Phylum Tardigrada: From the Ocean Floor to Earth’s Orbit

By Nick Wingfield

Tardigrade, moss piglet, waterbear, all synonyms for one of the most extreme microbial animals that you never knew existed. With an average size of only 0.5 millimeters(3) (barely larger than these periods), a person might ask “How can these guys be interesting or extreme?” Well, by the end of this entry you won’t just be interested in tardigrades, you’ll want to be one.

As a phylum, Tardigrada are most closely related to Onycophorans and Arthropods (3). Their appearance resembles something like an earth worm crossed with a shar pei, with a little bit of grizzly bear thrown in. They are cylindrical and bulky, with their bodies split into five sections; a defined head and four body sections (6). On the head you will find a simple eyespot, as well as a sucking pharynx and basic mouth parts (6). Due to their microscopic size, tardigrades usually feed on bacteria(6). There is however, one big mean eating machine known as Milnesium tardigradum, that is carnivorous and grows up to 1.2 millimeters long who is known for eating smaller invertebrates and other tardigrades (2).  Each body section contains a pair of short, stubby, clumsy looking legs equipped with some form of claws that can vary in structure between species(6). The rear pair of legs, however, are not like the others. They are attached backwards. These legs are generally used for grasping, and allows for tardigrades to contort and climb all around their environment(6).

Water bear (tardigrade), Hypsibius dujardini, scanning electron micrograph by Bob Goldstein and Vicky Madden, http://tardigrades.bio.unc.edu/. Used under the Creative Commons Attribution-ShareAlike 3.0 Unported license.

Unlike their insect cousins, tardigrades show much less diversity with approximately 1000 species discovered so far, possessing only 10 discernable physical characters (2,3). They are however found in much more diverse habitats. They are prevalent in Antarctica, found on mountain tops, barnacles, riverbanks, and even present on the ocean floor(2,3,6,7,8). They live amongst lichens, mosses, barks, various sediments, basically anywhere with food (6,7).

The ability of a tardigrade to undergo cryptobiosis is the reason for this huge habitat tolerance (7, 8). Basically, this means a tardigrade can shut down its body and stop the aging process until favorable conditions arise. This can be induced by freezing, osmotic changes, anoxia (lack of oxygen), or most famously, anhydrobiosis (almost total dehydration) (4). Through these techniques tardigrades can survive exposure to the vacuum of space, boiling ethanol, CO2,H2S, huge doses of radiation, and nearly a decade in -80 degrees Celsius (7). While it appears that the upper range of survival in cryptobiosis is a decade, there have been observations suggesting near rehydration from pieces of wood kept in a museum after as much as 100 years! (4)

The dominant speciesat utilizing cryptobiosis is again M. tardigradum. While the exact mechanism is not known, this species has adapted some cellular skill that allows it to survive the most extreme environmental stresses (8). When sent into low orbit, this species was the only one to survive vacuum and complete exposure to all levels of solar radiation, as well as their eggs still bearing young when exposed to space vacuum conditions (5).

Even more incredible than space survival is a tardigrade’s inert radiation tolerance. These little guys are tougher than the incredible hulk. A lethal dose of gamma radiation to a human is in the 7.5-10 Gy (j/kg) range, while a fly can live up to around 1400 Gy (J/kg), but astoundingly the average tardigrade can survive exposures of up to 6000 Gy(1). In a recent study however, it was seen that our friend M.tardigradum’s eggs (that’s right, unborn animals) can survive exposures of up to 500 Gy, by apparently adjusting the very mechanisms of mitosis(1). So, even if you still don’t want to become a tardigrade, I definitely wouldn’t mess with them; they’ll outlast you everytime.

References

(1) Beltran-Pardo E, Jonsson KI, Wojcik A, Haghdoost S, Harms-Ringdahl M, Bermudez-Cruz RM, and Villegas JEB. 2013. Effects of Ionizing Radiation on Embryos of the Tardigrade Milnesium cf. tardigradum at Different Stages of Development. PLoS ONE 8(9): e72098.doi:10.1371/journal.pone.0072098

(2) Blaxter M, Elsworth B, and Daub J. 2004. DNA taxonomy of a neglected animal phylum: an unexpected diversity of tardigrades. Proc Biol Sci.. 271:S189-S192

(3) Campbell LI,  Rota-Stabelli O, Edgecombe GD, Marchioro T, Longhorn SJ, Telford MJ, Phillippe H, Rebecchi L. Peterson KJ, and Pisani D. MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that velvet worms are the sister group of Arthropoda. PNAS 108(38):15920-15924.

(4) Jonsson KI, and Bertolani R. 2001. Facts and fiction about long-term survival in tardigrades. Journal of Zoology, London. 255:121-123

(5) Jonsson KI, Rabbow E, Schill, RO, Harms-Ringdahl M, and Rettberg P. 2008. Tardigrades survive exposure to space in low Earth orbit. Current Biology. 18(17):R729-R731

(6) Kinchin, I. M. 1994. The Biology of Tardigrades. London and Chapel Hill, N.C.: Portland Press.

(7) Sands CJ, McInnes SJ, Marley NJ, Goodall-Copestake WP, Convey P, and Linse K. 2008.Phylum Tradigrada: an “individual” approach. Cladistics 24:861-871.

(8) Wang C, Grohme MA, Mali B, Schill RO, and Frohme M. 2014 Towards Decrypting Cryptobiosis—Analyzing Anhydrobiosis in the Tardigrade Milnesium tardigradum Using Transcriptome Sequencing. PLoS ONE 9(3): e92663. doi:10.1371/journal.pone.0092663