Meiofauna: Biodiversity, Ecology, and Role in Ecosystems

The meiofauna is currently considered to be a group of microscopic metazoans ranging in size from 30 to 1000–2000 μm, forming a distinct size class [1,2]. Several taxa of high taxonomic rank are represented exclusively or almost exclusively by meiobenthos organisms of marine areas and freshwater reservoirs. These taxa are Tardigrada, Gastrotricha, Gnathostomulida, Kinorhyncha, and Loricifera. The main diversity of meiofauna in most biotopes consists of representatives of segmented worms (type Annelida mainly Subclass Oligochaeta), nematodes (type Nematoda), rotifers (type Rotifera), and arthropods: harpacticoids (class Copepoda, order Harpacticoida), ostracods (class Ostracoda), water mites (class Arachnida, order Trombidiformes) [3]. In addition, the size group of meiobenthos includes the larvae of some macrozoobenthos organisms (Subtype Crustacea, Types Mollusca and Echinodermata, etc.), which comprise the so-called «pseudomeiobenthos» group [4]. The number of known meiobenthic species in each of these groups ranges from hundreds to several thousands.

Among meiofauna groups, nematodes dominate in most biotopes. The proportion of nematodes can reach 90% of the total number of multicellular organisms [5]. Harpacticoids, kinorhynchs, tardigrades, turbellarians, and sometimes gastrotrichs are usually the second most abundant in marine areas, and in brackish and freshwater, they are joined by oligochaetes, rotifers, and ostracods.

Taxonomic composition, the average size of individuals, and a set of morphological and life cycle adaptations can serve as the criteria for distinguishing meiobenthos as an independent ecological group [2]. Among the adaptations associated with the miniaturization of organisms, low individual fertility—the small number of acts of reproduction and low egg numbers produced by an individual per year—should be noted [6]. In addition, the absence of pelagic larvae is typical of meiofauna. Another characteristic feature of meiobenthos is their short life cycles and, as a consequence, high sensitivity to the short-term variability of environmental factors [7]. Meiobenthic communities are characterized by a smaller scale variability of structure than macrozoobenthos communities, and seasonal changes in their structure occur as a result of changes in a number of generations. Thus, meiobenthos is not only a “technically” convenient size group that requires special methods to study organisms but also an independent structural unit of the bottom ecosystem with its own spatial and temporal characteristics.

The six papers in the Special Issue, which focuses on the biodiversity and ecology of meiofauna, fall under two main themes. Three of the papers are taxonomic, containing descriptions of organisms new to science. The other three papers are devoted to the ecology of meiofauna—the structure of communities and environmental factors regulating them.

The taxonomic articles deal with two groups of meiofauna: nematodes, whose diversity sometimes seems endless, and tardigrades. Tchesunov et al. [8] describe a new nematode species of the genus Acanthopharynx (family Desmodoridae), which is associated with sponges of the class Demospongiae. Their description includes an analysis of the morphology and morphometry of the species, as well as their molecular genetics. Such taxonomic descriptions are highly informative and provide an opportunity to distinguish the species from possible cryptic species. Many meiobenthic species that are considered cosmopolitan are in fact a group of morphologically similar but genetically distinct cryptic species [9,10]. This fact has determined the existence of the meiofauna paradox, where species have very large ranges with low dispersal ability [1]. Another article in this Special Issue [11] is also dedicated to describing a new nematode species of the genus Pseudochromadora (family Desmodoridae). This is the first record of this genus in the Black Sea. It is worth noting that the described species has intermediate characters between several genera of Desmodoridae, indicating the need for further genetic analysis of individuals. The article by Saulenko et al. [12] on tardigrades is devoted to the fauna of a poorly studied and difficult-to-access area—the abyssal plain adjacent to the Kuril–Kamchatka Trench (depth > 3200 m), located in the northwestern part of the Pacific Ocean. Two new species of Tardigrada belonging to the genera Coronarctus and Moebjergarctus were found. The analysis of a large amount of material collected during four deep-sea expeditions indicates a patchy distribution of tardigrades in the deep waters of the northwestern part of the Pacific Ocean.

Papers dedicated to the meiofauna ecology of the Siberian region, with its severe, sharply continental, arctic climate, are also presented. The article by Fefilova et al. [13] characterizes the deep-water relict fauna of the unique rift Lake Baikal. For the first time, the composition of harpacticoids of the soft sediments of the bathyal and abyssal zone (270–1623 m depth), including hydrothermal and methane seeps of that area, is described in detail. About 19 endemic species were found, of which more than half are potentially new to science. This fact confirms the high degree of endemism in the copepod fauna of Lake Baikal. Significant differences in the taxonomic composition of harpacticoids in the deep and littoral zones of the lake were noted. A high diversity of meiofauna was found unexpectedly in the high latitude regions of Siberia [14]. The biota of polar regions is under the constant influence of extreme environmental factors, causing changes in community composition, a reduction in species richness, diversity, and abundance, and a simplification of structural relationships [15]. This trend is particularly pronounced in areas consisting of Arctic tundras and deserts, Arctic islands, and subarctic mountain massifs [16]. At first glance, it seems that all high-latitude ecosystems should be impoverished. However, studies of the aquatic fauna of the Arctic Putorana Plateau suggest the opposite [14]. The unusually high taxonomic diversity of the fauna and the high species richness are thought to be due to the fragmentary glaciation of these areas during the last ice age. On the plateau, the glacial domes were located only on flat ridge tops, while the lakes in the valleys preserved the refugia of Pleistocene fauna. The article by Chertoprud et al. [17] describes a change in the complexes of dominant species in the communities of micro-, meio-, and macrozoobenthos, as well as zooplankton, during the development of thermokarst tundra water bodies in the Lena River delta. The meiobenthos in these water bodies is mainly represented by microcrustaceans (Cladocera and Copepoda). The change in community types with the increasing age of the water body has been demonstrated as primary succession. Communities of planktonic, micro-, and meiobenthic organisms with short life cycles were found to be more sensitive to changes in environmental factors than those of macroinvertebrates with long life cycles. Therefore, crustaceans can be a reliable indicator of ecosystem changes.

This Special Issue collates articles on a number of key topics in meiofaunal research. The taxonomy and morphology of several characteristic groups of marine and freshwater meiobenthos are covered, the factors regulating the structure of Arctic communities are described, and a comparative analysis of the formation of micro-, meio-, macrozoobenthic, and zooplanktic assemblages is performed. It is shown that freshwater meiobenthic organisms can be important indicators for monitoring Arctic ecosystem changes in the framework of global climate change.