Sunday, 18 September 2011

Water Bears, born to survive

Kingdom: Animalia
SuperPhylum: Ecdysozoa
Phylum: Tardigrada

Despite their misleading name, water bears (also known as moss piglets) are microscopic organisms that were first found and described by Johann August Ephraim Goeze during 1773. What is so unique about about them is their astonishing ability to thrive or at least survive in almost any kind of environment, even under conditions that would most certainly kill any other animal or organism in an instant

They can handle and survive in temperatures that are close to the absolute zero (−273 °C , or −459 °F), temperatures going as high as 151 °C (304 °F), survive a decade without water and handle at least 1,000 times more radiation than any other animal. Actually in an experiment performed during the September of 2007, tardigrades were placed into low Earth orbit and were exposed for 10 days in the vacuum of space. After the mission returned the majority of them had managed to survive and after a while they even mated and produced healthy offsprings!

The "water bear" name was given to these animals firstly because the way they walk is similar to how a bear walks, and secondly due to the fact that they prefer water-rich environments.
Echiniscus sp
Water Bear Description 
Water bears form the phylum known as Tardigrada, which is a part of the super phylum Ecdysozoa. There are more than 1,000 water bear species known and described as of today
Most of them have a total length of 0.3 to 0.5 millimeters (0.012 to 0.020 inches) with the largest species having a total length of 1.2 millimeters (0.047 inches). These microscopic animals have barrel-like segmented bodies with 8 stubby and joint less legs arranged in 4 pairs. Each foot has many claws ranging from 4 up to 8. Their body has always 4 segments or five if we include the head.

They are eutelic animals (they have a fixed number of cells), with the adults of some species having more that 40.000 cells, however the majority of them has far less. Water bears have an average life span of three to thirty months.
Their cuticle is mainly made out of chitin and is moulted (shed) up to twelve times during their lifetime. Although their bodies do have a circulatory system (haemocoel), true coelom exists only around their gonad. They have no respiration organs, with gas being exchanged all around their bodies. Their brain contains multiple lobes and is connected to a big ganglion found underneath the esophagus. From there a double ventral nerve runs through the entire body.

It has been found that a few water bear species have three tubular glands around the rectum. Their exact function is yet unknown although it is speculated that they are excretory organs

Water Bear Habitat 
Tardigrades can be found all over the world, from the highest peak at the Himalayas to the deepest ocean floors (below 4,000 meters (13,000 ft)) and from the poplars to the equator surviving in all kinds of environments including:
  • Deserts 
  • Beaches 
  • Marine, freshwater and ocean sediments 
  • Deep in the soil 
  • Hotspings 
  • Near volcanoes 
  • Solid Ice 
  • Lakes 
  • Ponds 
  • Meadows 
  • Human made structures (houses, walls, antennas) 
  • And probably any other place you can imagine 
Typically the best place for most species to thrive is on lichens and mosses.
Hypsibius dujardini
Water Bear Diet 
The majority of water bears is phytophagous (eating plant matter) or bacteriophagous (eating bacterias), however there are also some that are carnivorous (for instance the Milnesium tardigradum).
In order to feed, Water bears make use their tubular mouths (which are armed with stylets) to perforate plants, plant cells, algae and small invertebrates. Then they open their mouths sucking the fluids that are released from the piercing.

Water Bear Reproduction 
Most Tardigrades reproduce bisexually however there are also some species that reproduce via parthenogetic means (eggs are fertilized without the need of male sperm).
Male water bears have two ducts running from the testis, opening though a single pore in front of the anus while females have a single duct that opens either above the anus or directly inside it, thus forming a cloacae (opening for intestinal, reproductive, and urinary functions). They are oviparous animals with the fertilization typically taking place externally. Females release their eggs by moulting, with the eggs contained inside the shed cuticle. Afterwards males cover them with sperm
Regardless the species, it takes a maximum of 14 days for the eggs to hatch. As mentioned before water bears are eutelic animals and the hatchlings already have the same number of cells as adults do. Their development to adults is the result of these cells growing in size (a process known as hypertrophy) with no cell division whatsoever taking place.

Water Bears born to be survivors Water Bears have the ability to survive all kinds of harsh conditions by entering a state known as cryptobiosis. During this state their water content drops up to 1 % of their normal while their metabolism is suspended and lowered to less than 0.01 %. During the cryptobiosis state they can survive all kind of harsh conditions including: Extremely high or low temperatures: They can handle heat of 151 °C for many minutes or cold at -200 °C for many days and even a few minutes at -272 °C. Take into consideration that most organisms would die in just mere seconds if not instantly under these temperatures.
  • Pressure: They can easily handle extremely low and high pressures. As mentioned in the beginning water bears have even managed to survive in the low pressure of space, which is also accompanied by solar radiation.
  • Dehydration: Certain Tardigrades can survive 10 or more days without having access to sources of water
  • Radiation: Water bears can withstand doses of 5,000 Gy (of gamma-rays) and 6,200 Gy (of heavy ions). 5-10 Gy is considered to be lethal for humans while 600-900 is lethal for cockroaches

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1 comment:

  1. it says they can not have water for a decade, but then it says their life spawn is three to thirty months?!