Taxonomic descriptions of the brackish-water calanoid copepod <i>Acartia tsuensis</i> Ito from Japan and the closely related <i>A. ohnoi</i> n. sp. from the Philippines

Acartia (Acanthacartia) tsuensis Ito, 1956, a predominant planktonic calanoid copepod in the brackish-waters of western Japan, was originally described from Japan. This species has been recorded also from other East and Southeast Asian countries. However, a recent genetic study indicated cryptic speciation in specimens so far identified as A. tsuensis from Japan and the Philippines. The present study describes A. tsuensis s. str. based on specimens from Japan, including those collected by the original author of the species, and then the specimens from the Philippines as A. ohnoi n. sp. Previous descriptions of A. tsuensis from Korea and Vietnam somewhat differ from Japanese specimens. The new species is easily distinguishable from the co-occurring related species A. sinjiensis by the short caudal ramus in the female and strong spinules on the urosome in the male. Acartia tsuensis, A. ohnoi n. sp., and A. bilobata can be classified into the A. tsuensis species group because of close morphological similarity. The most distinctive features of the new species, separating it from the other members of the species group, are thick rostral filaments in the female and strong spinules on the urosome of the male. Their maxillule having a unique process on the praecoxal arthrite and their unique male leg 5 suggest a close relationship with a species of a different subgenus, Euacartia.