Macrophthalmus japonicus and M. banzai were classified as mature and immature using the R package “sizeMat”, and the relative growth of the male cheliped and female abdomen was evaluated to characterize the secondary sex characteristics of the closely related crab species. In males, mature M. japonicus appeared at a carapace width（CW）of 15.57 mm, while mature M. banzai appeared at a CW of 11.18 mm. In immature individuals before the onset of secondary sex characteristics, the allometry coefficient of the male cheliped of M. banzai was higher than that of M. japonicus. In females, mature M. japonicus appeared at a CW of 14.37 mm, while mature M. banzai appeared at a CW of 9.69 mm. In immature individuals, the allometry coefficients of the female abdomens of M. banzai and M. japonicus were not significantly different. However, the intercept was higher in M. banzai, suggesting that female M. banzai may reproduce soon after the onset of secondary sex characteristics.

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