This study reports the taxonomy, ecology and evolution of interstitial Ostracoda (Crustacea). Taxonomy and ecology of interstitial ostracods from river mouth were discussed. High salinity tolerance of the interstitial ostracods living river mouth was proven by field distribution and laboratory experiment. The author hypothesizes that ancestral marine interstitial species which has high salinity tolerance have been migrated to the river mouth environment where salinity varies drastically. From the mating experiment of Parapolycope spiralis, multi-step of pre-copulatory behavior was observed: (1) the male captures the female by a sucker on the antennula; (2) the male clasps and maintains the mating position using the endopodite claw and the exopodite of the antenna; (3) the male makes contact using the distal part of the upper lip; and (4) copulation takes place. The male upper lip shows species-specific and largely diverged morphologies in the Parapolycope species. The specific contact stimuli might be caused by various shapes of male upper lip and then females discriminate a favorable mating partner depending on these stimulations. The author concluded the species-specific morphologies of male upper lip have evolved by female mate choice. In addition, this study provides a brief overview of several taxonomic studies on marine ostracods.