Wednesday, April 7, 2010

Unnecessary adaptations?

The Tardigrade (or water bear) is a microscopic animal typically between 1.5 and .1mm. Despite their small size water bears are some of the most durable animals. They are able to live at temperatures as low as -273 degrees Celsius (thats 1 Kelvin almost absolute zero) and as high as 151 degrees Celsius (a 1000 times more radiation than humans can handle). Additionally water bears have survived 10 days in the space before coming back to earth and laying eggs hatching normal offspring making them the only known animals to have survived in a vacuum. Though impressive these adaptations do not seem to serve a practical purpose. There are no environments on earth that are found to be near absolute zero or comparable to space, so why do you think these adaptations evolved? More importantly how do you think these adaptations came about? What sort of conditions could lead to such extreme survival skills?


  1. You pose a good question. Let’s first start off with why they are capable of living in extreme temperatures. Tardigrades can live just about anywhere, spanning from the rainforest to Antarctica. In order to survive in all these places the tardigrade must be able to endure changes in temperature. With having an ability to withstand extreme hot and cold temperatures, they can cope within any environment without endangering themselves. You must also remember, though they survive these conditions, they aren’t still freely moving around unfazed. When conditions turn unfavorable they turn into what is called tuns “by replacing all the water in their bodies with a specialized sugar called trehelose which protects their cell membrane from damage,” (First Science). This hibernation state is what they use when enduring high pressures, high and low temperatures, low oxygen, or dryness. They can stay in this state for long periods of time until conditions become favorable again. A question that might appear is where they’d face these conditions. Water bears have been seen to live in the Himalayans and deep in the ocean where air pressure and water pressure is great, in water with low concentrations of oxygen, and when the moss they live in dries up they face the danger of dehydration. Like most adaptations the tardigrade’s extreme survival skills were probably formed after many generations of natural selection. The tardigrade wouldn’t have started off being able to withstand the heat of volcanoes or the coldness while living within ice. As Coyne discussed in his book about the perfect design, “mutations occur regardless whether they’d be useful for the individual” (118). In other words, a tardigrade might have had a mutation that made it more capable to withstand harsh conditions that improved its chances of reproducing and passing on the adaptation to its offspring. Eventually the adaptation, that would have probably been a lot more minor at the time, grew to what it is now.

    Now let’s address how being able to “live in space” could even be possible. According to an article about tardigrades and space research, the ability to withstand space could be a byproduct of other adaptations. In space a person would face freezing conditions, desiccation (drying out), and radiation (Spaceref). These are basic adaptations that tardigrades use in order to survive in environments on earth. When you phrase it as “survive space” it does sound extraordinary, but once what problems you’d face in space are broken down you can realize it doesn’t sound as incomprehensible. To go in to more detail, the normal hibernation state the tardigrade uses here on earth helps them survive in space. As brought up beforehand, in the tardigrade’s hibernation they form a sugar in their body called trehelose while the rest of their body dries out. This sugar, according to the same article, is protective from the irradiation that makes it possible for them to tolerate it more than hydrated animals.

    The book

  2. Tardigrades may not have to survive conditions such as absolute zero or vacuum on Earth, but they actually do live in similarly harsh conditions such as the Himalayas, hot springs, ice sheets, and ocean sediments (wikipedia). Survival in these harsh conditions, where there is virtually no competition from other organisms and thus an opportunity for a well-adapted organism to claim a unique niche, requires special adaptations that have been naturally selected over millions of generations. In the case of the tardigrade, this adaptation is the ability to reversibly suspend metabolism and go into a state of cryptobiosis, in which it almost completely shuts off metabolic activity and ensures its survival on almost no energy until environmental conditions improve. When this happens, the organism can return to its normal metabolic state (Wikipedia). It is through this process that tardigrades have survived absolute zero and space; they simply shut down, wait to return to less extreme conditions, and then resume life. In the Himalayas and other extreme habitats, conditions can obviously deteriorate quickly and render the place uninhabitable. By the process of natural selection, which undoubtedly killed off all tardigrades that had not developed cryptobiosis, evolution allows the tardigrade to survive when conditions become too harsh.