2.+Tartigrade

Water Bears
 * Tardigrade:**

__**1. Classification/Diagnostic Characteristics**__ Classification: Ecdysozoans
 * The size of tardigrades are usually between 0.05 mm and 1.2 mm. (2)
 * Under many conditions, they go into a state called cryptobiosis in order to survive. During this state, the metabolic rate slows down, and a sugary gel called trehalose protect their internal organs. (7)
 * Have eight legs (first three pairs for moving, last pair for grabbing). (2)
 * Very resilient animals that can survive harsh conditions (pressure, temperature, humidity). (3)
 * They can live without food or water for up to 30 years by undergoing cyclomorphosis. (4)

__**2. Relationships to Humans:**__
 * Can withstand 1k more radiation than other animals. (4)
 * Water bears have a protein that can prevent DNA damage from X-rays. Recently, scientists were able to transfer the that protein into human cells. (7)
 * Tardigrades have also been shown to survive the vacuum of space. (5)

__**3. Habitat and Niche:**__ They are all considered aquatic because they need water around their bodies to do gas exchange. They can be found on lichens and mosses and sand dunes, soil, sediments and leaf litter. Their niche is very broad, as they can be found everywhere around the world as long as there is some water. They can survive in mountaintops, deep seas, tropical rainforests, and even the Antarctic. They survive in temperatures ranging form 0 degrees celsius to 30 degrees celsius.

Water Bears have been found in the hot springs of the Himalayas, in ocean sediment, and even under thin layers of ice. They also can be found in less intense environments like ponds, grass, lakes, stone walls, and roofs. They are most common in moist environments, but thrive anywhere they can retain moisture. @https://www.livescience.com/57985-tardigrade-facts.html

__**4. Predator Avoidance:**__ The main mechanisms for avoiding predators include the tardigrade's minuscule size (at most 1.5 millimeters), ability to propel itself using its clawed arms, and ability to survive extreme conditions that allow them to outlast any possible predators. That said, besides the main predators of worms, mites, spiders, and insect larvae, some tardigrade species are carnivorous and consume other tardigrades. They also must contend with free-traveling amoebae, parasites, and fungi that more slowly consume some species; however, most species are only most vulnerable when molting and thus exposing themselves to attacks. They consume plant material @http://www.baertierchen.de/wb_apr04.html []

__**5. Nutrient acquisition**__ Can eat entire live organisms such as rotifers. Moss, algae, and lichen to feed. They need to eat fluid in order to survive. However some species are carnivores and even canibals. Tardigrades have stylets, which are small, sharp mouth parts that pierce plant cells and animal body cells to suck the liquids out. They also have a long, bulbous structure inside their bodies that allow them to suck in their food and slowly digest it inside the bulb. @https://www.livescience.com/57985-tardigrade-facts.html

__**6. Reproduction and life cycle**__
 * Have a wide variety of reproducing mechanisms that are both sexual and asexual. (1)
 * Normally, they have male and female versions and they reproduce sexually. (1)
 * Some species, however, don't have a male version. (1)
 * In sexual reproduction, the female will lay 1-30 eggs which they leave in their shield molt, and then males will come and fertilize those eggs by depositing their sperm. (1)
 * In asexual reproduction, there are no males so females undergo pathogenesis where the females will lay eggs without the need of fertilization.
 * Their development is inexistent and experiences no larval stages. (1)
 * They generally live between 2-18 months.
 * If the tardigrade uses cryptobiosis - it can live over a hundred years. (7)
 * They molt 4- 12 times throughout their life span. (5)
 * They reach sexual maturity after around the 2nd and 3rd molt. (5)
 * Tardigrades are found in multiple locations because the eggs laid by the females are lightweight and carried to far off places by other animals passing by. (9)
 * Eggs are carried inside of the body and take up a lot of the space, as seen below:
 * [[image:https://www2.palomar.edu/users/warmstrong/images2/tardigrade6c.jpg width="414" height="170" link="@https://www2.palomar.edu/users/warmstrong/images2/tardigrade6c.jpg"]]

__**7. Growth and Development:**__ Grow up to 1mm long. They have a distinguishing characteristic with their cuticle, a stiff external covering. They grow my shedding, or molting the cuticle. they are able to to grow and develop to their environment; able to adapt to their environment.

The length of a tardigrade will vary among the 800 species, but they typically are around 0.05 mm and range from 0.0f to 1.2 mm. While they are born essentially as adults because they are eutelic, tardigrades are born with their complete number of adult cells, typically around 4,000. They can grow in size, however, by increasing individual cell size - not by cell division or the creation of more cells. @https://www.britannica.com/animal/tardigrade

__**8. Integument:**__ Some have a hard body armor called chitinous. Its muscular system has single muscle cells extending outwards with muscle bands. Water Bears have extremely tough skin, as they are able of surviving in extremely brutal environments. This makes the water bear an extremophile. []

__**9. Movement:**__ .
 * Tardigrades have multiple legs which they use to swim. (1)
 * The term tardigrade comes from the Italian "tardigrada" meaning slow stepper. Tardigrades (also known as "water bears") use eight legs to move, mostly through water, by oscillating front legs on one side and then on the other to writher forward. In laboratories, they have been documented to reach speeds of 13.3 cm per hour. (http://www.baertierchen.de/wbwb.html - video clip of "walking" tardigrade). (6)

__**10. Sensing the Environment:**__
 * Tardigrades can survive under extreme conditions. They go into a state called cryptobiosis in which they curl up into a dehydrated ball and retracting their limbs. This defense mechanism allows them to survive under harsh conditions and come back to life within a few hours. (5)
 * They have a tun state where body dries out and curls up into a ball (tun). Can survive in tuns for years, decades even. They can be distributed by wind and water. (3)
 * In response to a dry and hot environment, some tardigrades will produce __trehalose__, a type of sugar, which wraps itself around water molecules inside the tardigrade to prevent them from expanding and rupturing cells. They will also produce antioxidants to get rid of dangerous chemicals while in the tun state, which will also restore its DNA after it recovers. (2)

__**11. Gas Exchange:**__
 * A thin cuticle allows the exchange of gases, minerals, and water across the membrane's surface. (3)
 * There are no real organs for respiration (they don't have their own version of lungs). (12)
 * The oxygen diffuses through integument to store itself in within hemocoel. (6)

__**12. Waste Removal:**__
 * The excretory system includes a dorsal excretory gland and a pair of Malpighian tubes (2)
 * The water bear also depends on other organs in the body. Simple wastes travel through the midgut wall and expels. However, other wastes are excreted by molting, which usually keeps waste in the cuticle. (3)

__**13. Environmental physiology**__
 * When there is not enough oxygen in water, they stretch out. The metabolic rate slows down, and the muscles enough water and oxygen essential for survival. (4)
 * During extremely cold weathers, they form a tun that helps in preventing the growth of ice crystals.

__**14. Internal circulation:**__
 * They do not have specific organs for circulation, but there is fluid that transports blood and oxygen. (5)
 * This fluid is located in their body cavity, which spans across their entire body.(12)

__**15. Chemical Control:**__
 * Tardigrades are able to survive under some of the most hostile conditions. (13)
 * We can also predict that tardigrade distribution in nature will be found in neutral soils with a pH range of 6-8. (13)

__**16. Review questions:**__
 * 1) Where can tardigrades live around the world?
 * 2) How are tardigrades beneficial to humans?
 * 3) How do tardigrades move themselves around?
 * 4) How long can a tardigrade live for?
 * 5) How do tardigrades remove waste?

__**17. Review question answers:**__
 * 1) Tardigrades can live nearly anywhere around the world. They can be found in moss, ferns, soils, dunes, and deserts all around the planet.
 * 2) As humans, we can use tardigrades for research and learn about how they can survive for so long under rough conditions.
 * 3) Tardigrades use their little legs to swim around. During dry seasons, they curl up into a little ball called a tun.
 * 4) Tardigrades can live 2-18 months. However as a tun, a tardigrade can survive for decades or even longer. According to Smithsonian, with some water, they live for more than 100 years as a tun.
 * 5) They have an excretory system including a dorsal excretory gland and a pair of Malpighian tubes (2).

__**Sources**__
 * 1) http://bioweb.uwlax.edu/bio203/s2008/shifflet_bran/naughtybits.htm (1)
 * 2) https://www.livescience.com/57985-tardigrade-facts.html
 * 3) https://www.livescience.com/57985-tardigrade-facts.html
 * 4) http://palaeos.com/metazoa/ecdysozoa/tardigrada/tardigrada.html
 * 5) http://www.baertierchen.de/wbwb.html - video clip of "walking" tardigrade
 * 6) https://www.britannica.com/animal/tardigrade
 * 7) https://www.nature.com/news/tardigrade-protein-helps-human-dna-withstand-radiation-1.20648
 * 8) https://www.sargentwelch.com/www.sargentwelch.com/images/Tardigrades_(Water_Bears).pdf
 * 9) https://www.livescience.com/57985-tardigrade-facts.html
 * 10) http://www.bbc.com/earth/story/20150313-the-toughest-animals-on-earth
 * 11) http://www.biodiversityexplorer.org/metazoa/tardigrades/index.h
 * 12) []
 * 13) https://archive.org/stream/reactionoftardig00razl/reactionoftardig00razl_djvu.txt