First Specimen-based Records of Redfin Emperor Monotaxis heterodon (Perciformes: Lethrinidae) from Japan, with New Diagnostic Characters Applicable to Identification of Preserved Specimens

Twenty-four specimens (184.6–243.6 mm in standard length) of Monotaxis Anonymous [Bennett], 1830, collected from the Ryukyu Islands, southern Japan, were identified as M. heterodon (Bleeker, 1854) (Perciformes: Lethrinidae). Although a widely distributed Indo-West Pacific species, all previous records from Japanese waters have been based solely on photographs, the present specimens therefore representing the first specimen-based records of M. heterodon from Japan. A detailed comparison between M. heterodon and its only congener, M. grandoculis (Forsskål, 1775), from which it has previously been distinguished primarily by scale rows below the lateral line and fresh or live coloration, revealed new diagnostic characters, most of which are applicable to preserved specimens. In addition to previously recognized characters, the two species can be distinguished by mid-dorsal snout profile in adults (concave vs. straight), snout length (excluding lips) [8.9–11.1 (mean 9.9) % of SL vs. 10.3–12.1 (11.2) % SL], spinous anal-fin base length [4.6–5.9 (5.0) % of SL vs. 3.9–4.9 (4.4) % SL], a distinct black blotch above the pupil (absent vs. present), a dark brown stripe across the post interorbital region in preserved specimens (present vs. absent), and preserved coloration of the pectoral-fin base inner surface (blackish brown vs. light brown).