Tardigrades are microscopic invertebrates comprising Panarthropoda with two other phyla, Arthropoda and Onychophora. More than one thousand tardigrade species are known worldwide from marine, freshwater, and limno-terrestrial environments. To facilitate beginners interested in tardigrade systematics, the collection methods for these three environments, the specimen preparation methods for light microscopy and scanning electron microscopy, the DNA extraction procedures, and the recent advances in the systematics of the four orders belonging to the two classes, Eutardigrada (Apochela and Parachela) and Heterotardigrada (Arthrotardigrada and Echiniscoidea), are reviewed. The present status of the Japanese tardigrade fauna is also mentioned in brief.

Baron Yoshio Tanaka (1838–1916), a famous historical researcher of natural history based in the “honzo-gaku” (traditional natural history) developed in the Edo Period in Japan, established himself as a talented and energetic higher governmental official in early modern Japan, establishing the National Museum. He also contributed much to the development of modern biological education by the compilation of wallcharts and translations of European and American textbooks into Japanese, and improved understanding of agricultural and fisheries products. He is therefore regarded to have bridged the gap between pre-modern and modern approaches in biology (especially botany), although his influence in the understanding of ascidians has been overlooked. However, a hand-written and previously unknown manuscript on this animal group by Tanaka (dated 1882), translated here into modern Japanese with detailed explanatory notes, included references to dissected museum specimens, evidence of Tanaka’s modern approach, since dissection and museum-deposition of specimens had not been practiced in “honzo-gaku”. The only cited western article in the manuscript, entitled “Cuvier’s molluscs”, was identified as Deshayes’ molluscan volume [completed in 1845 and including good figures of ascidian anatomy] included in the third edition of Cuvier’s “Le Règne Animal”, issued between 1836 and 1849. However, Tanaka made no reference to Lankester’s “urochordate” concept, published in 1877, and left no detailed figures of ascidian anatomy, necessary for modern taxonomy. In fact, the modern taxonomy of Japanese ascidians was begun in 1882 by foreign taxonomists, to be followed soon after by a Japanese researcher, Dr. Asajiro Oka. However, these endeavors did not avail themselves of Tanaka’s museum specimens, which were possibly unavailable even then, due to irregular museum deposition practice. It is clear that the gap between pre-modern and modern practices, was very significant in the case of ascidians.

An adult female of the argulid branchiuran Argulus coregoni Thorell, 1864 was collected from the dorsal body surface of an oily bitterling, Tanakia limbata (Temminck and Schlegel, 1846) (Cyprinidae: Acheilognathinae), in the lower reaches of the Asahi River, Okayama Prefecture, western Japan. This represents a new host record for A. coregoni and its new prefectural record in Japan. The external morphology of the female collected is reported in detail. Tanakia limbata is the second acheilognathine host of A. coregoni. The occurrence of A. coregoni on the oily bitterling is unusual because this parasite usually infects various salmonids and ayu, Plecoglossus altivelis altivelis Temminck and Schlegel, 1846 (Plecoglossidae), in the upper and middle or lower-reaches of rivers, respectively, in central and western Japan. The individual of A. coregoni probably parasitized the oily bitterling after detachment from ayu near the collection locality.