Bull. Jpn. Soc. Fish. Oceanogr. 78(1), Page 36-44, 2014   Larval morphology of Scomber japonicus and Scomber australasicus: Re-evaluation of morphological characteristics as species identifiers Chiyuki Sassa1†, Mami Saito2, Toru Kitamura2, Yoshinobu Konishi1 and Youichi Tsukamoto1 1 Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8 Taira-machi, Nagasaki 851-2213, Japan 2 JAPAN NUS Co., Ltd., Yokohama Kanazawa High-technology Center 5F, Fukuura 1-1, Kanazawa-ku, Yokohama 236-0004, Japan † E-mail: csassa@fra.affrc.go.jp Chub mackerel Scomber japonicus and spotted mackerel S. australasicus are important fishery resources in the western North Pacific and its adjacent seas. Larval morphology of these two species is similar, and no clear conclusion has been reached as to whether it is possible to distinguish larvae of the two species based solely on morphological characteristics. Using specimens fixed and preserved under the same conditions, we compared the larval morphology of S. japonicus and S. australasicus to investigate the possibility of morphological species identification. We used a total of 1,172 Scomber spp. larvae of <9 mm in body length (BL) collected in the southern East China Sea during February and March, 2004-2010. After observing morphometric characteristics and pigmentation, the species of each larva was identified by using polymerase chain reaction-restriction fragment length polymorphism analysis of mitochondrial DNA (mtDNA). We identified 590 S. japonicus and 582 S. australasicus larvae. There was no interspecific difference in BL at notochord flexion. In larvae of <7 mm BL, the frequency of occurrence of internal melanophores at the nape was significantly higher in S. australasicus than in S. japonicus. In larvae of 3.0-6.9 mm BL, the number of melanophores in a series along the ventral midline of the tail was significantly lower in S. australasicus than in S. japonicus. Pre-anal length relative to BL and head length relative to BL were significantly higher in S. australasicus than in S. japonicus in larvae of <4 mm BL and 4.0-6.9 mm BL, respectively. We applied discriminant analysis to use these characteristics to differentiate the species, but the accuracy rate was relatively low (76%) because of large overlaps in ranges of the characteristics for the two species. We conclude that mtDNA analysis is necessary to accurately identify larvae of these two Scomber spp. Key words: chub mackerel, spotted mackerel, larval morphology, species identification, southern East China Sea, mitochondrial DNA