Morphology and Systematics of the Order Plectida Malakhov, 1982 (Nematoda)

Nematodes constitute one of the largest animal phyla. Hitherto about 20.000 species have been described but there are estimates that this number is less than 2% of the total number of existing species. Although the larger animal and human parasites were known since ancient times the description of free living nematodes and plant parasites started around 1850. Although nematodes can be found in almost every habitat, they are still one of the least known groups of extant metazoans. In a hierarchical organisation (classification) of nematodes, as for some other groups of multicellular animals, the higher the level, the greater the confusion and discrepancy among different opinions. It is clear, that the applied, prognostic and educational value of such artificial classification is low and requires further improvement.

This thesis represents a small brick in the castle of humans' knowledge about nematode morphology, diversity and phylogeny. The author brings to light some new data and ideas about the form of some nematodes, how they develop and how they apparently evolved. Four main chapters deal with one of the smallest but morphologically and ecologically diverse orders, Plectida, whose members are found in marine, terrestrial and fresh water habitats.

At the first stage of studies, just one genus, Anonchus, is analysed. Among the six known species included in the analysis, however, Anonchus shows considerable variability in morphology of reproductive organs, of both females and males. It is worth mentioning that morphological peculiarities of female gonads and the male copulatory apparatus are often used to separate populations into different species and to create higher taxonomic categories. The discovery of two new species, A. venezolanw and A. winiszewskae, adds even more to understanding the structural diversity of this genus. The analysis of the morphology of Anonchus species results in considerable rearrangement of the classification of this genus and its allies, which are placed in the newly reinstated family Aphanolaimidae.

The main objective of the third chapter was to uncover the morphological diversity of representatives of mostly marine and presumably primitive groups of Plectida, and to propose a phylogenetic hypothesis of their relationships with each other and with their terrestrial allies (descendants ?). Additional descriptions are given for five species. Morphological and taxonomic analysis has been followed by the discovery of new taxa; the description of one, Paraplectonema loofi, is given in this chapter. Another, Procamacolaimus dorylaimus, is described in a separate article, but is also included in the phylogenetic reconstructions, which, on the whole, include 20 in-group laxa representing all distinct genera. A thorough systematic study required at least a brief discussion of phylogenetic affinities of the families Bastianiidae and Odontolaimidae and the genus Tobrilia which were previously included in the order Plectida. The taxonomic position of these taxa has become clearer in light of the new morphological data, particularly in the structure of male copulatory apparatus.

The subsequent part of the thesis deals with the few species of the subfamily Wilsonematinae, whose members are one of the smallest terrestrial nematodes. The taxonomy of the Wilsonematinae has been severely hampered by the limits of light microscopy for correct observation and interpretation of their highly elaborate, complex labial region. The discrepancy between recent data and previous published descriptions of the structure of the labial region illustrates the great importance of scanning electron microscopy (SEM) for morphological and taxonomic studies of the group in question. Keeping this in mind, the author provides detailed redescriptions of five species of the genera Wisonema, Ereptonema and Neotylocephalus on the basis of both type and new material from diverse habitats and localities throughout the world. Reappraisal of the earlier morphological descriptions of these taxa resulted in substantial changes in their classification, and critical revision of all allied species and genera. Ultrastructural studies (SEM) made it possible to propose a phylogenetic hypothesis for the subfamily Wilsonematinae.

The fifth chapter of this thesis includes a phylogenetic analysis of the largest terrestrial taxon of the order Plectida, superfamily Plectoidea. This analysis is based on published and updated morphological data for 35 in-group species. One of these species will be described as new in a separate publication. To better understand the character polarity, the postembryonic development of four species is described in detail. It was found that not only the general growth and development of reproductive system occurs during postembryonic development but that somatic sensilla, digestive and excretory systems also change morphologically during successive stages. Possible historical relationships of the superfamily Plectoidea are analysed using different methods and principles; one of the phylogenetic hypotheses was selected for thorough discussion, and as a basis for the revised classification of this taxon. Morphological data, which are elucidated and summarised in this chapter, provide the basis to further speculate on structural evidence for parthenogenetic reproduction in one of the studied genera, and the data also suggest several probable ways of niche partitioning in the superfamily Plectoidea.

The general discussion summarises all taxonomic changes and suggestions proposed in preceding chapters, and gives a revised and updated classification of the order Plectida. Possible morphological adaptations to the fresh water habitat and structural changes associated with the soil habitat are briefly discussed based on the proposed phylogenetic hypothesis. Since family Plectidae sometimes has been suggested to be closely related to the scientifically and economically important order Rhabditida (=Secernentea), the author proposed that morphology of plectid genus, Anaplectus, may be a source for future research on the origin and evolution of rhabditids.