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Two new species of cyclopoid copepod, Halicyclops setiformis and H. itohi, are described based on female specimens from the Tama River estuary in Tokyo Bay, central Japan. One of the defining characteristics of H. setiformis is the presence of a setiform lateral element on the third exopodal segment of legs 3 and 4. Among its congeners, only H. blachei Lindberg, 1952 has similar elements; H. setiformis differs from it in the shape of the third endopodal segment of leg 4. This new species is considered carnivorous based on the occurrence of a specimen biting a nematode and the presence of stout, claw-like spines on the oral appendages. The second new species, Halicyclops itohi, is distinguishable from most of its congeners in the absence of lateral protuberances on the genital double-somite, the absence of a terminal accessory seta on the caudal ramus, and a 3.4.4.3 spine formula on the third exopodal segment of legs 1-4. Congeners sharing these three morphological characteristics differ variously from H. itohi in their caudal ramus shape, urosomal frills, antennal setal number, medial elements of the third endopodal segment of leg 4, and/or the shape of leg 5. The hitherto known Halicyclops species from Japan are briefly reviewed, and a key to the Japanese species is presented to resolve previous questionable identifications.

Two New Species of Halicyclops (Crustacea: Copepoda: Cyclopidae) from Honshu with a Key to the Species of the Genus in Japan