigrade consists of a mouth which opens into a buccal tube that terminates in a muscular pharynx. Behind the pharynx are the midgut and hindgut which are similar to the stomach and intestines of humans. There is a stylet located on both sides of the buccal tube. The stylets are thrust forward like tiny daggers that pierce the cells and bodies of prey items allowing the tardigrade to drain the nutrient rich fluids. The pharynx is muscular and helps pull the food into the digestive tract. The pharynx may have placoids which are small mineralized structures that help grind up food items like the stones in the gizzard of a bird.
Classification
There are about 1000 different species of tardigrades that have been named and classified and there are probably many more undiscovered species. Tardigrades fit into two major classes: Heterotardigrada and Eutardigrada.
Heterotardigrades
Heterotardigrades have hair-like sensory structures unbranched claws or toe pads and the terrestrial ones have armored plates on their backs.
A Heterotardigrade (Echiniscus spp.) with characteristic armored plates.
Eutardigrades
Eutardigrades have branched claws and lack the armored plates found on the Heterotardigrades.
A typical Eutardigrade (Macrobiotus spp.). Notice the lack of dorsal armor plates unlike the Heterotardigrade above.
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