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A new species closely related to Acartia sinjiensis (Copepoda: Calanoida), from river estuaries of northern Luzon, the Philippines.

We describe a brackish-water calanoid copepod Acartia (Acanthacartia) cagayanensis sp. nov. collected from river estuaries in the northernmost Luzon, the Philippines. The new species has several characteristic features that are typical to the A. plumosa group (A. (A.) plumosa Scott T., 1894, A. (A.) sinjiensis Mori, 1940 and A. (A.) tropica Ueda Hiromi, 1987); specifically, a short apical spine on the long terminal segment of male left leg 5, which is unique to the group. The morphological features of A. cagayanensis sp. nov. different from those of the A. plumosa group are the barrel-shaped genital double somite and the cylindrical basal part of the terminal segment of female leg 5. Among the species in the group, A. cagayanensis sp. nov. is closest to A. sinjiensis in terms of the spinule patterns on the female antennule, the posterior corner of the prosome, and the male second urosomite. The genetic analysis using DNA sequences of mitochondrial gene cytochrome oxidase subunit I (COI) revealed that A. sinjiensis from Japan and A. cagayanensis sp. nov. differed by 16.5-16.9%, in contrast to a small variation (0.0-0.5%) within each population. We confirmed that previous records of A. sinjiensis from the Philippines were not A. cagayanensis sp. nov., and therefore, A. cagayanensis sp. nov. is the third species of the subgenus Acanthacartia Steuer, 1925, after A. sinjiensis and A. (A.) tsuensis Ito, 1956.