Elthusa phoenix sp. nov. (Crustacea: Isopoda: Cymothoidae), a branchial fish parasitic isopod, from the Sohachi flounder, Cleisthenes pinetorum Jordan and Starks, 1904 from off Sado Island, Japan

A new branchial fish parasitic cymothoid species, Elthusa phoenix sp. nov. (Crustacea: Isopoda: Cymothoidae), is described based on the ovigerous female collected from off Sohachi flounder, Cleisthenes pinetorum Jordan and Starks, 1904 obtained from Sado Island, Japan. Elthusa phoenix sp. nov. can be distinguished from all congeners by the following combination of features: body slightly twisted, elongated, dorsal surfaces smooth, 1.9 times as long as greatest width; pleon short, ∼11% body length, pleon 0.75 times as wide as pereon maximum width; cephalon symmetrical, sub-triangular 1.20 times wider than long, anterior margin rounded, turned down to form a rostrum; short uropods extending to middle of the pleotelson; pereopods 4–7 basis proximo-lateral margin with small carinate lobe; prominent fleshy lobes present on sternite of pereonite 7; antenna with 12 articles; maxilliped with oostegite lobes, and lateral margin with many plumose setae.