Milnesium tardigradum by Wade Schaeffer

Introduction:

Milnesium tardigradum are a species known as water bear or tardigrade. These microorganisms may not seem like much, but they may hold the future to space travel and resistance to extreme environments for humans. They are theorized to live very long in a dormant state, but only live for about a month and a half during active states. While thought to be a species found around the world in every environment, new evidence hints that this may not be the case.

Life history:

Tardigrades live in moist mosses that can dry out, making them form structures that resist dehydration called tuns. This form allows tardigrades to resist most extreme environmental factors, which is what most people know tardigrades for. The life cycle of tardigrades is a challenge to study, due to the size of the specimens (.7 millimeters in length) and their habitat is hard to recreate in a laboratory. Atsushi Suzuki found a rearing system that allowed for a tardigrade's life cycle to be studied from egg-laying through death.

In the sixteen female individuals studied, the average life span was 41 days, where sexual maturity was achieved on days 12-18. The mean clutch size was 6.9 eggs, and the average interval between clutches was 8.1 days, allowing for up to 6 theoretical clutches (5 were observed in the most productive individual, but she did produce 41 eggs). Right before laying the eggs, tardigrades sought out a structure to wedge themselves in, as the action of laying eggs made them vulnerable to possible predation. Laying the eggs lasted no longer than two minutes, where the female laid the eggs right in the cuticle. The females stayed inside the old cuticle for several hours after egg laying, where after ecdysis would occur, or the individual would die. The eggs would develop over the next six days, hatching and escaping the cuticle to repeat the process. Since Suzuki's experiment only studied female individuals, this life history may not be representative of males in Milnesium tardigradum. Her experiment also only studied individuals who were active throughout their entire life cycle, so the life span of 41 days only applies to active individuals.

Behavior:

Milnesium tardigradum behave much like most other microorganisms, avoid environmental stressors and predators, find food and mates, and conserve energy whenever possible. The average rate of movement in water bears is 23.3 millimeters per hour (only 33 body lengths per hour!), which agrees with their Latin etymology of the phylum name of "slow walker." The fastest a tardigrade was observed going was 1166.4 millimeters per hour, 50 times the average speed. This max speed can only be held for a few seconds, and is hypothesized to be used under negative environmental stimuli, such as predation. (Shcherbakov et al.)

Tolerance to radiation and heat stress:

As stated previously, tardigrades under extreme environmental conditions form a structure called a tun (pictured above). When in a tun state, tardigrades lose water from their cells and are able to conserve massive amounts of energy, almost like a state of hibernation. Milnesium tardigradum are able to survive up to 5000 grays of gamma radiation, and 6200 grays of heavy ion radiation in this tun form (one gray is equal to one Joule per kilogram of energy absorbed). To put that in perspective, a human exposed to 10 grays of gamma radiation will die within a few hours. The minimum amount of radiation where no adverse effects are observed is slightly larger than 1000 grays of gamma radiation, as that results in individuals becoming sterile. This result is replicated in the tun state as well. Eggs can be laid after exposure to greater than 1000 grays of gamma radiation, but there will be no development in them. (Horikawa et al.)

The protein hsp70 allows tardigrades to resist heat shock by allowing other proteins to fold under high amounts of heat. Water bears exposed to heat and allowed to go into cryptobiosis (tun state) and back out have large quantities of the hsp70 proteins, more than originally starting with. Hsp70 protein levels increased going into cryptobiosis, but no production of hsp70 occurred in the cryptobiosis stage. Coming out of the dormant state, hsp70 levels minorly decreased, hinting that some of the proteins fell apart during this transitional state, and no more hsp70 proteins were made as well. It is speculated that after being exposed to heat and going through and out of cryptobiosis, a gene is activated to produce excess hsp70 proteins, as individuals after exposure had triple the amount of hsp70 proteins to that of a tardigrade that was not exposed to heat. (Schill et al.)

A new discovery:

Milnesium tardigradum is widely considered cosmopolitan species, or a species that is found widely across most, if not all, regions of the world. This may not be true now, as there is more understanding of the tardigrade phylum and how to distinguish these tardigrades apart. When looking for water bears in Japan, Kenta Sugiura and other authors found Milnesium tardigradum, Milnesium inceptum, and Milnesium pacificum in various habitats. Sugiura confirmed that Milnesium tardigradum existed in Japan, but the number of confirmed sightings of this species is somewhat limited. These include Russia, Europe, and now Japan. What Sugiura noted was Milnesium tardigradum was only found in colder climates in Japan, something validated in the other confirmed sightings of tardigradum, hinting that this species prefers cold climates, where cryptobiosis is required during winter months. Although the confirmed sightings are only in cold climates, there may be other climates that Milnesium tardigradum can live in, but this hints at Milnesium tardigradumn not being a true cosmopolitan species.

Bibliography:

Atsushi C. Suzuki "Life History of Milnesium tardigradum Doyère (Tardigrada) under a Rearing Environment," Zoological Science 20(1), 49-57, (1 January 2003). https://doi.org/10.2108/zsj.20.49

Horikawa, Daiki D., et al. “Radiation Tolerance in the TARDIGRADEMILNESIUM TARDIGRADUM.” International Journal of Radiation Biology, vol. 82, no. 12, Dec. 2006, pp. 843–848., https://doi.org/10.1080/09553000600972956.

Kenta Sugiura, Hiroki Minato, Midori Matsumoto, and Atsushi C. Suzuki "Milnesium (Tardigrada: Apochela) in Japan: The First Confirmed Record of Milnesium tardigradum s.s. and Description of Milnesium pacificum sp. nov.," Zoological Science 37(5), 476-495, (27 August 2020). https://doi.org/10.2108/zs190154

Schill, Ralph O., et al. “Stress Gene (HSP70) Sequences and Quantitative Expression in Milnesium Tardigradum (Tardigrada) during Active and Cryptobiotic Stages.” Journal of Experimental Biology, vol. 207, no. 10, 15 Apr. 2004, pp. 1607–1613., https://doi.org/10.1242/jeb.00935.

Shcherbakov, Denis, et al. “Movement Behaviour and Video Tracking of Milnesium Tardigradum Doyère, 1840 (Eutardigrada, Apochela).” Contributions to Zoology, vol. 79, no. 1, 22 Jan. 2010, pp. 49–57., https://doi.org/https://doi.org/10.1163/18759866-07901002.