Diversity of Kinorhyncha in Japan and Phylogenetic Relationships of the Phylum

Kinorhyncha is a phylum of marine, meiobenthic, segmented, pseudocoelomate animals. To date, about 230 species of kinorhynchs have been reported worldwide, from habitats including coarse to fine sand, mud, and surfaces/interstices of algae or other invertebrates. Although they are broadly distributed, kinorhynchs have been scarcely studied in Japanese waters. Before the twentieth century, there were only six reports of kinorhynchs from Japan, totaling 4 species in 3 genera. Although the known kinorhynch diversity in Japanese waters has increased markedly in the past 15 years to 16 named and at least 17 unnamed species representing 14 genera, this is likely only a small portion of the actual species diversity in this region. Phylogenetic studies of kinorhynchs began only recently. Recent phylogenetic analyses of molecular data, or of combined morphological and molecular data, have revealed discrepancies between the old classification and phylogenetic relationships among higher kinorhynch taxa. On the basis of phylogenies, the kinorhynch classification has been revised to 2 classes, 3 orders, and 11 families. Although the higher-level phylogeny is fairly well resolved, relationships within and among some taxa remain unclear.