Discovered in 1773 by Johann August Ephraim Goeze, a German Zoologist, Tardigrades are arthropod-like micrometazoans with four pairs of legs (lobopods) particularly known for their ability to survive in various extreme conditions.
* The name Tardigrada, meaning "slow stepper" was given by Lazzoro Spallanzani (an Italian biologist) in 1776.
Based on morphological and molecular studies, the Tardigrade has been shown to be a phylum consisting of a variety of organisms.
The following is a biological classification of phylum Tardigrade:
To date, studies have identified three major classes of phylum Tardigrada. Each of the three classes is composed of a few Orders, which in turn consist of several families and genera. As such, phylum Tardigrada has been shown to consist of several hundred (over 700) known species that have been classified in the following categories:
Compared to the other two, Heterotardigrada is the most diverse class in phylum Tardigrada. It's further divided into two Orders (Arthrotardigrada and Echiniscoide) and further into Families that include Batillipedidae, Oreellidae, Stygarctidae and Halechiniscidae among a few others. These Families are further divided into well over 50 genera.
Some of the characteristics associated with class Heterotardigrada include gonoducts, cephalic appendages and separate claws (4) in their legs.
Other characteristics include:
Dentate collar on their posterior pair of legs
Pore patterns that vary between the species
Compared to the other Classes, Class Mesotardigrada is only divided into a single Order (Thermozodia), Family (Thermozodidae) and a single species (Thermozodium esakii). Thermozodium esakii was discovered in a thermal spring in Japan but no one species in the class has been identified.
The following are some characteristics of Class Mesotardigrada:
Each foot has six claws
Thermozodium esakii is an intermediate between the members of Heterotardigrada and Eutardigrada
Thermozodium esakii have cirri
Spines and claws resemble those of Heterotardigrada species
Their macroplacoids resemble those found in Eutardigrada
* Since their habitat was destroyed, Thermozodium esakii species belonging to the Class Mesotardigrada have been declared extinct. Currently, no other species belonging to this class has been identified.
Class Eutardigrada is divided into two Orders that include Parachela and Apochela. The two Orders are further divided into six Families that include Mineslidae, Macrobiotidae, Hypsibidae, Calohypsibidae, Eohypsibidae and Eohypsibidae. These Families are further divided into well over 35 genera with different types of species.
Some of the characteristics of Class Eutardigrada include:
Compared to the other two classes, members of Class Eutardigrada do not have lateral appendages
Have a smooth cuticle
Do not have dorsal plates
Gonoducts open into rectum
They possess double claws
Reproduction and life cycle among the members of phylum Tardigrade is largely dependent on their habitats. Given that the life of these organisms is largely characterized by inactivity and intermittent inactivity, researchers have concluded that it is essential for reproduction to occur rapidly in high numbers when conditions are favorable.
Depending on their environment, tardigrades may reproduce asexually (self-fertilization) in a process known as parthenogenesis or sexually where males fertilize the eggs (amphimixis).
Sexual reproduction in Tardigrades is common among dioecious species (with male and female with their appropriate sexual organs). Most of these organisms/species are found in marine environments and thus reproduce in marine environments.
Although the shape and size (morphology) of the gonads of Tardigrada is largely dependent on the species, sex and age etc of the organisms, microscopic studies have identified the following sexual organs in male and female Tardigrada:
A pair of vas deferens that open into the cloacae (at the hindgut)
Internal seminal vesicles
Female and Hermaphrodite Species:
A pair of oviduct that open into the cloacae
Seminal receptacles (In heterotardigrades)
Internal spermatheca (In eutardigrades)
During sexual reproduction among some members of Class heterotardigrades and eutardigrades, the eggs of the female are directly or indirectly fertilized. During direct sexual fertilization, the male Tardigrade deposits sperm into the seminal receptacle of the female, which allows the sperm to be transported to the eggs for fertilization.
During indirect fertilization, the male will deposit sperm into the cuticle of the female as the female molts. When the female sheds the cuticle, the eggs are already fertilized and develop over time. During molting, the female sheds the cuticle as well as some of the other structures such as the claws.
As is the case with spermatogenesis (production of spermatozoa) oogenesis also starts after molting
Among some members of Tardigrada, such as Isohypsibius nodosus, mating/courtship has been observed. During courtship, one or more males stroke the female using their cirri which in turn stimulates the female to lay eggs.
While some of the male will deposit their spermatozoa in the cloacol opening of the female, fertilization does not take place internally. Rather, the female lays her eggs externally for external fertilization.
While cross-fertilization is common, some species have also been shown to be capable of self-fertilization (hermaphroditic). In such cases, variations are not common except in mutation.
Also, cross-fertilization presents a big advantage in that it allows for genetic recombination from the fusing of different genomes.
Whereas sexual reproduction (amphimixis) takes place in marine environments during favorable conditions, parthenogenesis has mainly been observed in terrestrial environments as well as liming conditions.
In particular, Parthenogenesis is a common means of reproduction among the unisexual members of Tardigrades. For some of the species, there are no male tardigrades and thus female tardigrades reproduce through a process known as Parthenogenesis.
Here, the female produce and lay eggs and leave them to develop without being fertilized. As a result, this has been shown to result in the proliferation of female offspring only among the species.
Given that this form of reproduction takes place in unstable environments (compared to more favorable marine environments) it has been shown to be a beneficial mode of reproduction allowing the species to continue reproducing and thriving in such conditions while making it possible for the species to continue evolving as they invade new environments away from marine environments.
Depending on the species, eggs are either fertilized internally (e.g. in L. granulifer where oviposition takes place), externally (in most heterotardigrades) or simply released externally where they develop without being fertilized.
Although parental care of the eggs is rare, it has been observed in a few species such as Pseudobiotus Kathmanae. Here, the eggs remain attached to the caudal part of the female thus ensuring that the female Pseudobiotus Kathmanae cares for the eggs before they hatch.
Development of the eggs and organisms is yet to be fully understood. However, development of the young Tardigrade has been shown to take between 30 to 90 days. This is largely dependent on the species and conditions of the surrounding environment.
* When environmental conditions become increasingly unfavorable, egg development stops or slows down significantly. Development continues once conditions improve.
The following stages have been identified in postembryonic development:
Immediately they are hatched, the hatchlings do not have a visible anus or gonopore. However, they posses two claws on each leg (the claws are internal)
After undergoing the first molt, they develop an anus and four claws on each egg. At this stage, the gonopore starts to develop and may not be seen in some species.
The second molting produces a mature Tardigrade with fully developed gonopore, anus as well as fully developed claws (four) on each leg.
* The number and structure of the claws, however, is dependent on the species.
Essentially, Tardigrades are aquatic organisms given that water provides favorable conditions for processes such as gas exchange, reproduction and development. For this reason, active Tardigrades are often found in marine and fresh water as well as terrestrial environments with some water.
While they are considered aquatic, Tardigrades can also be found in many other environments including sand dunes, soil, rocks and streams among others. They can survive in films of water on lichens and mosses and thus are commonly found on these organisms.
The eggs, cysts and tuns of Tardigrades are also easily distributed by wind to different environments allowing the organisms to colonize new environments. According to research studies, Tardigrades have been discovered in various remote environments such as volcanic islands, evidence that wind and animals like birds widely disperse and distribute the organisms.
* They are also distributed by water, rain, melting snow and some insects.
Apart from favorable and less favorable environments/habitats, Tardigrades have also been discovered in various extreme environments such as very cold environments (as low as -80 degrees Celsius). Because of their ability to survive and even reproduce in these environments, Tardigrades are found in virtually all environments across the world.
Survival in Space and Extreme Environments
Tardigrades have been described as polyextremophiles because of their ability to survive in a variety of extreme environmental factors.
This has become one of their most defining characteristics and one of the most studied aspects of the phylum. While they are active during favorable conditions, Tardigrades have adopted a number of strategies that allow them to survive.
These strategies are typically known as quiescence (cryptobiosis) and include:
Anoxybiosis refers to a cryptobiotic state that is stimulated by very low or lack of oxygen among aquatic Tardigrades. When the levels of oxygen are significantly low, Tardigrades respond by becoming rigid, immobile and extended. This makes it possible for them to survive several hours (for extreme aquatic Tardigrades) to a few days without oxygen and ultimately become active when conditions improve.
This response to extremely low levels of oxygen has been shown to be particularly beneficial for Tardigrades that live in deep water or those in Antarctic lakes where levels of oxygen can vary from time to time.
Cryobiosis is a form of cryotobiosis that is influenced by low temperatures. When the temperature in their environment falls to freezing levels, Tardigrades react by forming barrel-shaped tuns and trehalose to protect the membrane.
Frozen in such states, Tardigrades can survive several years in their environments. This survival mechanism has been used to explain the presence of Tardigrades in such extreme environments as the Polar Regions that experience significantly low temperatures for extended periods.
In aquatic solution of high ionic strengths (such as high salt levels) some organisms are unable to survive and thus die off. However, a good number of Tardigrades found in freshwater environments and terrestrial habitats survive through a form of cryptobiosis known as osmobiosis.
Here, the organisms survive by developing contracted tuns. In this state, they can survive for a few days and resume activity when conditions improve.
Anhydrobiosis is a survival response to water loss by evaporation. For a variety of organisms, water is important for such processes as gaseous exchange and other internal mechanisms. For a majority of freshwater Tardigrades, survival during dehydration is not possible.
However, for a good number of eutardigrades, survival during such conditions is achieved by contracting and retracting of the head and legs. The organisms then turn into barrel-shaped tuns that are able to survive desiccation.
Moreover, the organisms also produce a number of compounds such as glycerol, heat-shock proteins and trehalose that protect the cell and enhance survival. In this state, Tardigrades have also been shown to survive a number of other extreme environmental conditions such as very high or low pressure and radiation among others.
Some of the other modes of survival (also known as diapause) include:
Encystment - This survival strategy is common among Tardigrades found in freshwater, soil and those that live on moss. When environmental conditions become increasingly unfavorable, species like Dactylobiotus and Bertolanius undergo morphological changes that result in the formation of a cyst that is capable of surviving such conditions.
Here, the organisms start by losing the sclerified followed by the development of three cuticles. The cysts then turn darker in complexion and immobile with an oval shape that can survive for elongated periods (months).
Resting eggs – Eggs that remain dormant and only develop once conditions become favourable.
Physiology and Adaptation
Because of their ability to survive various extreme conditions, Tardigrades have been found in such environments as hot springs, below thick layers of ice and Himalayan Mountains among others.
In most cases, Tardigrades form a shrunken structure referred to as a Tun that is capable of surviving for as long as several years. As the organisms form the Tun, they lose water (desiccation) which is replaced with trehalose, a disaccharide sugar. This stops the remaining fluid (about 1 percent water) from expansion as well as inhibiting metabolism.
In this state, Tardigrades are capable of surviving the following conditions:
-200 to 151 degrees Celsius
Some can survive low temperatures of -272 degree Celsius
They can survive in vacuum
Pressures 1,200 times that of atmospheric pressure
They can survive solar radiations for about 10 days
Space and Vacuum
Because of their ability to survive in very high and very low pressures (such as those of vacuum) some species of Tardigrades are theoretically said to be capable of surviving in space without any protection.
According to a 2011 study to determine whether Tardigrades can survive in space, Italian scientists discovered that microgravity and cosmic radiations did pose significant effects on the organisms. Based on the findings, they concluded that Tardigrades will prove increasingly useful in space research.
* In all studies where Tardigrades have been exposed to the vacuum of space, the extreme conditions did not affect their DNA, reproductive capabilities or ability to continue surviving.
For active Tardigrades, the lifespan has been shown to range from about 3 to 30 months. However, there are significantly extended latent periods where Tardigrades are able to survive for long periods of time (inactive). The active lifespan is therefore largely associated with aquatic Tardigrades that often live an active life in their aquatic environments.
Typically, Tardigrades feed on plants (microflora such as algae and mosses). Using their needle-like mouth (sharp stylets), they pierce and penetrate plant cells and consume their fluids. Some of the Tardigrades survive by feeding on other organisms such as bacteria, protozoa and detritus as well as dead tissue.
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