Limno-terrestrial Tardigrades Research Articles

Freeze tolerance, supercooling points and ice formation: comparative studies on the subzero temperature survival of limno-terrestrial tardigrades

Many limno-terrestrial tardigrades live in unstable habitats where they experience extreme environmental conditions such as drought, heat and subzero temperatures. Although their stress tolerance is often related only to the anhydrobiotic state, tardigrades can also be exposed to great daily temperature fluctuations without dehydration. Survival of subzero temperatures in an active state requires either the ability to tolerate the freezing of body water or mechanisms to decrease the freezing point. Considering freeze tolerance in tardigrades as a general feature, we studied the survival rate of nine tardigrade species originating from polar, temperate and tropical regions by cooling them at rates of 9, 7, 5, 3 and 1 degrees C h(-1) down to -30 degrees C then returning them to room temperature at 10 degrees C h(-1). The resulting moderate survival after fast and slow cooling rates and low survival after intermediate cooling rates may indicate the influence of a physical effect during fast cooling and the possibility that they are able to synthesize cryoprotectants during slow cooling. Differential scanning calorimetry of starved, fed and cold acclimatized individuals showed no intraspecific significant differences in supercooling points and ice formation. Although this might suggest that metabolic and biochemical preparation are non-essential prior to subzero temperature exposure, the increased survival rate with slower cooling rates gives evidence that tardigrades still use some kind of mechanism to protect their cellular structure from freezing injury without influencing the freezing temperature. These results expand our current understanding of freeze tolerance in tardigrades and will lead to a better understanding of their ability to survive subzero temperature conditions.

An attempt to revisit the global biogeography of limno-terrestrial Tardigrada

The major increase in distribution records of limno-terrestrial tardigrades over the last ten years has enabled us to reassess the global biogeography of the Tardigrada using cluster analysis, principal components analysis and parsimony analysis of endemism (PAE). Although the new clustergram topology shows a close correlation with those we originally presented in 1998, the PAE outputs warrant a radical reinterpretation of the results as they imply that the Laurasian fauna is derived and the Gondwanan groups basal. The distribution of endemic tardigrade genera and families provides some support for this argument though the findings should be viewed with some caution as PAE has its detractors and has not been previously applied on a 'global' scale.

Distribution of limnoterrestrial Tardigrada in Georgia and the Gulf Coast states of the United States of America with ecological remarks

This report is an effort to improve understanding of the distribution of limnoterrestrial tardigrades in Georgia and the states along the Gulf Coast of the United States of America. We collected 14 species of tardigrades from cryptogams (mosses, lichens, and liverworts) and leaf litter in a statewide survey of Louisiana and reviewed all publications and theses reporting tardigrade distributions in the Gulf Coast states. Statewide surveys have been also conducted in Alabama, Florida, and Texas, while sampling in Mississippi and Georgia has been more localized. Currently 51 species have been identified in the region: 19 in Texas, 16 in Louisiana, 10 in Mississippi, 33 in Alabama, 3 in Georgia, and 15 in Florida. These tardigrades have been collected from cryptogams (mosses, lichens, and liverworts) on trees and rocks, from soil and leaf litter, and from freshwater. Twenty species are widely distributed in the region (i.e., found in ≥ 2 non-contiguous states), while 27 have been found in only one state. Eighteen species are probably cosmopolitan. Seven species, widespread in the Gulf Coast states but unknown elsewhere in the Nearctic Region – Echiniscus kofordi, Echiniscus cavagnaroi, Parexapodibius pilatoi, Hexapodibius christenberryae, Biserovus bindae, Minibiotus fallax and a new Macrobiotus cf. hufelandi – may represent a distinctive regional fauna in cryptogams.

Analysis of the species description process for a little known invertebrate group: the limnoterrestrial tardigrades (Bilateria, Tardigrada)

This study was designed to examine the entire species description process of limnoterrestrial tardigrades up until the present, and to establish the effects of several variables on this process. Information from monographs, revisions and published papers related to limnoterrestrial tardigrades was assessed using generalized linear models (GLM) and variance partition. Our findings reveal the tardigrade species description process fails to show an asymptotic tendency. That is, at the current description rate, we will not get to know all the species in the short- or mid-term. The variables examined served to explain about half of the process variance. Species distribution areas and variables related to their presence in each region were the factors found to have the greatest explanatory capacity. Our analysis indicates that insufficient efforts have been invested in the tardigrades compared to other invertebrates (such as Insecta: Coleoptera), and that there is a need for more taxonomists and more extensive sampling areas. Studies such as this will be useful for planning future species sampling campaigns, and help us to improve our methodological approach to investigations aimed at conservation and the preservation of biodiversity.

Evolution of the Reproductive Mechanisms in Tardigrades — A Review

Although tardigrades can reproduce only through gametes they have exploited several modes of reproduction, which may be determined by their environment. Marine species (mainly heterotardigrades) are gonochoristic; hermaphroditism is only cited once, and parthenogenesis is unknown. In many cases females mature one egg at a time throughout adult life, whereas males are semelparous. Gonochorism is still present in limno-terrestrial species, while sporadic hermaphroditism occurs in several eutardigrade families. Thelytoky is the most common mode of reproduction in non-marine Tardigrada. Females are iteroparous, laying groups of eggs (free or in the exuvium), while males are semelparous (in a limnic species) or iteroparous with a continuous or cyclical maturation of the spermatozoa (in species from moss and leaf litter). Self-fertilisation appears to characterise hermaphroditic species, found in freshwater, mosses, leaf litter and soil. Egg maturation in these species is similar to that of the gonochoristic species, while spermatozoa mature in appreciable numbers before the oocytes, subsequently maturing continuously but in small numbers over the life of the animal. Parthenogenesis in limno-terrestrial tardigrades always appears continuous. In many species only females occur, but morpho-species populations may be found with both bisexual amphimictic (diploid) and unisexual thelytokous (often but not always polyploid) cytotypes. We can hypothesise that with the evolution of cryptobiosis and passive dispersal unstable and isolated habitats may favour parthenogenesis and self-fertilisation, as both reproductive modes allow colonisation of a new territory by a single individual. Parthenogenesis and hermaphroditism do not occur in the same species, and we can surmise that self-fertilisation will only evolve where parthenogenesis has never occurred.

Biogeography and Limno-terrestrial Tardigrades: Are They Truly Incompatible Binomials?

Due to the passive dispersal, tardigrades have been traditionally considered unsuitable for biogeographical studies. However, this paper provides some biogeographical and biological data supporting the possibility of using those animals to solve biogeographical problems at continental level.