The morphological differentiation of two horseshoe crab species,Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura), in Hong Kong with a regional Asian comparison

Two horseshoe crab species, namely Tachypleus tridentatus and Carcinoscorpius rotundicauda, have been identified from Hong Kong. This paper describes the morphological characteristics of the two species and the morphometrics employed to help distinguish them. Comparisons were also made between adults from local waters and those from other Asian geographic regions. Intra-specific variations were also investigated by comparing morphological data obtained for local T. tridentatus juveniles and individuals of comparable sizes from Fujian Province, China. Morphological differences between the species and sexes and significantly different morphometric ratios were identified for the two horseshoe crab species. Intra-specific variations in morphometric characteristics were also demonstrated between juveniles from local beaches and those from Fujian.