Hypsibius dujardini

Kingdom: Animalia

Phylum: Tardigrada

Class: Eutardigrada

Order: Parachela

Family: Hypsibiidae

Genus: Hypsibius

Species: Hypsibius dujardini

Introduction

Hypsibius dujardini is a species of tardigrade, also known as a water bear. Water bears are found in the Ecdysozoa, meaning they have the capability to molt. This places them in close proximity to both Arthropoda and Nematoda, two extremely important groups for laboratory study. Because of this, interest in tardigrades has expanded recently. The phylum Tardigrada is most notorious for its hardiness relative to other animal species, and H. dujardini has been used in multiple experiments examining this phenomenon (Goldstein, Blaxter).

In terms of appearance, tardigrades are microscopic organisms that are roughly half a millimeter long at the largest. H. dujardini is four segments long, and each segment has a pair of legs attached to it. H. dujardini is able to grip surfaces such as moss by using claws found on each foot. H. dujardini is transparent in coloration (Surette).

Development Research

Compared to other animal groups, the development of Tardigrada was a mystery. In the 1960s, Sydney Brenner, the scientist responsible for C. elegans' dominance as a model organism, almost selected tardigrades as his choice of study instead. However, he noticed that C. elegans was simpler in composition compared to tardigrades; primarily in the nervous system. Species in Tardigrada have received little attention from a developmental standpoint since.

Scientists decided to change this in 2007 and examined Hypsibius dujardini's early development. They found that H. dujardini has a compact genome. Interestingly enough, this genome is actually smaller than that of both Drosophila flies or C. elegans. They also discovered that H. dujardini generations are short, clocking in at around two weeks at room temperature. These factors make it a great candidate for a model organism.

They also studied early embryos of H. dujardini and saw asymmetrical cell divisions, nuclear migrations, and cell migrations. The study was finally able to chart a cell lineage for embryos of H. dujardini, presenting a clearer understanding regarding their development (Gabriel).

Cryptobiosis

Tardigrades are extremely well known almost solely due to their capability to undergo cryptobiosis. There are several forms of cryptobiosis, and each is used to adapt to different conditions.

• - Anhydrobiosis is an animal's response to a lack of water.

• - Cryobiosis is a response to extremely low temperatures.

• -Anoxybiosis is a response to the lack of oxygen

• - Osmobiosis is an animal's response to a change in salt concentration.

Some groups of animals can employ a singular form of cryptobiosis, but tardigrades are unique. They are capable of using many forms of cryptobiosis, and as a result, be ridiculously resilient to a variety of extreme environments. The ability to become dormant when faced with adverse conditions, potentially for years, is what allows this phylum, and this species, to colonize almost every type of habitat on earth.

When exposed to low amounts of water, tardigrades can reject up to 95% of the water in their body to enter a dormant state that withdraws its legs and contracts longitudinally. This form is known as a tun. Tuns are able to survive extreme conditions until they improve. A tardigrade can enter this phase of survival at any time during its life cycle, including when it is an egg. However, survivability has been shown to be higher in later stages than early ones.

H. dujardini is a freshwater tardigrade, and lives habitats with more consistent temperatures than hardier, terrestrial species. This creates an interesting situation regarding the species' capability to go through cryobiosis compared to other species. Scientists compared H. dujardini 's survivability when exposed to freezing temperatures to other species of tardigrades from a variety of habitats. They discovered that tardigrades from a terrestrial habitat are much more resistant to freezing temperatures than freshwater tardigrades. However, 20% of individuals were still able to survive at temperatures as low as -80 degrees Celsius. (Bertolani).

The physiological reasons for this phenomenon are of great interest to scientists in recent years. Members of Tardigrada's ability to restart metabolism many years after they have been inactivated creates a discussion on what it means for a tissue to be dead and if restarting metabolism is possible in other organisms (Møbjerg). Additionally, it is a mystery as to why the Tardigrade's ability to withstand extremes has allowed it to survive conditions that far exceed regular habitat fluctuations. The phylum, and H. dujardini in particular, is about to lead to a new phase of biological breakthroughs.

Gamma Radiation

Interestingly, the ability to resist dessication has also given Tardigrada the ability to resist gamma radiation. A study exposed H. dujardini to gamma radiation. The specimens were able to survive Gamma radiation up to around 4200 Gray, which is comparable to terrestrial species. Additionally, even though they did survive, fertility was drastically reduced to irradiated specimens, even those only irradiated to 100 Gray. Finally, newly created eggs were more likely to not hatch in earlier stages than later ones when exposed to radiation. (Beltran-Pardo).

Scientists were baffled by this phenomenon, as H. dujardini is a freshwater species. As demonstrated by the cryobiosis experiment, H. dujardini and other freshwater tardigrades are much less resistant to sudden habitat changes. The strangely high tolerance for gamma radiation suggests that the mechanisms used for other forms of cryptobiosis are independent from H. dujardini''s high gamma radiation resistance.

They theorize that this may be due to Tardigrada having barely any cells function during periods of inactivity. Gamma radiation would not be able to affect cells well since the processes it commonly affects, like cell division, are not happening.

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