Sexual dimorphism is the most conspicuous difference between the sexes. This study examines possible sexual dimorphism and the relative growth patterns of morphometric characteristics in the marine medaka, Oryzias dancena for their potential to help differentiate between males and females of this species. The von Bertalanffy growth parameters estimated by a non-linear regression method were L∞=30.2 mm, K=3.22/year, and τ0=-0.05. All 18 characteristics measured showed a difference between males and females from 70 days after hatching. Each of these characteristics were significantly different between sexes (ANCOVA, P<0.05), and the ratio of standard length between sexes showed that males were larger than females for all five morphometric measurements. Fin length measurements were taken for 21 distances of anal fin and 7 distances of dorsal fin between landmarks. There were all differences for all dorsal fin rays between the males and the females and there is significant difference in 70 days after their hatch when the sexual dimorphism is presented. The significant difference (P<0.05) in fin ray for male and female was more greatly seen as they grow. Male marine medaka showed more rapid growth than females, with longer length, dorsal fins and anal fins. Differences in these characteristics will be useful during experiments when it is necessary to differentiate between sexes of marine medaka.

• Jae Hyun Im(Inland Fisheries Research Institute, National Fisheries Research &#x26; Development Institute, Cheongpyeong 12453, Korea)
• Hyun Woo Gil(Division of Marine Environment and Bioscience, College of Ocean Science and Technology, Korea Maritime and Ocean University, Busan 49112, Korea)
• Tae Ho Lee(Division of Marine Environment and Bioscience, College of Ocean Science and Technology, Korea Maritime and Ocean University, Busan 49112, Korea)
• Hee Jeong Kong(Biotechnology Research Division, National Fisheries Research &#x26; Development Institute, Busan 46083, Korea)
• Cheol Min Ahn(Biotechnology Research Division, National Fisheries Research &#x26; Development Institute, Busan 46083, Korea)
• Bong Seok Kim(Future aquaculture research center, National Fisheries Research &#x26; Development Institute, Jeju 63610, Korea)
• Dong Soo Kim(Institute of Marine Living Modified Organisms (iMLMO), Pukyong National University, Busan 48513, Korea)
• Chang Ik Zhang(Division of Marine Production System Management, College of Fisheries Sciences, Pukyong National University, Busan 48513, Korea)
• In-Seok Park(Division of Marine Environment and Bioscience, College of Ocean Science and Technology, Korea Maritime and Ocean University, Busan 49112, Korea) Corresponding author