抄録
Two species of Amblygaster, A. leiogaster and A. sirm, are sympatrically distributed in the Ryukyu Islands and often caught by the same set net. Although both species are important resources for food and fishing bait, they are not distinguished by local fishermen. Since these two species are so similar to each other, identification errors have often occurred in publications. In this study, we re-evaluated the known identification characters (position of the dorsal fin, upper jaw length, number of gill rakers) and also searched for other distinguishable characters based on 52 specimens (110.3–226.0 mm SL) of A. leiogaster and 53 specimens (77.5–234.5 mm SL) of A. sirm. Amblygaster leiogaster is characterized by the origin of dorsal fin at the middle of snout tip and caudal fin base (predorsal length 48.5–52.6% SL), small upper jaw (26.2–35.7% HL), fewer gill rakers (31–36), dorsal sides of body blue when fresh, no oblique line above the upper jaw, while A. sirm is characterized by the origin of the dorsal fin closer to snout tip than caudal fin base (predorsal length 42.4–47.1% SL), larger upper jaw (32.2–38.7 % HL), more gill rakers (37–42), dorsal sides of body greenish blue when fresh, and an oblique line (greenish when fresh and black after being preserved) above the upper jaw. In addition, prevalence of Ryukyua globosa on two species of Amblygaster was examined based on 128 specimens (120.3–211.3 mm SL) of A. leiogaster and 117 (122.5–159.0 mm SL) of A. sirm. No parasitic R. globosa were found on specimens of A. leiogaster. In contrast, the parasite was found only on specimens of A. sirm (total 85%) and its prevalence increased with host body-size.
