Comparison of morphometric identification of species in juvenile mangrove crabs (Genus Scylla) by automated and local approaches

Morphological species identification in juvenile Scylla is now possible using image analysis of frontal lobe spines. The differences found are subtle and require automation using a mobile application. Considering the bulk of crablets to be sorted for trading, distinct features that can help stakeholders distinguish the species of juveniles at a glance is still needed. In this study, morphometric analysis of dorsal carapace features was performed to identify more obvious features for species identification. Principal component analysis of ten morphometric characters from 1186 juveniles revealed only two clusters with Scylla olivacea creating a separate group from S. tranquebarica and S. serrata. The most distinguishable ratio for S. olivacea, determined through discriminant analysis and ANOVA, was the ratio of the frontal median spine height to the frontal width at 0.035 (p = 0.002). There is an emerging market for S. olivacea in the soft-shell crab industry. Selection of this smaller but useful species can aid fishers in setting appropriate prices for traded crablets and minimize predation to the larger S. tranquebarica and S. serrata. Testing if the ratio can be detectable by visual approximation is ongoing as well as alternative features based on local practices.

Automated summary

Morphological species identification in juvenile Scylla is possible through image analysis of frontal lobe spines, with distinct features still needed for easy differentiation. This study identified the most distinguishable ratio for S. olivacea and highlighted its emerging market potential in the soft-shell crab industry. Ongoing tests are exploring visual detectability of the ratio and alternative features based on local practices.