The Internal Anatomy of a Water Bear

They are very small, have eight stubby legs, and are highly adaptable and surprisingly robust. Although tardigrades are also known as water bears, they can actually survive desiccation, as well as freezing temperatures and extreme heat – and even exposure to radioactivity doesn’t seem to do them much harm. So what does their interior anatomy look like? The American biologist and photographer Tagide deCarvalho used fluorescent dyes to answer that question in a spectacular fashion. The image won one of the top prizes in the Olympus Light Microscope Photo Competition 2019.

The photograph clearly shows the sturdy stylets (turquoise) in the oral cavity, which are toughened with crystals of aragonite. The tardigrade extends the stylets to grasp prey, which is then sucked into the pharyngeal bulb. The stylets are attached to appropriately powerful muscles, which can be seen near the salivary glands (green) in the image.

The water bear’s brain is a relatively simple affair. It consists of two cephalic ganglia, which form a nerve ring that encompasses the anterior end of the animal’s gut (blue). Several projections from the nerve ring connect it to the bilateral nerve cord consisting of four paired ganglia that innervate each of the legs. The gut occupies much of the middle of the body cavity and is associated with the excretory glands or Malpighian tubules, the tardigrade equivalent of the human kidney. The hindgut is lined with a layer of tough cuticle (bluish).

Some years ago, a group of German and Swedish biologists sent tardigrades into low Earth orbit on board an ESA satellite to test the limits of their surprising robustness. Not all of the animals survived the rigors of space flight, but some proved capable of withstanding both cosmic radiation and the chilling temperatures. When the researchers examined the biological freight on its return to Earth, survivors were found even among the group that had been subjected to the most extreme conditions. Not only that, these hardy returnees also reproduced normally, indicating that their genetic material had not suffered any significant damage during the trip. How tardigrades manage to protect the integrity of their DNA during periods of desiccation and exposure to radiation remains a mystery.