Morphometric parameters that reveal sex differences on the dorsal surface of the horsehair crab <i>Erimacrus isenbeckii</i> (Brandt, 1848) (Decapoda: Brachyura: Cheiragonidae)

Abstract Visual sex identification of the horsehair crab Erimacrus isenbeckii (Brandt, 1848), a popular food in Japan, on fishing vessels and in food-processing factories is important for preventing indiscriminate fishing of female crabs as most individuals settle with their dorsal sides upward when placed on a flat surface. We used the Mahalanobis-Taguchi system to identify the sex of horsehair crabs on the basis of images of their dorsal surfaces. The Mahalanobis-Taguchi system is a multivariate analytical method that can be used for effective cause analysis without supervision. The system was applied to photographs of 60 male and 60 female horsehair crabs captured in Funka Bay, southern Hokkaido, Japan. We determined the signal-to-noise (SN) ratio for five variables related to several morphological characters on the dorsal surface of the crabs. As a result, the widths of the carapace and posterior margin (i.e. the distance between the base of the coxae of the fifth pereiopods) were found to be the best combination among five variables related to the dorsal view, enabling effective male/female identification.