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Open-cast coal-mining creates a severe disturbance of ecological systems, resulting in post-mining areas that have lost their soils entirely and are exposed to extreme conditions. Nonetheless, these areas allow us to investigate the process of primary succession. We studied the first 12 years of soil tardigrade succession at ‘Chicken Creek’, an experimental site within the open-cast coal-mining area in Welzow Süd, Germany. A total of ten tardigrade genera and 13 species or species groups were identified at the site. Over time, the organic carbon content, proportion of silt and nutrient availability in the soil increased, and the soil particle size distribution and pH value changed significantly. The vegetation cover and nematode abundance increased, whereas tardigrade densities peaked in 2008 and decreased thereafter, as did their mean body size. Species replacement was obvious within this 12 year study period, and tardigrade species diversity increased, indicating that ecological niche diversity also increased throughout this time. Vegetation cover, soil particle size distribution and time were the most important factors influencing the tardigrade community. We also discuss further potential factors and the ability of terrestrial tardigrades to persist despite many environmental constraints, such as fluctuations in pH and moisture, and to colonize new habitats faster than most other animal groups.

The present study gives the first detailed results on the partner-finding behaviour of tardigrades. We investigated whether the finding of mating partners by Isohypsibius dastychi followed certain criteria that are expected if pheromone signals are involved. For this purpose, tardigrade pairs were positioned in an arena, at a range of distances from each other. Only those females that were ready for mating attracted the male. The smaller the initial distance between them, the more directly the male headed for the female. The route travelled and time taken increased exponentially with increasing initial distance between partners, which means that the male lost orientation with respect to the female the larger the distance between the pair became. The findings indirectly suggest a yet undetermined female pheromone, which produces a concentration gradient in the water and loses strength quickly in terms of distance. Our assumption is supported by rare observations of sperm release without direct contact between partners that nevertheless resulted in fertile offspring. The observation that the female partners in this no-contact mating behaviour then deposited their eggs (a behaviour that was never seen when males were absent) suggests that they, likewise, can sense a fresh ejaculate of spermatozoa in their direct vicinity.

Phylogenetic positioning within Eutardigrada is based mainly on claw and buccopharyngeal morphology. In tardigrades that lack claws, i.e. representatives of the genus Apodibius, buccopharyngeal structures are the only morphological characters used for systematic classification. Investigating the feeding apparatus of Apodibius confusus Dastych, 1983 with light microscopy and scanning electron microscopy (SEM) techniques, the present study added morphological affirmation to the recently published molecular classification of A. confusus as a representative of Isohypsibiidae. The present study aimed to disentangle which of the Isohypsibiidae genera A. confusus is most closely related to. From a morphological point of view and with the SEM images presently available, A. confusus appears closest to Hexapodibius and Doryphoribius. The present study once more revealed the need for investigations that combine molecular and detailed morphological data to finally disentangle the phylogeny of Isohypsibiidae.

In the present study we provide new insights into the mating behaviour of a bisexual tardigrade, Isohypsibius dastychi, revealing a process much more complex than expected. Mating included mutual stimulation that preceded semen ejaculation and egg deposition. If no mating occurred, egg absorption was observed. In addition, the life history and the influence of temperature on development were studied at two temperatures (12 and 20 °C). At both temperatures I. dastychi underwent three moults until adulthood with increasing inter-moult phases from the first to the third moult. As animals maintained at 12 °C became adults significantly later and also were significantly larger than animals at 20 °C, it is unlikely that either age or size determines the transition to adulthood. Generation time of females was significantly shorter than that of males, and both males and females were iteroparous. The consequences of our findings are discussed.