The species diversity, density and similarity of Gastrotrich fauna of bottom sediments to epiphytic fauna in three farm ponds were investigated. In the studied habitats 31 species of Gas-trotricha belonging to the family of Chaetonotidae were found altogether. In bottom sediments of the ponds there were 29 and on plants 17 species of Gastrotricha. Three species (Heterolepido-derma gracile Rmane, 1927, Chaetonotus disjunctus Greuter, 1917 and Ch. oculifer Kisielewski, 1981) were found to be dominants in bottom sediments with the dominance over 10.0%. H. gracile and Ch. oculifer also occur on vegetation, but their dominance is significantly lower. In turn three species (H. macrops Kisielewski, 1981, H. ocellatum (Mečnikow, 1865) and Lepidodermella squamata (Dujardin, 1841) proved to be dominant on water vegetation, with the dominance over 10.0% in all studied ponds. The value of species diversity index H’ including the number of spe-cies and uniformity of their dominance is from 2.76 to 2.93 for bottom sediments and from 2.60 to 2.72 for plants. The total density of gastrotrich fauna in bottom sediments fluctuated from 350.0 to 920.0 103 indiv. m-2 and on elodeids from 520 to 1110 103 indiv. m-2. The density of gastrotrich fauna of elodeids was higher than in bottom sediments in all the studied ponds. In each of the ponds examined, the differences are statistically significant. The similarity between bottom sediment fauna and epiphytic fauna in each of the studied ponds, cal-culated according to the homogeneity index, was very low and ranged from 44% to 48%.

The research was carried out on species diversity, density and similarity of Gastrotricha living on two species of macrophytes Myriophyllum spicatum L. and Elodea canadensis Michx. in a mesotrophic lake. Gastrotricha occurring on the studied macrophytes were represented by 23 species (21 on M. spicatum L. and 22 on E. canadensis Michx.). Species diversity (expressed by the Shannon index) for the Gastrotricha occurring on the two studied species of macrophytes amounted to 2.70 for M. spicatum L. and 2.81 for E. canadensis Michx., what suggests equal distribution of the gastrotrich species revealed in the studied species of macrophytes. The definite dominants inhabiting the two species of macrophytes were: Heterolepidoderma macrops Kisielewski, 1981, Heterolepidoderma ocellatum (Mečnikow, 1865), Lepidoderma squamata (Dujardin, 1841) and Aspidiophorus squamulosus Roszczak, 1936, and their inclusive percentage participation in the whole gastrotrich fauna for M. spicatum L. and E. canadensis Michx. was 41.8 and 40.3% respectively. The domination structure of the gastrotrich fauna of the two species of macrophytes was similar for each gastrotrich species (G-test, for all p > 0.05). Average density of Gastrotricha inhabiting M. spicatum L. amounted to 588.0 10³ indiv. m-2, and density of Gastrotricha occurring on E. canadensis Michx. amounted to 670.0 10³ indiv. m-2. Statistic analysis based on the Mann-Whitney test showed that differences of the gastrotrich densi-ty on the studied species of macrophytes: M. spicatum L. and E. canadensis Michx. (Z = -1.9282; p = 0.05) are statistically insignificant.

Species composition and density of Gastrotricha in bottom sediments were studied, on an annual basis, in the littoral of the mesotrophic Lake Piaseczno (Łęczyńsko-Włodawskie Lakeland). The number of species from spring to autumn remained on the same level (from 24 in spring to 22 in autumn), but in winter in littoral of the lake there were recorded only 8 species. Species diversity expressed by the Shannon index, in spring, summer and autumn was high and amounted 2.86; 2.69 and 2.77 respectively. In winter the value of the index was significantly lower and amounted 1.80. In individual seasons the mean density of Gastrotricha valued from 50 103 indiv. m-2 during winter to 1238 103 indiv. m-2 in spring. The density of Gastrotricha in spring was more than 24 times higher than that winter. There were no significant differences in species diversity Gastrotricha from spring to autumn, although the peak abundance was recorded in the spring. It is necessary to add, that the density of Gastrotricha during the spring peak was one order of magnitude higher from that in summer and autumn. The similarity of the fauna found in winter to the fauna occurring in other seasons, is significantly low, ranging from 24 to 27%. Seasonal changes in amount and diversity of the gastrotrich fauna are probably the result of temperature changes and food availability.

Despite great technological progress scientists still are not capable of ascertaining how many species are there on Earth. Systematic studies are not only time-consuming, but sometimes also significantly impeded by constraints of available equipment. One of the methods for morphology evaluation, which is gradually more often used for taxonomical research is microcomputed tomography. It’s great spatial resolution and ability to gather volumetric data during single acquisition without sectioning specimen are properties especially useful in evaluation of small invertebrates. Nondestructive nature of micro-CT gives possibility to combine it with other imaging techniques even for single specimen. Moreover, in case of rare organisms studies it allows to collect full structural data without fracturing their bodies. Application of proper staining, exposure parameters or specific sample preparation significantly improves quality of performed studies. The following article presents summary of current trends and possibilities of microtomography in morphology studies of small invertebrates.