Systematics of phylum Loricifera: Identification keys of families, genera and species

Loricifera is a phylum of free-living, marine animals of microscopic size, which is distributed worldwide at an extensive depth range. After the description of Nanaloricus mysticus Kristensen, 1983, a total of 36 loriciferan species have now been described. However, additional new species already known await formal description. Indeed, new loriciferans are frequently found during oceanographic expeditions, though they are usually accounted as “undescribed species” amongst the meiofauna collected. This is because their taxonomic identification is not easy, especially at the genus and species level, and the number of researchers familiarized with the loriciferan anatomy is reduced. Aiming at contributing with new tools for the identification of Loricifera, we present here the first hierarchical single-access and multiple entry keys based on morphological characters of adults (or postlarva) and the most prominent larval stage, the so-called Higgins larva. The single-access keys facilitate the identification at the species level, while the multiple entry key summarizes the diagnostic features for each genus of Loricifera. The two keys may be used separately or supplementary to each other. In order to facilitate the use of the keys, we provide a detailed description of the loriciferan morphology, as well as an updated list of the terminology used. In addition, we include a short description of the intermediate life cycle stages and the hypothetical life cycles of all families. Finally, we hope to encourage more researchers to study the enigmatic phylum Loricifera.