Horizontal distribution of moisture and Tardigrada in a single moss cushion

We carried out a study of the moisture distribution and the horizontal distribution of Tardigrada in a single moss cushion. During one field trip, we collected 25 equal samples (in a square of five rows × five columns) from a growth of the moss Hypnum cupressiforme Hedwig. Using two-way anovas without replications and linear regression analyses, we determined that there was no significant gradient of absolute or relative moisture along the moss slope. We isolated 224 specimens of seven Tardigrada species (Milnesium tardigradum Doyère, 1840, Hypsibius convergens (Urbanowicz, 1925), H. microps Thulin, 1928, Diphascon pingue (Marcus, 1936), Astatumen trinacriae (Arcidiacono, 1962), Macrobiotus hufelandi C.A.S. Schultze, 1833 and Minibiotus sp.) from the 25 samples. Using both chi-square tests of independence and chi-square goodness-of-fit tests as well as by calculations of the coefficients of dispersion, we found that the horizontal distribution of tardigrade specimens in general, as well as the distribution of each species, was aggregated. By contrast, species number was random in the observed moss samples. Based on the comparison of all polynomial regression models (third, second, and first order) with the null model and between each other, the distribution of Tardigrada specimens as a whole as well as the distribution of M. hufelandi was related neither to absolute nor relative amount of water in the moss cushion. Based on these results, we formulated a hypothesis explaining tardigrade heterogeneity in randomly sampled mosses, which stayed unexplained up to now. According to this hypothesis, there are two processes occurring at the same time: (1) random recruitment of specimens or eggs on a substratum and (2) subsequent establishment of their own micro-populations and gradual increase in their density in time of active periods with slow radiating of these micro-populations into the surroundings. The consequences of these processes are (1) larger substrata usually contain more tardigrades than smaller ones and (2) some parts of larger substrata can be without any tardigrades, while other parts can host rich tardigrade population(s); this within-substrate heterogeneity was ascertained for the first time in this investigation. We hypothesize that aggregated Tardigrada distribution in each moss cushion is the most likely reason for the large variability in tardigrade abundance in random samples taken from different moss cushions. Hence, we believe that introduced hypothesis and the resulting consequences have a considerable significance in the ecological studies based on random sampling