There are over 1,000 species of Tardigrades, also known as water bears or moss piglets, found in different environments throughout the world(Tsujimoto, Suzuki, & Imura, 2015). A variety of scientific studies agree that Tardigrades are considered to be the sister group of arthropods(Nelson, 2002), Acutuncus antarcticus is one of the fifty nine (59) species of Tardigrade found in Antarctica(Tsujimoto, Suzuki, & Imura, 2015). A. antarcticus is the most widespread and the most abundant tardigrade found in the soil and freshwater environments of Antarctica(Tsujimoto, Suzuki, & Imura, 2015). A. antarcticus has been found in east, west, and sub-Antarctic islands of Antarctica(Czechowski, et al., 2012). A. antarcticus was previously known as Hypsibius antarcticus and Macrobiotus antarcticus but was re-described, renamed and accepted as A. antarcticus by F. Richters in 1904(ITIS Report, 2017), and the classification of A. antarcticus is now under the class of Eutardigrada, the order of Parachela, and the family of Hypsibiidae (ITIS Report, 2017).
A. antarcticus along with many other tardigrade species are micro-metazoans which have: long cylindrical type bodies, a complete digestive system, a fluid-filled body cavity, a nonliving cuticle that is molted periodically, four pairs of unsegmented legs with sucking disks or asymmetric claws at the end, and a brain connected to a ventral nerve cord with fused ganglia(Nelson, 2002). The average adult body length is 0.3-0.5 mm(Kagoshima, Imura, & Suzuki, 2013), and they are transparent except when they have food in their intestine(Kagoshima, Imura, & Suzuki, 2013).Tardigrades have a complex feeding apparatus, typically a circular mouth with muscular pharynx to suck up their food(Guidetti, et al., 2012). Additionally, A. antarcticus are also characterized as herbivores. Food generally consists of chloroplasts and other cell content from moss, lichen, and algae(Nelson, 2002).
Furthermore, Tardigrades are considered to be aquatic animals because all Tardigrade species require a thin layer of water around the body to be active(Nelson, 2002). They also have the ability to survive extreme environmental conditions by entering different types of cryptobiosis, a form of dormancy when an organism’s bodily functions, such as metabolism and reproduction, slow down or stop completely during times of extreme environmental stress and return to normal when conditions become more favorable(Nelson, 2002).
Surprisingly, the average life span of A. antarcticus is only about 60-90 days not including anytime spent in a cryptobiosis state(Altiero, Giovannini, Guidetti, & Rebecchi, 2015). A. antarcticus is a parthenogenetic (eggs develop without being fertilized) species and can usually lay their first clutch of eggs as early as 9-10 days of being hatched(Tsujimoto, Komori, & Imura, 2016). They reproduce continuously and almost throughout their entire lives. They can reproduce as often as every 5 to 8 days with a normal clutch size of 3-6 eggs(Tsujimoto, Komori, & Imura, 2016). A distinguishing factor of A. antarcticus is the fact that they lay their eggs freely while other very similar species lay their eggs in exuviae (inside their molted exoskeleton; (Kagoshima, Imura, & Suzuki, 2013). The eggs of A. antarcticus are roughly 0.07-0.08mm in diameter(Kagoshima, Imura, & Suzuki, 2013). The eggshell is also transparent and has many spines or rods attached to its surface(Kagoshima, Imura, & Suzuki, 2013).
It has been hypothesized that A. antarcticus success in surviving in Antarctica is due to their ability to reproduce quickly and parthenogenetically and survive freezing or dry seasons by means of cryobiosis or anhydrobiosis, respectively(Tsujimoto, Komori, & Imura, 2016). However, because many individual Tardigrade species such as A. antarcticus are not well studied it is difficult to give a good estimate on their population size or conservation status(Nelson, 2002).
