Free living marine nematodes from the Southern Bight of the North Sea. 3. Species of the Monoposthiidae, Filipjev, 1934

The intraspecific variations of Monoposthia mirabilis Schulz, 1932 are examined. Different morphotypes are present in different habitats. The morphological variation is correlated with an environmental gradient, i.e. the median grain size of the sand fraction. A new species of the Monoposthiidae, Nudora gourbaultae sp. nov., is described; it is characterized by the broadened first and second cuticular annule, the presence of 12 costae, two pre-anal cuticular modifications and the shape of the gubernaculum