The Japanese fauna of Nereididae, which is the most successful annelid family in estuarine low-salinity and freshwater habitats, is reviewed here. A list of all nereidid species so far recorded from Japan (54 species belonging to 20 genera) is presented, with the information of type locality and distributional range in and around Japan for each species. Current topics on the estuarine nereidid fauna, consisting of 15 species, are the focus. The life-history modes of 4 species belonging to 2 genera (3 in Hediste and 1 in Tylorrhynchus) are compared. As do many marine nereidids, 2 of these Hediste and 1 Tylorrhynchus species show reproductive swarming preceded by epitokous metamorphosis. In these 3 species, however, the epitoky differs more or less from that into the common heteronereis form, which is almost uniform throughout the family. The adaptive and evolutionary significance of their unique epitoky are discussed in relation to their life-history characteristics in Asian estuaries. Some of the Japanese estuarine nereidids are in danger of extinction for reasons of severe habitat loss by recent anthropogenic land reclamation.
