The cladocerans of the order Ctenopoda are widely distributed and fairly well-known representatives of aquatic communities. Most of them inhabit continental waters of different types while two species of the subfamily Penilinae and family Pseudopenilidae occur in the pelagic zone of the world ocean and in the deep bottom sediments of the Black Sea, respectively. The group is most diverse and abundant in tropical, subtropical, and southern temperate latitudes but is represented in all climatic zones, and on all continents save Antarctica and New Zealand. Its representatives often dominate the pelagic and littoral zooplankton communities.
In spite of their comparatively low species richness, ctenopods demonstrate high morphological diversity and have developed various life strategies with adaptations to different habitats and predator avoidance. The peculiar adaptations of their representatives place them in a unique position within Cladocera and Branchiopoda.
Ctenopoda, especially those of the family Sididae, are among the most evolutionary primitive cladocerans. The detailed investigation of their morphology, including their embryology, ecology, and zoogeography contributes to a better understanding of the group and of evolutionary trends within the whole superorder Cladocera. On the other hand, more detailed morphological studies support the taxonomic exploration of cladocerans, which is evident from the recent key publications on the subject.
For a long time, the taxonomy of the Ctenopoda remained confused, until the beginning of their revision in 1970-1980s. Since the publication of identiﬁcation books on the world fauna of the order, further taxa have been either revised or described as new for science while some remain explored insufficiently. Consequently, some regions have appeared to be studied more intensively than others, e.g. Europe, Australia, and South-East Asia.
Analysis of novel information reveals that the order Ctenopoda currently includes more than 100 formal taxa of the species-group level whereas only 71 of them may be considered valid. The present work aims to summarize, at least brieﬂy, the available information on morphology, systematics, and other biological aspects of the group. It is hoped that this will stimulate further surveys of the world ctenopod diversity as well as of particular regional inland aquatic faunas.